EBioMedicine 70 (2021) 103514 Contents lists available at ScienceDirect EBioMedicine journal homepage: www.elsevier.com/locate/ebiom Review Personalised, image-guided, noninvasive brain stimulation in gliomas: Rationale, challenges and opportunities Giulia Sprugnoli a,b,c,d, Simone Rossi d, Alexander Rotenberg e, Alvaro Pascual-Leone f,g, Georges El-Fakhri h, Alexandra J. Golby c,1, Emiliano Santarnecchi a,1,∗ a Berenson-Allen Center for Noninvasive Brain Stimulation, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA b Radiology Unit, Department of Medicine and Surgery, University of Parma, Parma, Italy c Image Guided Neurosurgery laboratory, Department of Neurosurgery and Radiology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, USA d Brain investigation and Neuromodulation Laboratory (Si-BIN Lab), Department of Medicine, Surgery and Neuroscience, Neurology and Clinical Neurophysiology Unit, University of Siena, Siena, Italy e Department of Neurology and Division of Epilepsy and Clinical Neurophysiology, Boston Children’s Hospital, Harvard Medical School, Boston, MA, USA f Hinda and Arthur Marcus Institute for Aging Research and Center for Memory Health, Hebrew Senior Life, Boston, MA, USA g Guttmann Brain Health Institute, Institut Guttmann, Universitat Autonoma, Barcelona, Spain h Gordon Center for Medical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA a r t i c l e i n f o a b s t r a c t Article history: Malignant brain tumours are among the most aggressive human cancers, and despite intensive efforts Received 20 April 2021 made over the last decades, patients’ survival has scarcely improved. Recently, high-grade gliomas (HGG) Revised 12 July 2021 have been found to be electrically integrated with healthy brain tissue, a communication that facilitates Accepted 19 July 2021 tumour mitosis and invasion. This link to neuronal activity has provided new insights into HGG patho- physiology and opened prospects for therapeutic interventions based on electrical modulation of neural Keywords: and synaptic activity in the proximity of tumour cells, which could potentially slow tumour growth. Non- Brain tumours invasive brain stimulation (NiBS), a group of techniques used in research and clinical settings to safely Glioma modulate brain activity and plasticity via electromagnetic or electrical stimulation, represents an appeal- HGG ing class of interventions to characterise and target the electrical properties of tumour-neuron interac- NiBS tions. Beyond neuronal activity, NiBS may also modulate function of a range of substrates and dynamics Noninvasive brain stimulation Neuromodulation that locally interacts with HGG (e.g., vascular architecture, perfusion and blood-brain barrier permeabil- ity). Here we discuss emerging applications of NiBS in patients with brain tumours, covering potential mechanisms of action at both cellular, regional, network and whole-brain levels, also offering a concep- tual roadmap for future research to prolong survival or promote wellbeing via personalised NiBS inter- ventions. © 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 1. Introduction 18 months from diagnosis [2]. Many novel nonsurgical and non- pharmacologic therapies have been tested over the last decades as Due to limited therapeutic options, incidence in relatively adjuncts to the standard of care that includes surgery, radiation young people, delayed diagnosis, and infiltration into the brain and chemotherapy [3]. Among these, a tumour treating [electrical] parenchyma, malignant brain tumours rank fourth among all can- fields (TTFs) device (Optune- NovoTTF-100A System) that generates cers in terms of number of years of life lost, despite representing transcranial electrical stimulation utilising alternating current at a only 2% of all cancers [1]. Among primary brain cancers, glioblas- frequency of 200 kHz may work by disrupting mitosis in cancerous toma (GBM) is the most frequent and aggressive high-grade glioma cells. The device has been cleared by the FDA and recommended (HGG, WHO glioma IV), with a mean survival of approximately 16– in combination with chemotherapy (CHT) by the National Com- prehensive Cancer Network as a potentially effective treatment for ∗ patients with newly diagnosed GBM [4]. However, the benefit of Corresponding author. TTF in terms of patient survival has been modest (20.9 months in E-mail address: esantarn@bidmc.harvard.edu (E. Santarnecchi). 1 These authors contributed equally to the work. the TTF-CHT group vs 16.0 months in the CHT group [5,6]), and no https://doi.org/10.1016/j.ebiom.2021.103514 2352-3964/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 benefits have been reported for patients with recurrent GBM [6]. 2. Transcranial stimulation Thus, novel therapeutic strategies for HGG remain an unmet need. The recent discovery of neuron-to-glioma synaptogenesis of- TMS leverages the principle of electro-magnetic induction by fers the possibility for novel strategies to interfere with this new which an intracranial electric field (E-field) is induced by a rapidly pathophysiological behaviour [7,8], also delineating the new field fluctuating (i.e., 300–350 μs) magnetic field that penetrates into of cancer neuroscience [9]. While the concept of HGG promot- the brain through the scalp and skull [15], focally depolarising ing neuronal excitability is not new [10], the existence of synaptic neurons (Fig. 1A). TMS was introduced in 1985 and several TMS neuron-to-tumour connections, which lead HGG cells to depolar- devices are currently approved by the FDA and other regulatory ize in response to neuronal spiking and proliferate, is completely agencies worldwide for the treatment of drug-resistant depres- novel. Glioma cells reflect cellular subpopulations at various stages sion, obsessive compulsive disorder and migraine with aura, as of astrocytic and oligodendrocytic differentiation, such as oligoden- well as for presurgical mapping of eloquent areas including motor droglial precursor cells (OPCs) [11]. Normal OPCs are able to form and language areas [13]. When multiple stimuli are applied in a synapses with neurons, a communication that regulates progenitor repetitive fashion (repetitive TMS - rTMS), the stimulation induces cell proliferation, migration, differentiation and even synaptic plas- long-term plastic changes in the brain, modifying the efficacy of ticity in the human brain [11]. Interestingly, only OPCs and stem synaptic communication by triggering LTP or long-term depres- and progenitor cells (NPCs) are provided with a subtype of amino- sion (LTD)-like mechanisms, depending on the specific frequency 3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPA-R, applied. In particular, high-frequency rTMS (>1Hz) or intermittent ionotropic glutamate receptors) lacking of a GluR2 subunit that TBS – iTBS (short trains of impulses) usually increases cortical ex- makes the receptors Ca2+ -permeable, in contrast to the AMPA- citability and causes LTP-like effects, while low frequency rTMS R type of the differentiated neurons that are not permeable to (≤ 1 Hz) or continuous TBS – cTBS (single train of pulses) more Ca2+ [11]. Of note, Ca2+ -permeable AMPA-R are also physiologi- frequently causes a decrease of cortical excitability and eventually cally expressed in interneurons and characterised by fast kinetics LTD-like effects (Fig. 1A) [15]. Physiological LTD induction is de- thought to be crucial for neural development and synaptic plastic- pendant on N-methyl-D-aspartate (NMDA) receptors that are usu- ity, in terms of triggering long-term potentiation (LTP) and defining ally mildly stimulated via low frequency stimulation (LFS), leading synaptic efficacy [12]. Surprisingly, glioma cells express the same to a modest intracellular Ca2+ elevation that in turn activates pro- excitatory synaptic structures consisting of Ca2+ -permeable AMPA- tein phosphatases responsible of downregulation of AMPA-R [20]. R observed in normal OPCs [7,8]. Also, similarly to the normal OPCs LTD (and LTP)-like phenomena induced by TMS have been related forming the axoglial synapses, glioma cells have been observed to glutamatergic NMDA-mediated transmission and relative influx only in the postsynaptic part of these neuron-glioma synapses [7,8] of Ca2+ into the post synaptic cells that downregulate the AMPA-R, and the depolarising currents cause a calcium-ion influx into the both in vitro and in vivo [21]. Apart from LFS, LTD can be physiolog- glioma cells that ultimately promote tumour proliferation and in- ically also induced via baseline synaptic stimulation contempora- vasiveness [7,8]. neously with depolarisation (i.e., pairing), by administration of an The above findings highlight a positive feedback mechanism appropriate receptor agonist or via timed back-propagating action between increased neuronal excitation triggered by gliomas and potentials (i.e., spike-timing dependant plasticity - STDP) [20]. TMS its impact on mitosis and migration, suggesting that controlling protocols have been adapted to match each of these LTD-inducing neuronal excitability in patients with HGG may inhibit tumour mechanisms (for a comprehensive review see [21]). Finally, long- growth and proliferation, ultimately prolonging patient survival. To term effects of TMS seem to depend also on activity-dependant this end, we aim to bring attention to novel therapeutic oppor- brain-derived neurotrophic factor (BDNF) plasticity, gene induction tunities offered by noninvasive brain stimulations (NiBS; Fig. 1), and modulation of multiple neurotransmitter levels [22]. Therefore, a group of neurostimulation techniques that include transcranial TMS is thought to act via numerous mechanisms and pathways magnetic stimulation (TMS), and transcranial electrical stimulation to modify synaptic plasticity, potentially relevant to decrease the (tES) [13,14]. The possibility to interfere/interact with ongoing neu- neuronal-induced tumour growth. ronal activity and impact behaviour/cognition supported by a spe- In contrast to TMS that depolarises neurons thus generating cific brain area has led to the implementation of ad-hoc neurostim- action potentials, tES involves almost imperceptible electrical cur- ulation protocols to affect motor and/or cognitive functions in the rents (~2 mA) delivered by scalp electrodes, that reach the cortex healthy brain, as well as FDA-approved devices and protocols for where they modulate the resting membrane potential, thus affect- treatment of certain neurological and psychiatric conditions. NiBS ing the excitability of pyramidal cortical neurons [23] without di- protocols are safe and well-tolerated and have demonstrable ca- rectly inducing neuronal firing (Fig. 1B). Optimised tES protocols pacity to modulate cortical excitability [13,14], intracortical excita- enable multielectrode solutions to target cortical regions with a tion/inhibition (E/I) balance and cortical plasticity [15], even in a few centimeters resolution. Among tES methods, transcranial di- long-lasting manner via modifications of synaptic strength and ef- rect current stimulation - tDCS, involves subthreshold depolarisa- ficacy mediated by mechanisms resembling LTP and long-term de- tion of neurons under the anode (generally resulting