2573 - Feasibility and Outcomes of Navigated Transcranial Magnetic Stimulation Based Diffusion Tensor Imaging Tractography of Motor Pathways in Patients Undergoing Stereotactic Radiosurgery

J. McDonald¹, J. K. Bronk¹, M. Muir², H. Michener³, D. Mackin⁴, W. Talpur¹, T. Beckham¹, A. J. Ghia¹, S. L. McGovern¹, C. Wang¹, K. D. Woodhouse¹, D. N. Yeboa¹, M. Farhat¹, A. M. Elliott¹, J. Li¹, S. Prabhu², S. Prinsloo^2,3, and C. Chung¹; ¹Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ²Department of Neurosurgery, The University of Texas MD Anderson Cancer Center, Houston, TX, ³Department of Palliative, Rehabilitation, and Integrative Medicine, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): There are no stereotactic radiosurgery (SRS) dose constraint guidelines for the motor cortex and tracts as this region of interest (ROI) is difficult to delineate using conventional imaging techniques and anatomic landmarks. Navigated transcranial magnetic stimulation (nTMS) is a novel non-invasive tool using electromyographic signal and magnetic resonance diffusion tensor imaging (DTI) to functionally map cortical motor tracts (MT). While nTMS is used to identify the MT before tumor resection, it has not been implemented in SRS planning. The objectives of this study were to (i) determine the feasibility of performing nTMS-based DTI in patients with previously SRS treated brain metastases in or near the MT, (ii) measure concordance of nTMS seeded MT (TMS-MT) versus anatomically seeded MT (A-MT) and (iii) evaluate the relationship between radiation dose to TMS-MT and patient outcomes measured by objective hand function testing and patient reported outcomes (PROs). Materials/

Methods: 15 patients with brain metastases proximal to anatomically defined MT previously treated with single-fraction SRS were enrolled on an IRB-approved cross-sectional study. At a median 4.9 months after SRS, patients underwent nTMS testing of the upper extremities and brain DTI. Functional outcomes testing (Grip Dynamometer) and quality-of-life (QOL) PROs (EQ-5D-5L) were acquired. Tractography was generated on treatment planning software and imported as an ROI into the treatment planning system for volumetric and dosimetric evaluation.
Results: Functional testing, PRO questionnaires, and nTMS scheduling was successful for 15/15 (100%) of enrolled patients and TMS-MT was successful in 14/15 (93.3%). Examination of size and location of A-MT versus TMS-MT revealed that A-MT were larger (median volume 17.85cc versus 7.1cc for TMS-MT). There was a median volumetric overlap of 4.16cc (32.1%) between the TMS-MT and A-MT on the patient’s treated side (range 16.4-46.3%). Strength deficits occurred in 5/10 (50%) patients treated unilaterally (unaffected extremity testing measures used as a baseline control), and 60% of patients experienced coordination deficits. Median Dmax to TMS-MT was 12.5Gy for patients with deficits in grip strength compared to 4.2Gy for those without detected deficits. For all patients, a decrease in EQ5D5L score trended linearly with increase in Dmax to the TMS-MT.
Conclusion: In this completed feasibility study, nTMS testing in the radiation oncology clinic met criteria for feasibility. Dmax to the TMS-MT correlated with magnitude of negative impact in motor function and overall QOL. The notable differences in location and size of the TMS-MT vs. A-MT suggest that additional functional imaging may improve MT delineation to inform clinical decision making and SRS treatment planning.