P004 Application of TMS and DTI in pre-radiotherapy planning and the effect of radiation on motor function: A pilot study with two patients

Abstract

Question Although radiotherapy (RT) applied to treat brain tumors is a standard clinical procedure, the influence of RT on neural functions and structures nearby the tumor and the treatment volume has not widely been investigated. Thereby, functional areas are commonly not avoided as organs of risk. In this pilot study we evaluated whether TMS and DTI are beneficial in RT planning. Also, we aimed to receive indications on how RT affects the motor function. Methods The study included 2 patients with a subcortical tumor in the immediate vicinity of the corticospinal motor tracts, but a few centimeters from the motor cortex. Measurements were conducted before, after and 3 months from RT with navigated TMS and MRI. Following parameters were determined: resting motor threshold (rMT), upper threshold (UT) from Mills–Nithi algorithm ( Mills and Nithi, 1997 ), threshold for a silent period of 30 ms in duration (SPT30) ( Kallioniemi, 2014 ), anisotropy index (AI) at UT ( Kallioniemi, 2015 ) and size of the cortical motor representation mapped with UT. TMS-mapped cortical motor area and DTI-derived corticospinal motor tracts were defined as organs of risks in RT planning and given dose limits, although scattering radiation could not be avoided. A reference plan in which the motor areas were not avoided was also done, but was not used in the RT. Results Utilizing TMS and DTI in RT planning was able to reduce the maximum dose in the motor areas and corticospinal tract by 6–54% (Table 1) in comparison to reference plan. The characteristics of motor function are presented in Table 2. No clear trends of changes in rMT, UT, SPT30 or AI can be seen, whereas the size of the motor area seems to change after RT. Conclusion This pilot study shows that utilizing TMS and DTI in RT planning may be beneficial. Estimation of the full potential of the added information will require patients with varying tumor location and radiation dose under controlled conditions. The preliminary data implies that radiation may have influenced the size of the motor area through damage in the corticospinal tract. In patient 2, however, the motor area size returned back to pre-RT level 3 months from RT. Download : Download high-res image (315KB) Download : Download full-size image Download : Download high-res image (580KB) Download : Download full-size image