in an in- pression (LTD) [16], thus may offer an interesting opportunity for crease of their excitability) and hyperpolarisation of those under modulation of neuron-to-glioma functional circuitry activity. the cathode (decrease of excitability), respectively leading to en- In light of recent findings of electrical HGG integration, NiBS hancement or suppression of regional brain activity usually par- can be regarded as a potential tool to suppress the proposed alleled by cognitive/behavioural modifications linked with the role neuron-to-glioma communication by, for instance, inhibiting neu- of the targeted regions/networks [24]. Short-term effect of tDCS in- ronal populations surrounding HGG via LTD-like effects. On the volves voltage-dependant ion channels, while stimulation extend- other hand, NiBS could also affect tumour growth via non-neuronal ing over a few minutes promotes LTP or LTD-like plasticity that can effects. Indeed, NiBS has recently been found to modify cere- also affect interconnected cortical and subcortical areas [23], again bral and intratumoural perfusion [17], the permeability of the relying on NMDA receptors and Ca2+ influx [25]. In addition, tDCS blood brain barrier (BBB) [18], and to interact with microglia [19], acts on cortical excitation/inhibition balance via a modulations of suggesting additional interventional -still unexplored- noninvasive γ -aminobutyric acid (GABA), BDNF and glutamate/glutamine con- stimulation strategies for patients with brain tumours. centrations [23]. Other forms of tES that deliver different type of current (i.e., alternating current or random noise) have been de- veloped and tested during the last years among healthy subjects 2 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 Fig. 1. NiBS techniques. (a) Transcranial Magnetic Stimulation (TMS) can be applied as a single stimulation pulse (single pulse TMS, spTMS), pairs of stimuli separated by variable intervals (paired pulse TMS, ppTMS) delivered to the same or different brain areas, or as trains of repetitive stimuli at various frequencies (repetitive TMS, rTMS) to respectively measure cortical excitability, excitation/inhibition balance and induce long-lasting neuromodulation effects and changes in plasticity. Outside the motor cortex, accurate targeting is guaranteed by a neuronavigation system that provides, on the basis of individual MRI data, the spatial coordinates of a target area allowing the coil to be held in the correct position during a stimulation session, as well as in subsequent ones. Long Term Depression (LTD) and Long Term Potentiation (LTP)- like effects are mediated by multiple mechanisms, such as actions on GABA and NMDA receptors, gene induction and modulation of numerous neurotransmitters. When spTMS is applied to the motor cortex, TMS elicits motor evoked potentials (MEPs) recorded via surface electromyography, whose amplitude reflects the excitability and integrity of the corticospinal system, the conduction properties along peripheral motor pathways, and the degree of excitability of the motor cortex itself [27]. When ppTMS is delivered, depending on the specific stimulation parameters, intracortical Inhibition and Facilitation can be assessed, respectively measuring the activity of GABAergic and glutamatergic (inter)neurons as well as excitation/inhibition balance (E/I). (b) Transcranial electrical stimulation (tES) can be applied using 2 electrode or adopting multifocal montages that allow for finer customisation of stimulation protocol and targeting accuracy over the region(s) of interest. tDCS: Direct Current Stimulation causes a subthreshold depolarisation of neurons under the anode (increased excitability) and hyperpolarization of neurons under the cathode (decreased excitability). Transcranial alternating current stimulation (tACS) is capable of modulating cortical rhythms, i.e. to entrain neuronal firing at a specific frequency and causing enhancement of related brain functions [28,29]. tACS depolarised and hyperepolarised neurons at a specific frequency (e.g., 10Hz), increasing the probability of neuronal spiking in response to other inputs during their depolarised phase via stochastic resonance mechanism [23]. As per the STDP law, synapses of neuronal network that have a resonance frequency matching that of repetitive inputs are strengthened. Transcranial random noise stimulation (tRNS) delivers electrical noise in a wide frequency band (1–640 Hz) to modulate cortical excitability. The injection of noise is thought to promote the excitability of pyramidal cells via stochastic resonance mechanism, but activation of voltage-gated sodium channels has been documented as well [23]. tRNS after-effects seems to be mediated by GABA receptors and voltage-gated sodium channels [23] to increase their cognitive abilities as well on patient populations has a good safety profile, with no adverse effects other than occa- to restore physiological neural activity [23] (see Fig. 1B). sional scalp itching and redness, especially for tES [14]. As for TMS, LTD-like changes in synaptic excitability could be relevant in the risk of a seizure induction is low, even in patients with pre- the new neuron-to-glioma context, where reduced probability of existing epileptic conditions or who are taking medications which neuronal firing after a presynaptic event would reduce the activa- potentially lower the seizure threshold [13]. Also, it must be no- tion of Ca2+ AMPA-R in the post-synaptic glioma cell, leading to a ticed that patients with brain tumour usually assume antiepileptic limited inflow of Ca2+ signal mitosis-promoting and thus poten- treatment lowering the possibility of the occurrence of seizures. tially limiting the neuronal contribution to glioma growth [7–9]. Moreover, NiBS protocols that increase the risk of seizure are those Notably, NiBS protocols differ conceptually from the currently-used inducing hyperexcitability in the brain, that reasonably will not be TTF paradigm in which alternating current is delivered at extra- those applied in the case of patients with brain tumours since the physiological frequencies (200 kHz) and is aimed at directly inter- aim is to suppress the neuron-to-glioma communication via in- fering with cancer cell mitosis, rather than modulating neuronal hibitory protocols of stimulation, thus further lowering the possi- activity [26]. TTFs requires patients to shave their head and wear bility of inducing seizures. the devices at least 18 h/day in order to be effective, whereas tES In conclusion, many of the neurophysiological peculiarities of- offers a light and highly portable alternative that typically pro- fered by NiBS can potentially impact tumour pathophysiological duces neurophysiological (i.e., modulation of cortical excitability) mechanisms that are currently not targeted by other therapeu- and cognitive effects (i.e., performance increase) even with single, tic approaches, including TTF that supposedly affects tumour mi- relatively short (i.e., 30 min) sessions. Finally, a considerable per- tosis. Considering the standard of care provided to patients with centage of patients receiving TTFs (up to 43%) reports dermatolog- HGG, consisting in surgical excision, radiotherapy plus concomi- ical adverse effects such as dermatitis, erosions, ulcers, and infec- tant chemotherapy and adjuvant chemotherapy, a specific tempo- tions due to the continuous wearing of the device [26], while NiBS ral framework can be proposed for the implementation of NiBS 3 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 Fig. 2. Temporal Framework for potential NiBS Applications. Potential applications of NiBS in patients with brain cancers are presented, considering the timeline of standard of care therapy (e.g., surgical excision, radiotherapy plus concomitant chemotherapy, adjuvant chemotherapy). NiBS could be carried out via repetitive application in a long- term perspective to cause LTD-like effects, while other applications could be limited to a defined temporal window matching standard therapies, whose efficacy could be enhanced by concomitant application of NiBS, e.g. promoting drug delivery during CHT. Given the portability of tES in particular, extended home-use may be feasible. Baseline assessment should include neuroimaging and/or nuclear imaging data for image-guided personalised interventions to maximise effects towards relevant areas with minimal side effects, e.g. targeting the solid mass to reduce intra-tumoural perfusion, or the surrounding brain regions to inhibit tumour-promoting neural activity. Finally, amelioration of neurological, cognitive and psychiatric symptoms – thereby avoiding or reducing additional pharmacological treatments — could also be considered in patients with HGG, given the positive effects documented in other populations of patients. Note: CHT = chemotherapy, MRI = Magnetic Resonance Imaging, PET = Positron Emission Tomography, TAMs = Tumour-Associated Macrophages. (Fig. 2). We foresee a roadmap of the many opportunities through ing long cellular processes crucial for tumour invasion and allow- which NiBS applications could be offered to patients at multiple ing the connection of glioma cells into a functional communicat- timepoints in their clinical course, possibly in a personalised man- ing network, essential for transferring growth elements and factors ner (e.g., considering tumour location, type, size, extension of per- favouring treatment resistance [8]. The investigators observed fast itumoural infiltrated tissue; Fig. 2). In the following sections, bio- excitatory postsynaptic current propagating inside the HGG cells, logical mechanisms for local and network-based NiBS applications mediated by AMPA receptors and time-locked with the neuronal are presented, with suggestions for potential clinical trial protocols spiking of the presynaptic neurons. This fast response is followed in patients with HGG. by a long-lasting depolarizing current, probably depending on ex- tracellular concentration of potassium ions rather than on synap- 3. Local therapy tic activity. In total, approximately 31% of the observed GBM cells showed at least one of these electrical responses [8]. Additionally, NiBS may be used to modulate/suppress synaptic signalling of electrical currents were found to spread in the network of inter- neurons surrounding an HGG tumour, therefore slowing down its connected glioma cells via gap junctions, strengthening also the mitosis and migration rate. Moreover, additional, non-neuronal ef- connectivity between the cells’ network. Finally, depolarisation cur- fects of NiBS have begun to emerge, such as modulation of perfu- rents cause a calcium transient inside the cells, ultimately driving sion and permeability, as well as activation of microglia, each one mitosis and invasiveness. Pharmacologic and genetic blockage of potentially relevant for brain tumour management. AMPA receptors, as well as drug targeting gap junctions, reduced the invasion and mitosis of glioma cells, in turn promoting mouse 3.1. Suppression of neuronal activity-regulated cancer growth survival. Interestingly, such neuron-to-glioma communication is not uni- Recent pivotal work by two independant groups has demon- lateral. In clinical