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Personalized treatment of pain in brain tumor patients with tDCS: Are we there?

Zoi Manoli1, 2* and Theodoros Samaras1, 3
  • 1 Department of Physics, Aristotle University of Thessaloniki, Greece, Greece
  • 2 THESS S.A., Thessaloniki, Greece, Greece
  • 3 Department of Physics, University of Malta, Malta, Malta

1. Introduction: Transcranial direct current stimulation (tDCS) is a non- invasive neuromodulatory technique which is widely adopted in clinical cases such as stroke recovery, Parkinson’s disease and many neuropsychiatric conditions etc [1-2]. Furthermore, in recent years tDCS was used in patients with tumors to relieve them from the cancerous pain [3-4]. Until now there are few reports [5] about the use of tDCS technique on the cancerous pain control in patients with brain cancer. Our study intends to show the impact of the lesion area by estimating computationally the electric field distributions inside a realistic head model, including a brain tumor when receiving tDCS treatment. A secondary objective is to examine the variations in the electric field, due to the different grades of the brain tumor (I- IV) with the use of different electrical conductivities, as they are used in the respective literature of clinical studies. 2. Materials and Methods 2.1. The realistic head model In the present study a realistic human head model with a brain tumor was used. The MRI image data were obtained from the Cancer Imaging Archive [6] which provides collections of subjects who have a cancer type in lung, brain etc. In our study a female subject (sample HF0960) with an age between 30 and 34, diagnosed with a grade II oligodendroglioma in the left frontal brain, was used. The human- head dataset contained 240 x 240 x 116 slices in the transverse, sagittal and coronal axes, respectively [6-7]. The iSeg v3.8 tool (ZMT, Zurich, Switzerland), a medical image segmentation tool specially designed to construct realistic 3D human models was used for the segmentation of the human tissues. First, a semiautomatic generation of white matter, fat, muscles, cerebrospinal fluid (CSF) and skin was done. A manual correction was performed, to segment the brain tumor, the grey matter and the skull. The volume of this tumor was 4.8x10^5 mm^3. The entire process of segmentation was completed after 30 hours (Figure 1). With the help of the image processing toolbox of MATLAB (The MathWorks, Inc., Natick, MA) and a brain atlas, the electrodes were placed over the left M1 (primary motor cortex) (anode) and the Fp2 (contralateral supra- orbital region) (cathode) according to the 10/20 EEG international system [8], to stimulate the primary motor cortex for the cancer pain control [9]. A pair of electrodes with dimensions of 5 cm x 5 cm was modelled. The electrodes consisted of solid copper (electrical conductivity 5.8 × 10^7 S/m) and a part of saline-soaked sponge (electrical conductivity 1.4 S/m). The whole surface of the electrodes (sponge part) was in touch with the skin of the model. Finally, a current of 2 mA was applied between the anode and cathode electrode. 2.2. Numerical technique In order to perform the simulations, the computational platform SEMCAD-X v14.8 (SPEAG, Zurich, Switzerland) was used. In particular, the stationary currents electro quasi-static low- frequency solver was used. The Laplace equation (1) was solved and the electric potential (φ) distribution inside the human head models was determined. ∇(σ∇φ)=0 (Eq.1) At the outer boundaries of the computational domain a Dirichlet boundary of grounding (φ = 0 V) was assumed, whereas the lower boundary was set to a homogeneous Neumann condition (insulation).The mesh step was set at 1 mm to allow a good discretization of the human head model, the brain tumor and the electrodes. 2.3. Dielectric properties In this study, seven tissue regions were segmented (grey and white matter, CSF, skin, brain tumor, fat and muscle). For the conductivities of grey matter, white matter and CSF, the values used by Datta et al [10] were assigned: grey matter: 0.276 S/m, white matter: 0.126 S/m and CSF: 1.65 S/m. Skin was assigned the conductivity of muscle, since according to Geddes and Baker [11], the skin/scalp conductivity is very close to the average conductivity reported for muscle. Moreover, the muscle, eyes, fat and blood vessels were assigned the same tissue properties as that of scalp [12]. Therefore, in the present study, the conductivity of the skin/scalp was set to 0.20 S/m (i.e., equal to the conductivity of muscle at 10 Hz, as proposed by Gabriel et al [13]). Furthermore, the conductivities of the brain tumor was set according to Song et al [5] as shown in Table 1, which also summarizes the features of the eight simulation cases. All tissues were considered homogeneous and isotropic. 3. Results For all simulations (i.e. different grade and conductivity of the brain tumor) the electric field distributions were computed and analysed in brain tissues close to the brain tumor. In particular, both the 99th percentile and the tissues volumes with electric field intensity over 50% and 70% of the 99th percentile value [14] in grey matter which is close to the brain tumor, were analyzed. The 99th percentile was chosen to avoid possible computational artifacts in single, isolated voxels, which could originate from tissue segmentation. The influence of the brain tumor in growing grade and different conductivities creates the changing of the electric field distributions inside the brain of the patient. The peak value of the electric field in the area close to the brain tumor increases, as the grade of the tumor is increased. Moreover, from the results we can assume that it is safe to stimulate the primary motor cortex of patients with a brain tumor to relieve them from the cancerous pain and/or to help them with their neuropsychiatric disorders. 4. Discussion and conclusions Transcranial direct current stimulation can be considered as a promising technique of controlling the cancerous pain and/or a technique which patients with a brain tumor can use to help their neuropsychiatric conditions. This is a first approach of an individualized treatment of cancerous pain. In future studies, stimulation with varied montages of electrodes can be examined. It is clear that the existence of the brain tumor influences the electric field distibutions inside the brain of the patient. Moreover, as the grade of the tumor increases the electric field distribution affects more some deeper brain tissues.

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References

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Keywords: tDCS, personalized treatment, brain tumors, Pain, computational modeling

Conference: SAN2016 Meeting, Corfu, Greece, 6 Oct - 9 Oct, 2016.

Presentation Type: Poster Presentation in SAN2016 Conference

Topic: Posters

Citation: Manoli Z and Samaras T (2016). Personalized treatment of pain in brain tumor patients with tDCS: Are we there?. Conference Abstract: SAN2016 Meeting. doi: 10.3389/conf.fnhum.2016.220.00086

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Received: 29 Jul 2016; Published Online: 01 Aug 2016.

* Correspondence: Ms. Zoi Manoli, Department of Physics, Aristotle University of Thessaloniki, Greece, Thessaloniki, Greece, zomanoli@physics.auth.gr