Possibilities of using navigational transcranial magnetic stimulation as an element of radiotherapy preparation of patients with brain tumors: a pilot study
Authors: Kartashev A. V. , Ponezha T. Y. , Gilvanova E. V. , Makarov V. E. , Ilyin N. V. , Vinogradova Yu. N. 
Published: January 2023

INTRODUCTION: Navigational transcranial magnetic stimulation (NTMs) is used in neurosurgical practice to identify functionally important motor areas of the brain for the purpose of safe resection of a tumor focus. At the same time, when planning radiation therapy (LT) and determining the amount of irradiation, until recently, functionally significant zones of topical representation of skeletal muscles in the cerebral cortex were not taken into account. This study evaluates the use of mapping motor areas of the cerebral cortex using NTMs when planning LT in patients with malignant gliomas affecting the motor cortex.

OBJECTIVE: To evaluate the possibilities of navigational transcranial magnetic stimulation in the pre-radiation preparation of patients with brain tumors.

MATERIALS AND METHODS: The study included 31 patients with gliomas of a high degree of malignancy who received radiation and/or chemoradiotherapy at the RSCRT from 2020 to 2022. The median follow-up was 12.1 months (3.0–24.5 months), while all patients were followed up to the control, 3-month follow-up period after the end of the course of radiation therapy, 23 patients showed up for control after 6 months (two patients died between 3-month and 6-month control).

Statistics: analysis was performed using the Statistica 10.0 software (StatSoft, Inc., USA).

RESULTS: Three out of 31 patients had improved motor function, while eight out of 31 patients had decreased motor function. Moreover, when comparing the preoperative condition with the condition during control examinations six months after radiation therapy, motor function improved in three patients, and decreased in eight. Maps of motor zones of NTMs overlapped PTV by 41.2%. The average dose of Dmean maps of NTMs was 42.3 Gy (3.7–61.1 Gy) and 37.0 Gy (3.6–55.8 Gy) with a limit of 45 Gy per motor zone. Thus, the average dose of Dmean according to the maps of the motor zones of NTMs was significantly reduced by 5.3±3.3 Gy (14.3%, p<0.05). The average dose of Dmean on NTMs cards was reduced by 4.6±3.5 Gy (12.8%, p<0.05) to 37.8 Gy. The average volume of NTMs cards receiving a dose equal to or greater than 45 Gy and 55 Gy can be significantly reduced by 11.3% and 8.4%, respectively, with dose restriction (p<0.001). Anatomical ORS (optic nerve chiasm and brain stem) were not affected in relation to the average dose of Dmean or the maximum dose with an increase in the dose to GTV.

DISCUSSION: In our study, it was shown that maps of the motor zones of NTMs can be used in the planning of LT patients with gliomas of high malignancy. To date, the primary motor cortex has not been considered as an OR in such patients. The results of the study demonstrated that the dose in relation to the maps of the motor zones of NTMs can be significantly reduced without affecting the therapeutic doses for PTV.

CONCLUSION: LT planning using NTMs motor maps can help reduce the radiation dose to the motor cortex in primary LT, although the motor cortex appears morphologically rather insensitive to LT compared to speech zones and areas of high-order cognitive processes, such as the temporal and limbic cortex.