settings, glioma patients have an increased risk strated that neuronal activity promotes the proliferation and in- of seizures, and worsening seizure control correlates with recur- vasiveness of HGG in vivo [7,8]. One of the two groups, led by rence [10]. Even if glioma cells are not able to spike, they have Michelle Monje, has shown that neural activity promotes the mi- been found to promote neuronal firing in order to create a posi- tosis of cancer cells via a specific pathway involving synaptic pro- tive feedback with neurons for their further activation via multiple tein neuroligin-3 (NLGN3) for adult and paediatric GBM, diffuse in- mechanisms, such as synaptogenic factor secretion, non-synaptic trinsic pontine gliomas (DIPG), and anaplastic oligodendrogliomas glutamate release, and by reducing the activity of inhibitory in- [30]. NLGN3, secreted in an activity-dependant manner by neu- terneurons in the surrounding microenvironment [10]. Accordingly, rons, inversely correlates with overall survival (OS) of adult GBM epileptiform activity has been found to arise in the infiltrated patients [30], and patient-derived orthotopic xenografts of paedi- parenchyma of glioma xenotransplants [32], strongly dependant on atric GBM, DIPG and adult GBM are unable to grow in NLGN3- glutamate release [10]. Monje and colleagues further confirmed knockout mice [31]. These findings were recently complemented and extended these data by showing increased high gamma (γ , by the observation of synaptic structures between HGG cells and 70–110 Hz) activity–an index of neuronal activation— in infiltrated surrounding neurons in multiple models of HGG and DIPG, such tissue of patients with HGG via intraoperative electrocorticography as in patient-derived xenografts, resected human tumour tissue, [7]. Overall, HGG is integrated into the brain and is even capable and genetic mouse models [7,8]. As anticipated, these neuro- of initiating a vicious circle to promote its mitosis and invasiveness gliomal synapses show the characteristics of glutamatergic chem- by amplifying the excitability of the surroundings neurons. Further ical synapses, specifically involving Ca2+ -permeable AMPA recep- research is needed, including investigations directed at LGG, con- tors, with the glioma cells being exclusively postsynaptic. AMPA- R have been observed along the tumour microtubes, represent- 4 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 sidering that LGG–and not HGG- are more frequently associated cant benefit in OS in newly diagnosed nor in recurrent GBM, prob- with epilepsy in the clinical settings [33]. ably due to the single pathway targeting that causes a compen- Given the ability of rTMS and tDCS of causing long-lasting sup- satory increase in other neoangiogenesis strategies [41]. pression of neuronal spiking/excitability via LTD-like effects, these Electrical current has been shown to reduce intratumoural per- techniques could represent safe and useful tools to modulate neu- fusion in extracranial tumours (i.e., breast and lung cancer, liver ronal firing and thus, hopefully, limit glioma mitosis and invasive- metastases) when applied via two or more platinum electrodes lo- ness. In support of this, it has already been shown that both high- cated directly inside of the tumour or in the surrounding tissue. In frequency rTMS (600 pulses over 30 min daily) and low-frequency the last two decades, the application of electrical stimulation (Elec- rTMS (1800 pulses over 30 min daily at 1 Hz) protocols applied trical Treatment, ET) to malignant visceral tumours has been found to the left or right dorsolateral prefrontal cortex for drug-resistant to reduce tumour perfusion via a vasoconstriction phenomenon, depression are able to induce long-lasting changes in local corti- and even cause tumour necrosis when applied over multiple days cal activity, connectivity, perfusion, and even affect structural brain [42]. In addition, ET (usually delivering 10 0 0 V/cm at 5 kHz) seems properties [34,35]. Given the focality of TMS electric fields (E-field, to potentiate the effect of chemotherapy (CHT), by decreasing tu- 1-2 cm3 ), rTMS could be applied to several locations surround- mour blood flow, which in turn prolongs the contact with and con- ing a small cortical lesion, consequently targeting a relatively lim- sequently the action of CHT agents on tumour cells. ET also mod- ited bordering neuronal population; alternatively, a local or distant ulates tumour trans-membrane permeability favoring drug inter- single area could be targeted based on significant communication nalization [42], aligning with tDCS evidence in rat and endothelial with the HGG cells detected via neuroimaging modalities such as monolayer models [18,43]. For these reasons, many trials applying functional MRI-based connectivity. electrochemotherapy, as it is now known, are being conducted on Compared to TMS, tDCS applies a broader E-field that can be colorectal tumours, basal cell carcinoma and even spine metastases useful in the presence of larger lesions receiving inputs from mul- [44]. Targeted delivery of electrical stimulation to the solid tumour tiple other brain regions. Despite not being able to directly in- mass could, in theory, reduce intratumoural perfusion similarly to hibit the spiking of pyramidal neurons, the cathodal field can re- that observed in extracranial tumours and eventually even induce duce the probability of the neuronal spiking in the targeted cor- tumour cell necrosis. tical areas via LTD-like effects as well, representing an appeal- A recent pilot study by our group tested this possibility in pa- ing approach to interrupt diffuse neuron-to-glioma communication tients with GBM (n = 6) and lung metastasis in the brain (n = 2) (see also Network-based therapeutic opportunities paragraph). Ad- [17]. Multifocal tDCS was delivered for 20 min with an MRI- ditionally, both techniques can be safely combined in the same compatible device while the patient was inside the MRI scanner protocol to induce a synergistic effect on brain excitability [13]. A allowing contemporaneous assessment of perfusion variation via recent study simultaneously applying rTMS at inhibitory frequency a CBF-sensitive sequence (ASL) in a single experimental session. and cathodal tDCS over the motor cortex showed a stronger in- Direct current stimulation was applied according to individualised hibitory modulation of motor evoked potential (MEPs amplitude, biophysical models based on manually traced tumour masks of the an index of corticospinal excitability) compared to each technique necrotic core, solid tumour and T2-hyperintense region, in order to separately [36]. Finally, direct modulation of glioma cellular depo- maximise the E-field over the solid tumour mass. All the patients larisation by NiBS could not be a-priori excluded, i.e., making the completed the study without reporting any adverse effects, and a glioma cells less able to depolarise in response to neuronal spiking. decrease in intratumoural perfusion (-36% of CBF respect to base- The potential decrease of tumour responsiveness to neuronal spik- line) was observed, while no significant changes were detected in ing via NiBS need to be assessed, possibly starting from in vitro the surrounding edema, necrotic core nor other control regions of and preclinical models. the brain. Importantly, 3 patients also underwent another single session of tDCS inside the MRI scanner in the post-surgical setting, 3.2. Perfusion and permeability demonstrating the safety of applying tDCS in patients with skull breaches and skull fixation hardware [17]. Despite being a prelim- Recently, extra-neuronal effects of NiBS have begun to receive inary investigation, carried out with a single stimulation session attention, with animal and human studies exploring the effect of instead of repeated daily applications, this study supports the pos- tES on vascular brain components. The first study exploring perfu- sible role of tDCS as a novel therapeutic approach for brain cancers, sion effects of tDCS in humans was conducted on healthy subjects especially if combined with standard CHT regimens and applied for receiving direct current stimulation (tDCS) during the acquisition repeated sessions, as currently performed with electrochemother- of a perfusion-sensitive Magnetic Resonance Imaging scan (Arterial apy in extracranial tumours where intratumoural perfusion reduc- Spin Labelling, ASL [37]). The authors showed modulation of Cere- tion seems to prolong drug persistence in the tumour vessels, pos- bral Blood Flow (CBF) in the targeted cortical regions, with a pos- sibly boosting their actions [42]. Given the perfusion reduction ob- itive correlation to stimulation intensity. In particular, both anodal served following a single session of tDCS (20 minutes), it is rea- and cathodal stimulation induced a significant CBF increase with sonable to assume a greater – and clinically meaningful - decrease respect to baseline level, with anodal stimulation having a stronger following repeated sessions of tDCS, as observed in extracranial and more reliable vascular modulatory effect across repeated stim- tumours with electrochemotherapy that can even lead to necro- ulation blocks [37]. In gliomas, tumour perfusion is positively cor- sis [45]. However, potential negative effects need to be carefully related with WHO grade and negatively with survival [38–40]. Par- taken into consideration before scaling up this type of investiga- ticularly for GBM, neoangiogenesis and high perfusion are the most tion in brain tumours patients, such as the theoretical reduction distinctive histopathological features, related to extreme invasive- of cancer sensitivity to radiotherapy treatment (due to the lower ness and aggressive growth, as well as with markers of cell pro- oxygen concentration consequent to the perfusion reduction [46]), liferation (e.g., Ki67 index [38,39]). CBF and CBV (Cerebral Blood potential perfusion decrease in the healthy brain tissue surround- Volume) represent validated and reliable imaging markers of tu- ing the lesion, or the promotion of the recently discovered neuron- mour progression, with increased CBF on perfusion-sensitive MRI to-glioma communications. For these reasons, tDCS targeting the sequences predicting shorter PFS and OS [40]. Therefore, consider- tumour perfusion could be tested at recurrence when radiother- ing the potential importance of inhibiting tumour perfusion, an- apy usually has been already delivered and therapeutic options are tibodies targeting neoangiogenesis pathways (e.g., Bevacizumab) very limited [47], possibly in combination with antiepileptic treat- have been developed and tested, without finding however signifi- ments (e.g., Perampanel) aiming to decrease neuronal excitability. 