(Sunday, 7/29/2018) 2:05 PM - 3:00 PM
Room: Karl Dean Ballroom A2
Purpose: A recently installed MR-linac couples real-time, non-ionizing tumor tracking with double stacked, double-focused MLCs, making it a perfect candidate for high precision radiation therapy. However, establishing a robust patient-specific quality assurance (PSQA) program for the MR-linac is essential because of the unique machine configuration and presence of the magnetic field. This work describes the commissioning and implementation of our MR-linac PSQA program.
Methods: Thirteen IMRT patients (6 prostate, 4 lung, 2 abdomen, and 1 partial breast) with target sizes ranging from 5.2 to 13.5 cm, 3 to 10.5 cm and 5 to 10.5 cm in the superior-inferior, anterior-posterior, and left-right directions, respectively, were evaluated. Each plan was evaluated in two ways: (1) 2D radiochromic film measurement using a customized 15 cm solid water phantom and (2) 3D diode array. Dose was calculated using a Monte Carlo algorithm (1mm/1% accuracy). Point dose measurements were obtained using an MR-compatible micro-ionization A26 chamber for both configurations. Gamma passing rates were calculated and compared between film and diode measurements.
Results: Ionization chamber measurements agreed well with computed doses in both phantoms with percent differences of -0.34% ± 0.92% (range=-1.96% to 1.38%) and 0.76% ± 0.99% (range=-0.86% to 2.48%) within the diode array and solid water, respectively. Film and diode array results had excellent agreement with TPS calculation. Gamma passing rates were 98.37% ± 0.98% (n=13; range=96.5%-99.9%) and 99.9 ± 0.3% (n=13; range=99%-100.0%) for film and diodes, respectively, at 3%/3mm. The diode gamma passing rate was 98.62 ± 1.06% (n=13; range=95.6%-100.0%) at 2%/2mm, which yielded similar sensitivity level to film at 3%/3mm.
Conclusion: We have developed a PSQA program for the MR-linac. Results indicate that both 3D diodes and GafchromicTM film are suitable for routine PSQA. Future work will involve data collection in a larger cohort to establish action levels.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.
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