Commissioning and validation of patient-specific quality assurance on an MR-Linac.
Du D, Kim J, Glide-Hurst C, Doemer A, Wen N, Movsas B, Dragovic J, and Chetty I. Commissioning and validation of patient-specific quality assurance on an MR-Linac. Med Phys 2018; 45(6):e143.
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: (a) 2D radiochromic film measurement using a customized 15 cm solid water phantom and (b) 3D diode array. Dose was calculated using a Monte Carlo algorithm (1 mm/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%/3 mm. The diode gamma passing rate was 98.62 ± 1.06% (n = 13; range = 95.6%-100.0%) at 2%/ 2 mm, which yielded similar sensitivity level to film at 3%/3 mm. 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.