5 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 As for the theoretical reduction of radiotherapy sensitivity, no data an increase in permeability and concentration of molecules up to are available to date, but hypoxia could be assessed and monitored 70 kDa [18,43] while the standard chemotherapy agent for GBM via 18 F-Fluoromisonidazole (18 F-FMISO) PET scan. F-MISO corre- (i.e. Temozolomide) involves a small molecule of only 192 Daltons. lates with VEGF expression, can distinguish between LGG and HGG, is able to predict poor prognosis and to estimate chronic tissue 3.3. Microenvironment hypoxia [48]. GBM is characterised by vascular proliferation and necrosis, with the latter representing a consequence of the extreme Malignant brain cancers, such as GBM, are capable of interact- hypoxia in the tumour core due to high proliferation and lack of ing with the entire tumour microenvironment, ranging from pre- adequate metabolic supply [39]. A possible solution to avoid the existing vessels to the immune system, in order to promote tu- hypoxia induced mechanisms with NiBS intervention, could be rep- mour growth and resistance to treatments [53]. tES techniques resented by the application of repetitive and prolonged sessions of seem to have the potential to interfere with at least some of the stimulations that do not allow the tumour to sufficiently activate cellular components of the tumour microenvironment, raising the the neoangiogenesis response and rapidly prompt its necrosis, as possibility to counteract the self-promoting action of cancer cells observed in extracranial tumours [45]. and restore more physiological cellular function of the tumour With regard to the generalised perfusion decrease, accurate bio- environment. GBM is characterised by the ability to form a vir- physical modelling to enable precise targeting of the solid mass tual cellular continuum to transfer inorganic and genetic molecules would ensure personalised stimulation to the tumour mass with- into surrounding healthy tissue via tumour microtubes, thereby out significantly affecting the surrounding brain, especially with changing the phenotype of the microenvironment cells towards the adoption of multifocal montages controlling E-field distribu- one promoting tumour resistance and survival [53]. Microglia, the tion. Assuring physiological blood flow in peri-tumoural regions innate immune cells of the brain, are involved in the defense could be fundamental not only to maintain basic physiology and against pathogens and toxins. By disrupting the BBB, GBM al- metabolism via the delivery of adequate amount of oxygen and lows monocytes and macrophages -recruited by the tumour cy- nutrients, but also to ensure the delivery of drugs/CHT in the infil- tokine and chemokine gradient— to infiltrate the lesion, form- trated tissues that, in case of an impaired perfusion, could not suf- ing, together with the resident microglia, the so called “tumour- ficiently reach that region. The absence of significant CBF variation associated macrophages” or “myeloid cells”–TAMs [54]. TAMs sup- in the rest of the brain -when adopting multifocal montages selec- ports tumour growth by (i) producing matrix metalloproteinases tively targeting the solid mass– needs to be confirmed in larger pa- involved in extracellular matrix degradation, which is essential tient samples via non-invasive MRI assessment (e.g., non-contrast to GBM migration and invasion, and (ii) promoting angiogenesis enhanced perfusion acquisition such as ASL, and contrast-enhanced via the secretion of VEGF-A and other molecules [53]. Therefore, perfusion scan such as Dynamic Susceptibility Contrast–DSC). tDCS the suppression of pathological TAM activation could represent a could even exert a spontaneous dissociable effect on brain tissue therapeutic option, with some antibodies targeting their recruiting regarding perfusion modulation. In particular, tDCS could increase pathway currently under investigation [54]. Microglia modulation CBF in the healthy brain characterised by normal, intact vessels, via tES has been observed in healthy preclinical models [19], and and at the same time induce a perfusion decrease in the tumour promising evidence is emerging for an effect in Alzheimer’s Dis- tissue region, due to its abnormal vessels’ micro- and macrostruc- ease (AD) [55,56]. A recent preclinical study in an AD mouse model ture (i.e., impaired shunts, lack of smooth muscle cells in the walls, showed that exogenously-induced increases in brain γ oscillations irregular macrovascular architecture [42,49]). Overall, exploring the (i.e. gamma band) via optogenetic stimulation modulates the activ- exact boundaries of perfusion reduction obtained with tDCS in ity of microglia, modifies inflammatory brain processes, and leads brain tumours is necessary to assure safety and maintain the ef- to clearance of β -amyloid and p-tau deposition [55,56]. Now, pilot ficacy of other concurrent therapy such as CHT. In particular, it is studies from our group (NCT03880240, NCT04425148) are investi- crucial to accurately define the limits of perfusion reduction in the gating translation of these findings in patients with neurodegener- case of GBM, whose recurrence often takes place within 2 cm of ative disease such as AD and Frontotemporal Dementia by deliver- the resection margins [50]. Thus, a controlled reduction of perfu- ing tACS capable of entraining neurons at the provided frequency, sion in the immediate vicinity of the macroscopic tumour borders such as in the gamma band (i.e. 40 Hz) with the goal of restoring could be useful to constrain the progression of the disease. In that microglial function [57]. Considering the promising results in AD case, instead of restricting the targeting to the macroscopic tumour models and preliminary evidence in tumor animal models, human area, NiBS could be targeted to the surrounding areas following an translational research is needed to explore eventual modulation of approach analogous to radiotherapy, for instance including 2 cm3 microglia and/or of specific neuron type (i.e., parvalbumin-positive of radiographically healthy tissue [17,51]. interneurons) via one or more types of tES in brain tumour pa- Beyond effects on perfusion, tDCS has been shown in rats to tients [58]. transiently increase the permeability of the BBB to small and large molecules after only 20’ of stimulation [43], with the increase 3.4. Galvanotaxis in water and solute flux being confirmed by subsequent experi- ments in endothelial monolayers [18]. The magnitude of the in- The discovery that cells can be oriented and guided in their mi- duced flow was found to linearly correlate with the intensity of gration when exposed to electric fields –or galvanotaxis— dates to applied current and to be caused by an electroosmotic mechanism. the nineteenth century [59]. This phenomenon has been explored The changes in permeability were observed at electrical intensities and confirmed in a variety of physiological and pathological con- that are usually reached on the cortex via transcranial stimulation texts, such as embryonic development, nerve cell growth, angio- in humans (0.5 mV) [18]. Therefore, tDCS seems to affect vascular genesis, wound healing, and cancer cell migration [59]. The mag- brain components in –at least— two ways: (i) by varying perfusion nitude of migration, orientation and reactivity to electrical charges at the macroscale level (i.e. CBF), and (ii) by changing BBB per- varies across cell types and a major role for asymmetric ionic meability at the microscale (Fig. 3A). Both mechanisms could be flux and redistribution of charged membrane particles is seen at extremely relevant in the context of brain tumour management, the core of this phenomenon [59]. Generally, both healthy and the former for decreasing intratumoural perfusion and the latter cancerous cells align and migrate towards the cathode, although for enhancing the delivery of drugs through the BBB [52]. Interest- the opposite (e.g. anodal migration) is also possible, as observed ingly, in vitro as well as animal models of tDCS have demonstrated in metastatic lung or breast cancer cells [59]. GBM cells follow 6 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 Fig. 3. Local and network-based NiBS application. (a) Local (tumoural-peritumoural) approaches of different NiBS protocols for brain cancer management can include: (i) reduction of tumour perfusion to slow down cancer growth via tDCS, (ii) increase of BBB permeability to enhance drugs delivery via tDCS, (iii) migration control of further spreading leveraging galvanotaxis via tDCS, (iv) stimulation of TAMs tumour suppressive phenotype via tACS, and (v) suppression of neuronal tumour-promoting activity via inhibitory NiBS protocols. (b) Additionally, network-based approached based on advanced MRI and PET imaging could be developed to optimise interventions aimed at restoring cognition and/or controlling neuronal tumour-promoting activity, optimising growth control, and slowing spread of cancer cells. On top of using the aminoacidic tracer to delineate tumour extent, PET imaging optimize treatment planning and assessment of response to therapy [65], and could also be tested as predictor of migration and/or sustainment of tumour growth by looking at the classic 18 F-2-fluoro-2-deoxy-D-glucose (18 F-FDG)-PET, whose regional standard uptake volume might unveil the brain regions characterised by the highest glucose uptake [66], possibly having the strongest influence on neuronal-promoted tumour growth. Perfusion-weighted imaging (PWI) could refine targeting by identifying recently discovered resting-state perfusion networks, beyond their utility as measures of target engagement by NiBS [67,68]. Note: red dots = tumour; orange dots = site of migration and thus of neuronal suppression; blue dots = most impaired brain regions leading to cognitive deficits; BBB = blood brain barrier; dMRI = diffusion MRI; fALFF = fractional Amplitude of Low-Frequency Fluctuations; fMRI = functional Magnetic Resonance Imaging; PET = Positron Emission Tomography; rs-fMRI = resting-state fMRI; TAMs = Tumour-Associated Macrophages. the same migration routes of immature neurons and stem cells, gration and infiltration across the brain. Also, human peripheral that is along brain vessels and white matter tracts (e.g. the so- blood mononuclear cells (i.e., lymphocytes and monocytes) have called Scherer’s structures [60]). Additionally, GBM cells tend to been found to migrate towards the cathode as well. T cells are typ- reside in perivascular niches to easily extract nutrients from the ically downregulated in GBM by the tumour-promoting microglia bloodstream, particularly the subpopulation of glioma cells capa- phenotype [64]. A potentially beneficial colocalisation of T cells in ble of self-renewing (Brain Tumour Initiating cells – BTICs) that ul- the tumour regions could be obtained in the regions targeted by timately seems to be responsible for the constant self-renewal ca- the cathode, with the aim of promoting the immunological anti- pacity of GBM [61]. A recent study tested the migration pattern of tumoural response [64]. This could promote the immunotherapy BTICs from three different human GBM types, in both 2D and 3D treatments which currently have not shown positive results, due to environments under the influence of an E-field (EF = 0,5–1 V/cm the “cold” aspect of GBM (insensitive to immunotherapy) related to [62]). The investigators showed that the migration pattern is af- multiple mechanisms, such as strong immunosuppressive tumour fected by the environment, namely that glioma cells migrate to- action, impaired tumour antigen presentation, and