PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s):Radiosurgery (SRS) has been used with success to manage tremor in select patients with medically refractory tremor who cannot or do not want to undergo deep brain stimulation. Because of high doses, small target, and required precision, a system with a bolted frame has been the traditional platform. Our first objective was to develop and evaluate a safe, effective, and precise alternative on the linear accelerator without frame or cone. Our second objective was to couple treatment with high-resolution functional imaging of thalamic nuclei. We present here a pre-clinical evaluation of the technique, pilot treatment, and recently-opened phase II evaluation trial.
Materials/Methods:Patients’ pre-treatment tremor is evaluated with FTM score and PROMIS index. Patients are imaged on a 3T MRI and a 7T MRI, to generate MPRAGE, diffusion-weighted tractographic, and resting-state fMRI sequences. The VIM is identified via thalamic parcellation and compared to stereotactic reference location. The scan is fused to a thin-slice CT simulation obtained with patient immobilized in a Qfix Encompass rigid thermoplastic mask. The VIM is targeted to 130Gy Dmax. SRS is delivered on a linac with high-definition multi-leaf collimator (HDMLC) and intrafraction optical surface monitoring (OSMS) to ensure patient stationariness. Treatment is delivered in 20 flattening-filter free non-coplanar (half) arcs with fixed-MLC position and pre-determined beam modulation (Virtual Cone), resulting in spherical dose distribution equivalent to 4mm SRS shot. Post-treatment imaging and FTM/PROMIS scores are compared to pre-treatment baselines at scheduled intervals.
Results:In the pilot treatment, a patient underwent left VIM thalamotomy as above. QA revealed treatment accuracy to 0.3mm. At 3 month follow-up, patient had experienced significant tremor improvement in right upper extremity, with new T1-enhancing lesion corresponding to isocenter position. Our IRB-approved phase II trial has begun accruing as of December 2017 (https://clinicaltrials.gov/ct2/show/NCT03305588).
Conclusion:We believe advanced imaging has significant potential for improved VIM targeting and tremor reduction in radiosurgical thalamotomy in selected patients, and that the use of this novel approach maximizes patient comfort and treatment efficiency without compromise to accuracy.
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