Dose Reduction to Motor Structures in Adjuvant Fractionated Stereotactic Radiotherapy of Brain Metastases: nTMS-Derived DTI-Based Motor Fiber Tracking in Treatment Planning

SIMPLE SUMMARY: Management of brain metastases adjacent to the motor cortex and pyramidal tract bears an elevated risk for treatment-related morbidity and, hence, a reduced quality of life. For safe resection, navigated transcranial magnetic stimulation (nTMS) motor mapping combined with diffusion tensor imaging (DTI)-based fiber tracking (DTI-FT(mot.TMS)) is used to guide the neurosurgeon to preserve motor function. So far, during radiotherapy motor structures are not respected as organs at risk. In this study, the practicability of DTI-FT(mot.TMS) in adjuvant radiotherapy planning of brain metastases was investigated, demonstrating a potentially significant dose reduction to cortical and subcortical motor structures. ABSTRACT: Background: Resection of brain metastases (BM) close to motor structures is challenging for treatment. Navigated transcranial magnetic stimulation (nTMS) motor mapping, combined with diffusion tensor imaging (DTI)-based fiber tracking (DTI-FT(mot.TMS)), is a valuable tool in neurosurgery to preserve motor function. This study aimed to assess the practicability of DTI-FT(mot.TMS) for local adjuvant radiotherapy (RT) planning of BM. Methods: Presurgically generated DTI-FT(mot.TMS)-based corticospinal tract (CST) reconstructions (FT(mot.TMS)) of 24 patients with 25 BM resected during later surgery were incorporated into the RT planning system. Completed fractionated stereotactic intensity-modulated RT (IMRT) plans were retrospectively analyzed and adapted to preserve FT(mot.TMS). Results: In regular plans, mean dose (D(mean)) of complete FT(mot.TMS) was 5.2 ± 2.4 Gy. Regarding planning risk volume (PRV-FT(TMS)) portions outside of the planning target volume (PTV) within the 17.5 Gy (50%) isodose line, the DTI-FT(mot.TMS) D(mean) was significantly reduced by 33.0% (range, 5.9–57.6%) from 23.4 ± 3.3 Gy to 15.9 ± 4.7 Gy (p < 0.001). There was no significant decline in the effective treatment dose, with PTV D(mean) 35.6 ± 0.9 Gy vs. 36.0 ± 1.2 Gy (p = 0.063) after adaption. Conclusions: The DTI-FT(mot.TMS)-based CST reconstructions could be implemented in adjuvant IMRT planning of BM. A significant dose reduction regarding motor structures within critical dose levels seems possible.