the highly hy- wards the anode in a 2D environment and towards the cathode poxic and necrotic environment [64]. when posed in a 3D extracellular matrix [62]. Even if the inten- In conclusion, the potentials of NiBS techniques need to be ex- sity of the electric field typically used to induce the galvanotaxis tensively explored for their local therapeutic applications at mul- phenomenon is notably higher than the one reached in the hu- tiple levels and timing in patients with brain cancers (Fig. 3A). man brain with tDCS (around 0,008 V/cm in tDCS compared to These approaches could be investigated in combination with exist- 0,5–1 V/cm in galvanotaxis), a similar phenomenon during tDCS ing treatments such as CHT and TTF to act on tumour management cannot be excluded, especially in the case of repetitive tES ses- at multiple scales. NiBS applications in pilot human studies, in an- sions to obtain long-lasting changes of excitability/perfusion. Ac- imal and cellular models support the possibility of (i) suppressing cordingly, neuronal stem cells were found to increase their migra- neuronal activity-regulated cancer growth and slowing glioma mi- tory activity under the influence of tDCS delivered within typical tosis/invasiveness, (ii) reducing tumour perfusion and potentially human parameters in a rat model, even if a direct migration trend decreasing cancer growth, (iii) increasing vascular permeability and towards the cathode or electrode was not detected [63]. promote drug delivery, (iv) restoring microglia function, and (v) In this framework, the cathodal field generated by tDCS in the controlling cancer cell migration, with a favorable safety profile region surrounding the tumour aiming to control neuronal ex- and minimal side effects for patients. citability could also benefit patients by restricting migration of cancer cells localised in the cathodal field (i.e., in the immediate vicinity of the tumour borders) and potentially interfering with mi- 7 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 4. Network-based therapeutic opportunities ing to the restoration of the initial network state after cessation of stimulation [82]. However, the cortical excitation/inhibition (E/I) Anatomical and physiological studies have demonstrated that ratio is typically altered in patients with gliomas, as well as in cognitive and motor processes arise from the interaction between other neurological and psychiatric conditions, often associated with multiple and distant brain regions, forming so-called “large scale cognitive deficits and symptoms [82]. In patients with gliomas, the brain networks” [69]. The first type of brain network to be stud- E/I imbalance is involved with the emergence of epileptiform ac- ied has been the anatomical/structural “connectome”, that defines tivity, and with subsequent neuronal death, paralleled by tumour the structural connections – white matter fiber tracts- between progression [83]. Due to NiBS action on membrane channels, it has brain regions. In GBM patients, the structural connectome iden- been suggested that tDCS can also be useful in restoring the E/I tified via diffusion MRI (dMRI) has gained particular attention balance of brain network(s) [84]. In particular, daily application of since GBM cells are known to spread along white matter fiber tDCS with a large cathodal field to inhibit tumour-promoting neu- tracts [70]. Multiple models have been proposed to describe the ronal activity could also exert beneficial effects on cortical E/I bal- growth/migration pathways of GBM cells, however without appli- ance, hopefully leading to an interruption or even prevention of cability to clinical practice, beyond the important application to epileptogenesis [83], a common and debilitating symptom of the guiding surgical resection (i.e. sparing eloquent fiber tracts) [71]. disease that may also induce a positive feedback on neurons to In addition to structural connectomics, “functional networks” promote tumour growth and invasiveness, as previously discussed have been identified on the basis of the synchronous interactions [7,8]. Related encouraging results are emerging from the applica- between brain areas, independently of their anatomical connec- tion of tDCS in drug-resistant childhood epilepsy, with data show- tions [72]. Typically, functional networks are extracted from func- ing a decrease of at least 42% in seizure frequency after 10 sessions tional MRI (fMRI), by analyzing correlations in the fluctuations of of tDCS (1h/day) and pilot data showing a similar trend for epilep- the blood oxygen level dependant (BOLD) signal, and thus obtain- tic patients with previous surgical interventions [85]. ing the functional connectivity – FC (i.e. the temporal synchroni- Finally, TMS could provide markers of tumour progression sation between the activity of brain areas [72]). fMRI can be ac- and/or diagnosis by assessing the neuronal response to pertur- quired with subjects performing a specific task or, as emerged in bation of the surrounding apparently healthy regions, i.e., assess the last decade, during a resting-state condition (i.e., with a sub- cortical excitability and excitation/inhibition balance in the peritu- ject lying in the MRI scanner without performing any task), al- moural areas (Fig. 1). To this end, TMS could be combined with si- lowing the identification of several so-called resting-state networks multaneous electroencephalography (EEG) for measuring local and (RSNs) [73]. Each RSN is composed of regions involved in spe- distant brain responses to stimulation, given that a focal TMS pulse cific functions, ranging from the sensory domain (i.e., visual, mo- typically evokes activation in secondary interconnected cortical ar- tor, and auditory) to high-order cognitive processes (i.e., reason- eas (e.g. TMS-evoked potentials). In this context, TMS-EEG has ing and attention). Importantly, neurological and psychiatric con- emerged as a method to study not only local cortical reactivity, ditions have been linked to specific functional network alterations but also the causal communication between distant brain regions including Schizophrenia, Depression and Alzheimer’s Disease [74]. at high temporal resolution lacking in the context of fMRI stud- Considering that inter- and intra-network activity is responsible for ies. TMS-EEG could provide insights into the mechanisms of effec- human cognition and action, modulating the specific interplay be- tive connectivity (i.e. the influence of one neural system over an- tween two or more brain regions could promote (or inhibit) the other), and into higher-order cognitive processes at the individual function controlled by the targeted brain regions. In this context, level [86]. If integrated with imaging data and prediction theories, NiBS has been successfully used in healthy subjects, to promote TMS-EEG could provide temporally and spatially-optimised mark- intra- and inter-network connectivity [75] – and thus enhancing ers of neuron-to-glioma communications and eventually of glioma subject performance during cognitive [76] and motor tasks [77]. spreading trajectory, promoting the development of strategies to This approach has also been applied in patients, in an attempt control this new pathophysiological mechanism. to restore proper network dynamics, as in the case of Alzheimer’s Disease and Depression [78]. 5. Network-based approaches for symptom management The possibility of interacting with an entire network rather than with a single brain area could help in suppressing the neural ac- Resting-state connectivity has also recently been used to map tivity regulating cancer growth and tumour spread (Fig. 3B). In- neurological and psychiatric symptoms, providing a novel frame- hibition of an entire network could be more effective in slow- work to understand the basis of altered behaviour, especially in ing neuronal-related cancer growth, with tools derived from Net- relation to classic anatomical lesion studies [87]. In this approach, work Control Theory potentially representing a valuable approach the connectivity of the lesions located in different parts of the to select the most relevant stimulation targets [79]. Additionally, brain causing the same symptomatology is mapped to find out network-based approaches mapping network topology and evo- the common region/network whose connectivity is altered. This lution could help predict tumour spread (Fig. 3B). As previously approach, called “Connectome lesion-based mapping”, has helped mentioned, dMRI based white matter mapping is essential to visu- disentangle the pathophysiological correlates of many neurological alise the white matter fibers near the tumour and guide the surgi- and psychiatric conditions, such as amnesia, Parkinsonism, cervical cal resection [71]. Therefore, dMRI could also be crucial to identify dystonia and delusional misidentifications [87]. the anatomic pathways along which tumour cells can spread, guid- The connectome lesion-based mapping approach could be also ing the placement of stimulation electrodes, especially if combined applied in patients with brain tumours to possibly identify core with functional imaging [80,81] (Fig. 3B). Of course, possible cogni- regions whose functionality is altered, causing the core symptoma- tive/motor/physiological side effects of network inhibition need to tology presents in patients. As recently suggested, symptoms of pa- be carefully monitored and patient wellbeing should be considered tients with gliomas are an integral part of the disease, but their a primary aim in addition to the promotion of survival. characterisation has been largely ignored in favor of molecular On the other hand, the dynamics of regional brain activity rely classifications, consequently limiting the therapeutic opportunities on a complex and balanced interplay between many directly or in- for symptoms relief [88]. Developing a symptom-sensitive thera- directly connected areas and networks. Therefore, as in any other peutic strategy could be valuable to improve patients’ quality of physiological process, a homoeostatic response is frequently ob- life in addition to prolonging survival (considering that cognitive served after perturbation of a limited neuronal population, lead- impairment has been recognised as a significant prognostic fac- 8 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 tor for patient survival [89]), and for those who cannot receive or proaches based on general templates, thus allowing to reach un- refuse aggressive oncologic treatments. NiBS could offer a safe tool precedented fine and predictable electric field (Fig. 4B). to help patients mitigate cognitive impairment present in approxi- Finally, NiBS has the potential to induce cortical plasticity and mately 80% of adult glioma patients as a consequence of radiother- reorganisation into the healthy brain potentially enabling greater apy, CHT, surgery, and the tumour itself [90]. Gliomas can directly surgical resection in eloquent cortex affected by the tumour, as alter brain connectivity and cause cognitive deficits via: (i) mass demonstrated in a few seminal studies [97,98]. The extent of resec- effect, impairing the activity of neighboring brain regions via com- tion is a strong predictor of prognosis and the proximity or direct pression, (ii) infiltration and spreading along white matter tracts, presence of the tumour in eloquent cortices (e.g., motor cortex, (iii) reduction of perfusion in nearby areas due to increased tu- language areas) profoundly limits the tumour resection, even when mour vascularisation [90]. In general, gliomas – especially GBM- the most advanced techniques are used intraoperatively. LGGs can seem to induce local and widespread, network functional changes induce plasticity and reorganisation in cortical areas distant to the as visualised by FC fMRI analysis [91]. Decreased FC has been ob- lesion enabling the maintenance of a function, due to their slow served both in close proximity to the tumour as well as in distant growth, but may also harbor critical structures within the glioma regions, including the contralateral hemisphere [91]. A longitudi- itself [99]. Suppression of the eloquent cortex near/infiltrated by nal study in three GBM patients revealed that lesions located in the glioma (WHO grade II and III) paired to an intensive training of close proximity to a functional hub (i.e. a region among those with the targeted function (median training: 16 days) allowed a greater highest density of functional connections with other brain regions) resection associated with cortical reorganisation with new, distant strongly affect long-range connectivity, while the same effect has brain activation and no significant performance deficits detected at not been seen in the case of lesions involving a peripheral node follow-up assessments [98]. NiBS could be implemented to induce of the same network [92]. Surgery and radiotherapy interventions such cortical plasticity in LGG and HGG in order to maximise the were found to restore physiological connectivity to a certain ex- resection and prolong the survival, while also limiting functional tent in the affected regions, with the effect potentially due to the deficits. release from compression as well as to reduction of edema and in- Given the extensive and successful research made in the last flammation. However, after the initial treatment period, FC usually decade on different clinical and non-clinical populations via NiBS, decreases again, possibly indicating the progression of the disease network approaches integrating multiple stimulation modalities and further spreading of the tumour, along with the deleterious could be leveraged to tackle cognitive symptomatology and func- effects of treatments themselves [92]. tional deficits, which negatively impacts patients’ quality of life. In a group of patients with HGG and LGG followed for 1 year, memory and attention deficits were reported in patients with le- 6. Conclusion sions overlapping with the Default Mode and Attention Network, respectively [93]. Additionally, neurite density (a marker of axon Cellular, animal, and human models suggest realistic NiBS and dendrite concentration derived from dMRI) was also found to roles in targeting a range of glioma pathophysiological substrates, correlate with memory and attention recovery after surgery [93]. ranging from neuron-to-glioma synaptic communication, tumour As for patients with metastatic disease, there is a specific cognitive microenvironment, perfusion to galvanotaxis. Combined with its decline starting 4–6 months after whole brain radiation therapy safety, noninvasiveness and tolerability, NiBS may offer novel ther- and continuing indefinitely, mainly impacting memory and learn- apeutic approaches to control cancer growth and ameliorate pa- ing performance [94]. It can be speculated whether NiBS could tients’ disability. In particular, as demonstrated in multiple trials counteract these functional declines especially at the beginning of [100], tES is also suitable for home-based intervention, a signifi- treatment or in those who cannot undergo RT/surgery. Indeed, en- cant benefit in the present neurooncological landscape. hancement of behavioural performance has been demonstrated in In this promising scenario, there are some challenges that need many domains by NiBS [24], ranging from motor to cognitive, and to be addressed in order to optimise the application of NiBS in also including complex processes such as decision making and so- brain tumour patients. For instance, neuromodulation effects could cial behaviour, in patients as well as healthy subjects [23]. Recently be reduced by the antiepileptic therapy usually received by pa- promoted network-based targeting approaches for tES can refine tients for seizure prevention. Indeed, antiepileptic drugs act on var- the effects obtained with classical neuromodulation applications, ious ion channels, the same target of NiBS [101]. However, while which have been shown to improve performance in healthy sub- the blockage of voltage dependant Na+ and Ca2+ channels respec- jects and patients in domains such as abstract reasoning, mem- tively eliminates or decreases the hyperexcitability caused by an- ory, attention and motor function [95] (Fig. 3B). Importantly, ex- odal stimulation, the same effect is not observed during cathodal citatory protocols (i.e. tDCS, tRNS, high-frequency rTMS) are usu- stimulation [101]. In addition, the most frequently administered ally applied to enhance cognitive functions. Therefore, the stim- antiepileptic drug in glioma patients is levetiracetam which does ulation site(s) should be carefully selected to avoid those con- not inhibit voltage-dependant Na+ channels or GABAergic trans- nected with the glioma cell population to avoid a potential pro- mission, but binds a synaptic vesicle glycoprotein (SV2A) and in- motion of cancer growth as a collateral effect. Accurate selection hibits presynaptic Ca2+ channels [102]. Therefore, the modulatory of patients on the basis of the localisation of the lesion respect action of levetiracetam could be of no -or minimal- significance to the target brain area (i.e., lesion not in the immediate proxim- in brain tumour patients undergoing neuromodulation protocols, ity of the target region) would be of extreme importance along given the limited reduction of NiBS effects observed with Ca2+ with the adoption of personalised stimulation protocols. Indeed, channels blockers. Moreover, the protocols aimed at inhibiting the current flow across the brain can be accurately modelled and pre- neuron-to-glioma communications would see the application of dicted when considering all the cranial compartments, the tumour cathodal stimulation, reducing the implications of the assumption mass and/or surgical cavity as well as metallic clips to obtain a of antiepileptic drug. personalised image-guided stimulation ([96], Fig. 4A). Neurostimu- On the other hand, in the case of application of excitatory pro- lation techniques have gained more reliability and efficacy thanks tocol, for instance to restore cognitive and/or motor ability, the ad- to their integration with imaging techniques (anatomical MRI, CT, ministration of such antiepileptic drugs could partially limit the fMRI, PET) to personalise the stimulation solution based on each expected excitatory effects induced by NiBS, but, at the same time, patient’s anatomy and physiology, rather than using standard ap- control for potential collateral effects (i.e., seizure) and tumour- promoting neuronal activity. This scenario is probably the most dif- 9 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 Fig. 4. Personalised image-guided biophysical modeling. (a) After MRI/CT acquisition, images can be segmented into different tissues (skin, air, bone, cerebrospinal fluid- CSF, gray matter - GM, white matter - WM), along with the tumour masks. All the tissues (manually or semi-automatically created), as well as metallic device and skull holes/breaches, are imported into the 3D volume rendering of the single subject brain anatomy and conductivity values are attributed to each tissue [17]. (b) Examples of the best multielectrode montage maximizing the normal E-field (En) on the tumour target selected among all the 10/20 EEG combinations available for electrode’s positioning in a presurgical (left) and a post-operative (right) patient. ficult to optimise, and must include a multidisciplinary effort in- (5) Can NiBS in combination with neuroimaging and electrophysi- volving, for instance, expertise related to biophysical modelling in ological data improve the understanding of glioma pathophysi- order to maximize targeting, as well as knowledge from the anato- ology? mopathologist to estimate the percentage of glial cell in the surgi- cal/biopsy tissue samples actually forming synapses with the sur- Search strategy and selection criteria roundings neuronal tissue, and therefore estimate risk of possible tumour proliferation induced via NiBS along with the differential Data for this review were identified by searches of PubMed contribution to proliferiation by the different neurons. and references from relevant English articles using the search Multiple complementary possibilities in terms of targeting ap- terms “glioma”, “brain cancer”, “fMRI”, “NIBS”, “brain stimulation”, proaches, stimulation parameters, and timing of the intervention “tDCS”, “tACS”, “tRNS” without any type of restriction. Selection of should be tested in animal models, whereas some interventions the most appropriate references was made by the authors due to might be ready for first-in-human trials. A collaborative effort be- the journal constraints. tween neurooncologists, neurosurgeons, neurologists and neuro- physiologists, neuroradiologists, physicists, engineers, neurobiolo- Declaration of Competing Interest gists and neuroscientists is needed to rapidly evaluate the potential of NiBS in brain tumour patients. APL is listed as an inventor on several issued and pending patents on the real-time integration of noninvasive brain stimu- lation with electroencephalography and magnetic resonance imag- Outstanding questions ing and he is a member of the scientific advisory board of Starlab, Magstim and MedRhythm; APL is co-inventor of Linus Health and In the light of recent findings of electrical glioma integration, TI Solutions. The other authors declare no conflicts of interest. NiBS can be regarded as a potential tool to suppress neuron-to- glioma communication, in addition to other potential effects on Acknowledgements tumour features and microenvironment. Scientists and physicians should cooperate to operationalise the above formulated priorities E.S. is supported by the NIH (R01 MH117063-01, R01 in order to potentially provide a new therapeutic and safe option AG060981-01) and by the Alzheimer’s Drug Discovery Founda- to glioma patients: tion (ADDF) & Association for Frontotemporal Dementia (AFTD) via GA 201902-2017902. A.J.G. is supported by NIH P41EB015898, the (1) To what extent can NiBS affect the newly discovered neuron- Haley Distinguished Chair in the Neurosciences, the Jennifer Op- to-glioma communication and the related tumour progression? penheimer Cancer Research Initiative, the NIH via R21CA198740. (2) Can the NiBS-induced tumour perfusion reduction and its ef- A.P.L. is supported by NIH via R24AG06142, R01AG059089 and P01 fects on BBB permeability lead to tumour shrinkage if applied AG031720. G.E-L. is supported by NIH via P41EB022544. in a daily protocol basis? References (3) How NiBS affects the glioma microenvironment? (4) Can NiBS be applied to improve the quality of life of glioma [1] Gould J. Breaking down the epidemiology of brain cancer. Nature patients, especially to control cognitive and motor deficits? 2018;561:S40–1. doi:10.1038/d41586- 018- 06704- 7. 10 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 [2] Nayak L, Reardon DA. High-grade Gliomas. Contin Minneap Minn transcranial oscillatory potentials. Curr Biol 2013;23:1449–53 CB. doi:10.1016/ 2017;23:1548–63. doi:10.1212/CON.0 0 0 0 0 0 0 0 0 0 0 0 0554. j.cub.2013.06.022. [3] Bunevicius A, McDannold NJ, Golby AJ. Focused ultrasound strategies for [29] Santarnecchi E, Sprugnoli G, Bricolo E, Costantini G, Liew SL, Musaeus CS, brain tumor therapy. Oper Neurosurg 2019 Hagerstown Md. doi:10.1093/ons/ et al. Gamma tACS over the temporal lobe increases the occurrence of Eu- opz374. reka! moments. Sci Rep 2019;9:5778. doi:10.1038/s41598- 019- 42192- z. [4] Fabian D, Eibl MD PGP, Alnahhas I, Sebastian N, Giglio P, Puduvalli V, et al. [30] Venkatesh HS, Johung TB, Caretti V, Noll A, Tang Y, Nagaraja S, et al. Neu- Treatment of glioblastoma (GBM) with the Addition of tumor-treating fields ronal activity promotes glioma growth through neuroligin-3 secretion. Cell (TTF): a review. Cancers 2019;11. doi:10.3390/cancers11020174. 2015;161:803–16. doi:10.1016/j.cell.2015.04.012. [5] Stupp R, Taillibert S, Kanner A, Read W, Steinberg D, Lhermitte B, et al. Ef- [31] Venkatesh HS, Tam LT, Woo PJ, Lennon J, Nagaraja S, Gillespie SM, et al. fect of tumor-treating fields plus maintenance temozolomide vs maintenance Targeting neuronal activity-regulated neuroligin-3 dependency in high-grade temozolomide alone on survival in patients with glioblastoma: a randomised glioma. Nature 2017;549:533–7. doi:10.1038/nature24014. clinical trial. JAMA 2017;318:2306–16. doi:10.1001/jama.2017.18718. [32] Köhling R, Senner V, Paulus W, Speckmann EJ. Epileptiform activity preferen- [6] Stupp R, Wong ET, Kanner AA, Steinberg D, Engelhard H, Heidecke V, et al. tially arises outside tumor invasion zone in glioma xenotransplants. Neurobiol NovoTTF-100A versus physician’s choice chemotherapy in recurrent glioblas- Dis 2006;22:64–75. doi:10.1016/j.nbd.2005.10.001. toma: a randomised phase III trial of a novel treatment modality. Eur J Cancer [33] Venkataramani V, Tanev DI, Kuner T, Wick W, Winkler F. Synaptic input to Oxf Engl 2012;48:2192–202 1990. doi:10.1016/j.ejca.2012.04.011. brain tumors: clinical implications. Neuro Oncol 2020. doi:10.1093/neuonc/ [7] Venkatesh HS, Morishita W, Geraghty AC, Silverbush D, Gillespie SM, Arzt M, noaa158. et al. Electrical and synaptic integration of glioma into neural circuits. Nature [34] Notzon S, Steinberg C, Zwanzger P, Junghöfer M. Modulating emotion per- 2019;573:539–45. doi:10.1038/s41586- 019- 1563- y. ception: opposing effects of inhibitory and excitatory prefrontal cortex stim- [8] Venkataramani V, Tanev DI, Strahle C, Studier-Fischer A, Fankhauser L, ulation. Biol Psychiatry Cogn Neurosci Neuroimaging 2018;3:329–36. doi:10. Kessler T, et al. Glutamatergic synaptic input to glioma cells drives brain tu- 1016/j.bpsc.2017.12.007. mour progression. Nature 2019;573:532–8. doi:10.1038/s41586- 019- 1564- x. [35] Singh A, Erwin-Grabner T, Goya-Maldonado R, Antal A. Transcranial mag- [9] Monje M, Borniger JC, D’Silva NJ, Deneen B, Dirks PB, Fattahi F, et al. Roadmap netic and direct current stimulation in the treatment of depression: basic for the emerging field of cancer neuroscience. Cell 2020;181:219–22. doi:10. mechanisms and challenges of two commonly used brain stimulation meth- 1016/j.cell.2020.03.034. ods in interventional psychiatry. Neuropsychobiology 2019:1–11. doi:10.1159/ [10] Buckingham SC, Campbell SL, Haas BR, Montana V, Robel S, Ogunrinu T, et al. 0 0 0502149. Glutamate release by primary brain tumors induces epileptic activity. Nat [36] Han T, Xu Z, Liu C, Li S, Song P, Huang Q, et al. Simultaneously applying catho- Med 2011;17:1269–74. doi:10.1038/nm.2453. dal tDCS with low frequency rTMS at the motor cortex boosts inhibitory af- [11] Jung E, Alfonso J, Osswald M, Monyer H, Wick W, Winkler F. Emerg- tereffects. J Neurosci Methods 2019;324:108308. doi:10.1016/j.jneumeth.2019. ing intersections between neuroscience and glioma biology. Nat Neurosci 05.017. 2019;22:1951–60. doi:10.1038/s41593- 019- 0540- y. [37] Zheng X, Alsop DC, Schlaug G. Effects of transcranial direct current stimula- [12] Liu SJ, Zukin RS. Ca2+ -permeable AMPA receptors in synaptic plasticity and tion (tDCS) on human regional cerebral blood flow. NeuroImage 2011;58:26– neuronal death. Trends Neurosci 2007;30:126–34. doi:10.1016/j.tins.2007.01. 33. doi:10.1016/j.neuroimage.2011.06.018. 006. [38] Hu LS, Eschbacher JM, Dueck AC, Heiserman JE, Liu S, Karis JP, et al. Cor- [13] Rossi S, Antal A, Bestmann S, Bikson M, Brewer C, Brockmöller J, et al. Safety relations between perfusion MR imaging cerebral blood volume, microvessel and recommendations for TMS use in healthy subjects and patient popula- quantification, and clinical outcome using stereotactic analysis in recurrent tions, with updates on training, ethical and regulatory issues: expert guide- high-grade glioma. AJNR Am J Neuroradiol 2012;33:69–76. doi:10.3174/ajnr. lines. Clin Neurophysiol 2020. doi:10.1016/j.clinph.2020.10.003. A2743. [14] Antal A, Alekseichuk I, Bikson M, Brockmöller J, Brunoni AR, Chen R, et al. [39] Louis DN, Perry A, Reifenberger G, Von Deimling A, Figarella-Branger D, Cave- Low intensity transcranial electric stimulation: safety, ethical, legal regulatory nee WK, et al. The 2016 world health organization classification of tumors of and application guidelines. Clin Neurophysiol 2017;128:1774–809 Off J Int Fed the central nervous system: a summary. Acta Neuropathol 2016;131:803–20 Clin Neurophysiol. doi:10.1016/j.clinph.2017.06.001. (Berl). doi:10.10 07/s0 0401-016-1545-1. [15] Klomjai W, Katz R, Lackmy-Vallée A. Basic principles of transcranial mag- [40] Qiao XJ, Ellingson BM, Kim HJ, Wang DJJ, Salamon N, Linetsky M, et al. Arte- netic stimulation (TMS) and repetitive TMS (rTMS). Ann Phys Rehabil Med rial spin-labeling perfusion MRI stratifies progression-free survival and corre- 2015;58:208–13. doi:10.1016/j.rehab.2015.05.005. lates with epidermal growth factor receptor status in glioblastoma. AJNR Am [16] Cirillo G, Di Pino G, Capone F, Ranieri F, Florio L, Todisco V, et al. Neurobiolog- J Neuroradiol 2015;36:672–7. doi:10.3174/ajnr.A4196. ical after-effects of non-invasive brain stimulation. Brain Stimulat 2017;10:1– [41] Chen Z, Xu N, Zhao C, Xue T, Wu X, Wang Z. Bevacizumab combined 18. doi:10.1016/j.brs.2016.11.009. with chemotherapy vs single-agent therapy in recurrent glioblastoma: evi- [17] Sprugnoli G, Monti L, Lippa L, Neri F, Mencarelli L, Ruffini G, et al. Reduction dence from randomized controlled trials. Cancer Manag Res 2018;10:2193– of intratumoral brain perfusion by noninvasive transcranial electrical stimu- 205. doi:10.2147/CMAR.S173323. lation. Sci Adv 2019;5:eaau9309. doi:10.1126/sciadv.aau9309. [42] Ciria HMC, González MM, Zamora LO, Cabrales LEB, Sierra González GV, De [18] Cancel LM, Arias K, Bikson M, Tarbell JM. Direct current stimulation of en- Oliveira LO, et al. Antitumor effects of electrochemical treatment. Chin J Can- dothelial monolayers induces a transient and reversible increase in trans- cer Res 2013;25:223–34 Chung-Kuo Yen Cheng Yen Chiu. doi:10.3978/j.issn. port due to the electroosmotic effect. Sci Rep 2018;8:9265. doi:10.1038/ 10 0 0-9604.2013.03.03. s41598- 018- 27524- 9. [43] D.W. Shin, N. Khadka, J. Fan, M. Bikson, B.M. Fu Transcranial direct current [19] Gellner AK, Reis J, Fritsch B. Glia: a neglected player in non-invasive direct stimulation transiently increases the blood-brain barrier solute permeabil- current brain stimulation. Front Cell Neurosci 2016;10:188. doi:10.3389/fncel. ity in vivo. Proc. SPIE 9788, Medical Imaging 2016: Biomedical Applications 2016.00188. in Molecular, Structural, and Functional Imaging, 97881X (29 March 2016). [20] Collingridge GL, Peineau S, Howland JG, Wang YT. Long-term depression in https://doi.org/10.1117/12.2218197. the CNS. Nat Rev Neurosci 2010;11:459–73. doi:10.1038/nrn2867. [44] Esmaeili N, Friebe M. Electrochemotherapy: a review of current sta- [21] Huerta PT, Volpe BT. Transcranial magnetic stimulation, synaptic plastic- tus, alternative igp approaches, and future perspectives. J Healthc Eng ity and network oscillations. J Neuroengin Rehabil 2009;6:7. doi:10.1186/ 2019;2019:2784516. doi:10.1155/2019/2784516. 1743-0 0 03-6-7. [45] Jarm T, Cemazar M, Steinberg F, Streffer C, Sersa G, Miklavcic D. Perturba- [22] Bolognini N, Pascual-Leone A, Fregni F. Using non-invasive brain stimulation tion of blood flow as a mechanism of anti-tumour action of direct current to augment motor training-induced plasticity. J Neuroengin Rehabil 2009;6:8. electrotherapy. Physiol Meas 2003;24:75–90. doi:10.1186/1743-0 0 03-6-8. [46] Kelley K, Knisely J, Symons M, Ruggieri R. Radioresistance of brain tumors. [23] Reed T, Cohen Kadosh R. Transcranial electrical stimulation (tES) mechanisms Cancers 2016;8. doi:10.3390/cancers8040042. and its effects on cortical excitability and connectivity. J Inherit Metab Dis [47] Weller M, Cloughesy T, Perry JR, Wick W. Standards of care for treatment of 2018. doi:10.1007/s10545- 018- 0181- 4. recurrent glioblastoma–are we there yet? Neuro Oncol 2013;15:4–27. doi:10. [24] Polanía R, Nitsche MA, Ruff CC. Studying and modifying brain function with 1093/neuonc/nos273. non-invasive brain stimulation. Nat Neurosci 2018;21:174–87. doi:10.1038/ [48] Muzi M, Peterson LM, O’Sullivan JN, Fink JR, Rajendran JG, McLaughlin LJ, s41593- 017- 0054- 4. et al. 18F-fluoromisonidazole quantification of hypoxia in human cancer pa- [25] Huang YZ, Lu MK, Antal A, Classen J, Nitsche M, Ziemann U, et al. Plasticity tients using image-derived blood surrogate tissue reference regions. J Nucl induced by non-invasive transcranial brain stimulation: a position paper. Clin Med 2015;56:1223–8 Off Publ Soc Nucl Med. doi:10.2967/jnumed.115.158717. Neurophysiol 2017;128:2318–29 Off J Int Fed Clin Neurophysiol. doi:10.1016/ [49] Jarm T, Cemazar M, Miklavcic D, Sersa G. Antivascular effects of elec- j.clinph.2017.09.007. trochemotherapy: implications in treatment of bleeding metastases. Expert [26] Mrugala MM, Ruzevick J, Zlomanczuk P, Lukas RV. Tumor treating fields Rev Anticancer Ther 2010;10:729–46. doi:10.1586/era.10.43. in neuro-oncological practice. Curr Oncol Rep 2017;19:53. doi:10.1007/ [50] De Bonis P, Anile C, Pompucci A, Fiorentino A, Balducci M, Chiesa S, et al. s11912- 017- 0611- 8. The influence of surgery on recurrence pattern of glioblastoma. Clin Neurol [27] Rossi S, Hallett M, Rossini PM, Pascual-Leone A. Safety of TMS consensus Neurosurg 2013;115:37–43. doi:10.1016/j.clineuro.2012.04.005. group. safety, ethical considerations, and application guidelines for the use [51] Burnet NG, Thomas SJ, Burton KE, Jefferies SJ. Defining the tumour and target of transcranial magnetic stimulation in clinical practice and research. Clin volumes for radiotherapy. Cancer Imaging 2004;4:153–61 Off Publ Int Cancer Neurophysiol 20 09;120:20 08–39 Off J Int Fed Clin Neurophysiol. doi:10.1016/ Imaging Soc. doi:10.1102/1470-7330.20 04.0 054. j.clinph.2009.08.016. [52] Bender E. Getting cancer drugs into the brain. Nature 2018;561:S46–7. doi:10. [28] Santarnecchi E, Polizzotto NR, Godone M, Giovannelli F, Feurra M, Matzen L, 1038/d41586- 018- 06707- 4. et al. Frequency-dependant enhancement of fluid intelligence induced by [53] Broekman ML, Maas SLN, Abels ER, Mempel TR, Krichevsky AM, Breake- 11 G. Sprugnoli, S. Rossi, A. Rotenberg et al. EBioMedicine 70 (2021) 103514 field XO. Multidimensional communication in the microenvirons of glioblas- bined fMRI and DTI examination of functional language lateralisation and ar- toma. Nat Rev Neurol 2018;14:482–95. doi:10.1038/s41582- 018- 0025- 8. cuate fasciculus structure: Effects of degree versus direction of hand prefer- [54] Poon CC, Sarkar S, Yong VW, Kelly JJP. Glioblastoma-associated microglia ence. Brain Cogn 2010;73:85–92. doi:10.1016/j.bandc.2010.03.004. and macrophages: targets for therapies to improve prognosis. Brain J Neurol [82] Tatti R, Haley MS, Swanson OK, Tselha T, Maffei A. Neurophysiology and regu- 2017;140:1548–60. doi:10.1093/brain/aww355. lation of the balance between excitation and inhibition in neocortical circuits. [55] Iaccarino HF, Singer AC, Martorell AJ, Rudenko A, Gao F, Gillingham TZ, et al. Biol Psychiatry 2017;81:821–31. doi:10.1016/j.biopsych.2016.09.017. Gamma frequency entrainment attenuates amyloid load and modifies mi- [83] Pallud J, Le Van Quyen M, Bielle F, Pellegrino C, Varlet P, Cresto N, et al. Cor- croglia. Nature 2016;540:230–5. doi:10.1038/nature20587. tical GABAergic excitation contributes to epileptic activities around human [56] Adaikkan C, Middleton SJ, Marco A, Pao PC, Mathys H, Kim DNW, et al. glioma. Sci Transl Med 2014;6 244ra89. doi:10.1126/scitranslmed.3008065. Gamma entrainment binds higher-order brain regions and offers neuropro- [84] Krause B, Márquez-Ruiz J, Cohen Kadosh R. The effect of transcranial direct tection. Neuron 2019. doi:10.1016/j.neuron.2019.04.011. current stimulation: a role for cortical excitation/inhibition balance? Front [57] Thomson H. How flashing lights and pink noise might banish Hum Neurosci 2013;7:602. doi:10.3389/fnhum.2013.00602. Alzheimer’s, improve memory and more. Nature 2018;555:20–2. [85] Targeted Non-Invasive Neurostimulation Meaningfully Reduces Seizure Fre- doi:10.1038/d41586- 018- 02391- 6. quency in Refractory Epilepsy Patients with Prior Surgical Interventions. Py- [58] Tantillo E, Vannini E, Cerri C, Spalletti C, Colistra A, Mazzanti CM, et al. Differ- zowski P, Ruffini G, Salvador R, Rotenberg A, Kaye HL, Shafi M, San J.D. Ab- ential roles of pyramidal and fast-spiking, GABAergic neurons in the control stract presented at the annual meeting of the American Epilepsy Society of glioma cell proliferation. Neurobiol Dis 2020;141:104942. doi:10.1016/j.nbd. (AES) on December 09, 2019. 2020.104942. [86] Ozdemir RA, Tadayon E, Boucher P, Momi D, Karakhanyan KA, Fox MD, et al. [59] Cortese B, Palamà IE, D’Amone S, Gigli G. Influence of electrotaxis on cell Individualised perturbation of the human connectome reveals reproducible behaviour. Integr Biol 2014;6:817–30 Quant Biosci Nano Macro. doi:10.1039/ biomarkers of network dynamics relevant to cognition. Proc Natl Acad Sci U c4ib00142g. S A 2020;117:8115–25. doi:10.1073/pnas.1911240117. [60] Cuddapah VA, Robel S, Watkins S, Sontheimer H. A neurocentric perspective [87] Fox MD. Mapping symptoms to brain networks with the human connectome. on glioma invasion. Nat Rev Neurosci 2014;15:455–65 10.1038/nrn3765. N Engl J Med 2018;379:2237–45. doi:10.1056/NEJMra1706158. [61] Calabrese C, Poppleton H, Kocak M, Hogg TL, Fuller C, Hamner B, et al. A [88] Armstrong T. See brain cancer as more than just the sum of biology. Nature perivascular niche for brain tumor stem cells. Cancer Cell 2007;11:69–82. 2018;561:S45. doi:10.1038/d41586- 018- 06706-5. doi:10.1016/j.ccr.2006.11.020. [89] Klein M, Postma TJ, Taphoorn MJB, Aaronson NK, Vandertop WP, [62] Huang YJ, Hoffmann G, Wheeler B, Schiapparelli P, Quinones-Hinojosa A, Muller M, et al. The prognostic value of cognitive functioning in the Searson P. Cellular microenvironment modulates the galvanotaxis of brain tu- survival of patients with high-grade glioma. Neurology 2003;61:1796–8 mor initiating cells. Sci Rep 2016;6:21583. doi:10.1038/srep21583. 10.1212/01.wnl.0 0 0 0 098892.33018.4c. [63] Keuters MH, Aswendt M, Tennstaedt A, Wiedermann D, Pikhovych A, Rot- [90] Lee J, Chaloner Winton Hall R. The impact of gliomas on cognition and capac- thues S, et al. Transcranial direct current stimulation promotes the mobility ity. J Am Acad Psychiatry Law 2019;47:350–9. doi:10.29158/JAAPL.003841-19. of engrafted NSCs in the rat brain. NMR Biomed 2015;28:231–9. doi:10.1002/ [91] Ghinda DC, Wu JS, Duncan NW, Northoff G. How much is enough-can rest- nbm.3244. ing state fMRI provide a demarcation for neurosurgical resection in glioma? [64] Lim M, Xia Y, Bettegowda C, Weller M. Current state of immunother- Neurosci Biobehav Rev 2018;84:245–61. doi:10.1016/j.neubiorev.2017.11.019. apy for glioblastoma. Nat Rev Clin Oncol 2018;15:422–42. doi:10.1038/ [92] Tuovinen N, de Pasquale F, Caulo M, Caravasso CF, Giudice E, Miceli R, et al. s41571- 018- 0 0 03-5. Transient effects of tumor location on the functional architecture at rest in [65] Werner JM, Lohmann P, Fink GR, Langen KJ, Galldiks N. Current landscape glioblastoma patients: three longitudinal case studies. Radiat Oncol Lond Engl and emerging fields of PET imaging in patients with brain tumors. Molecules 2016;11:107. doi:10.1186/s13014-016-0683-x. 2020;25 Basel Switz. doi:10.3390/molecules25061471. [93] R. Romero-Garcia, J. Suckling, M. Owen, M. Assem, R. Sinha, P. Coelho, et al. [66] Kim MM, Parolia A, Dunphy MP, Venneti S. Non-invasive metabolic imag- Memory recovery is related to default mode network impairment and neu- ing of brain tumours in the era of precision medicine. Nat Rev Clin Oncol rite density during brain tumours treatment 2019. https://doi.org/10.1101/ 2016;13:725–39. doi:10.1038/nrclinonc.2016.108. 19008581. [67] Patel P, Baradaran H, Delgado D, Askin G, Christos P, John Tsiouris A, et al. [94] Valiente M, Ahluwalia MS, Boire A, Brastianos PK, Goldberg SB, Lee EQ, et al. MR perfusion-weighted imaging in the evaluation of high-grade gliomas after The evolving landscape of brain metastasis. Trends Cancer 2018;4:176–96. treatment: a systematic review and meta-analysis. Neuro Oncol 2017;19:118– doi:10.1016/j.trecan.2018.01.003. 27. doi:10.1093/neuonc/now148. [95] Santarnecchi E, Brem AK, Levenbaum E, Thompson T, Kadosh RC, Pascual- [68] Li Z, Vidorreta M, Katchmar N, Alsop DC, Wolf DH, Detre JA. Effects of rest- Leone A. Enhancing cognition using transcranial electrical stimulation. Curr ing state condition on reliability, trait specificity, and network connectivity of Opin Behav Sci 2015;4:171–8. doi:10.1016/j.cobeha.2015.06.003. brain function measured with arterial spin labeled perfusion MRI. NeuroIm- [96] Song B, Wen P, Ahfock T, Li Y. Numeric investigation of brain tumor influence age 2018;173:165–75. doi:10.1016/j.neuroimage.2018.02.028. on the current distributions during transcranial direct current stimulation. [69] Bassett DS, Sporns O. Network neuroscience. Nat Neurosci 2017;20:353–64. IEEE Trans Biomed Eng 2016;63:176–87. doi:10.1109/TBME.2015.2468672. doi:10.1038/nn.4502. [97] Barcia JA, Sanz A, González-Hidalgo M, de Las Heras C, Alonso-Lera P, Díaz P, [70] Colombo MC, Giverso C, Faggiano E, Boffano C, Acerbi F, Ciarletta P. Towards et al. rTMS stimulation to induce plastic changes at the language motor area the personalised treatment of glioblastoma: integrating patient-specific clin- in a patient with a left recidivant brain tumor affecting Broca’s area. Neuro- ical data in a continuous mechanical model. PloS One 2015;10:e0132887. case 2012;18:132–8. doi:10.1080/13554794.2011.568500. doi:10.1371/journal.pone.0132887. [98] Rivera-Rivera PA, Rios-Lago M, Sanchez-Casarrubios S, Salazar O, Yus M, [71] Golby AJ, Kindlmann G, Norton I, Yarmarkovich A, Pieper S, Kikinis R. Inter- González-Hidalgo M, et al. Cortical plasticity catalyzed by prehabilitation en- active diffusion tensor tractography visualisation for neurosurgical planning. ables extensive resection of brain tumors in eloquent areas. J Neurosurg Neurosurgery 2011;68:496–505. doi:10.1227/NEU.0b013e3182061ebb. 2017;126:1323–33. doi:10.3171/2016.2.JNS152485. [72] Biswal B, Yetkin FZ, Haughton VM, Hyde JS. Functional connectivity in the [99] Robles SG, Gatignol P, Lehéricy S, Duffau H. Long-term brain plasticity al- motor cortex of resting human brain using echo-planar MRI. Magn Reson lowing a multistage surgical approach to world health organization grade Med 1995;34:537–41. II gliomas in eloquent areas. J Neurosurg 2008;109:615–24. doi:10.3171/JNS/ [73] Zhang D, Raichle ME. Disease and the brain’s dark energy. Nat Rev Neurol 2008/109/10/0615. 2010;6:15–28. doi:10.1038/nrneurol.2009.198. [100] Sandran N, Hillier S, Hordacre B. Strategies to implement and moni- [74] Van den Heuvel MP, Scholtens LH, Kahn RS. Multiscale neuroscience of psy- tor in-home transcranial electrical stimulation in neurological and psychi- chiatric disorders. Biol Psychiatry 2019. doi:10.1016/j.biopsych.2019.05.015. atric patient populations: a systematic review. J Neuroengineering Rehabil [75] Santarnecchi E, Momi D, Sprugnoli G, Neri F, Pascual-Leone A, Rossi A, et al. 2019;16:58. doi:10.1186/s12984-019- 0529- 5. Modulation of network-to-network connectivity via spike-timing-dependant [101] Nitsche MA, Fricke K, Henschke U, Schlitterlau A, Liebetanz D, Lang N, et al. noninvasive brain stimulation. Hum Brain Mapp 2018;39:4870–83. doi:10. Pharmacological modulation of cortical excitability shifts induced by tran- 1002/hbm.24329. scranial direct current stimulation in humans. J Physiol 2003;553:293–301. [76] Momi D, Neri F, Coiro G, Smeralda C, Veniero D, G S, et al. Cognitive en- doi:10.1113/jphysiol.2003.049916. hancement via network-targeted cortico-cortical associative brain stimulation. [102] Sills GJ, Rogawski MA. Mechanisms of action of currently used antiseizure Cereb Cortex 2019;1991 N Y N. doi:10.1093/cercor/bhz182. drugs. Neuropharmacology 2020;168:107966. doi:10.1016/j.neuropharm.2020. [77] Fiori F, Chiappini E, Avenanti A. Enhanced action performance following TMS 107966. manipulation of associative plasticity in ventral premotor-motor pathway. NeuroImage 2018;183:847–58. doi:10.1016/j.neuroimage.2018.09.002. [78] Fox MD, Buckner RL, Liu H, Chakravarty MM, Lozano AM, Pascual-Leone A. Resting-state networks link invasive and noninvasive brain stimulation across diverse psychiatric and neurological diseases. Proc Natl Acad Sci U S A 2014;111:E4367–75. doi:10.1073/pnas.1405003111. [79] Liu YY, Slotine JJ, Barabási AL. Controllability of complex networks. Nature 2011;473:167–73. doi:10.1038/nature10011. [80] O’Donnell LJ, Rigolo L, Norton I, Wells WM, Westin CF, Golby AJ. fMRI-DTI modeling via landmark distance atlases for prediction and detection of fiber tracts. NeuroImage 2012;60:456–70. doi:10.1016/j.neuroimage.2011.11.014. [81] Propper RE, O’Donnell LJ, Whalen S, Tie Y, Norton IH, Suarez RO, et al. A com- 12