Purpose: The Elekta Unity MR-Linac has a fixed isocenter with couch movement in the superior-inferior direction only. The treatment isocenter in an MR-Linac plan could be far from the machine isocenter. Traditionally, patient-specific QA using a Sun Nuclear ArcCheck MR mounted on the Elekta QA platform aligns the phantom to the machine isocenter; however, this setup becomes an issue when the treatment isocenter is far from the machine isocenter laterally. We propose a modified QA setup and procedure to account for these off-centered plan measurements.
Methods: A custom QA platform consisting of a machined acrylic holder and etched MR safe exact bars for indexing was created in-house for easily setting up the ArcCheck MR at the center of the couch or shifting the phantom laterally by 10 cm. The new procedure was tested on a clinical breast plan in which the center of the target was 12.5 cm to the left of the machine isocenter. IMRT QA plans with couch positions corresponding to the central and shifted setups were created in the Monaco treatment planning system. IMRT QA measurements were analyzed with gamma criterion of 3mm/3% dose difference and a 10% dose threshold.
Results: The prescription dose was 750 cGy per fraction. The maximum dose delivered to the phantom was 494 cGy and 672 cGy to for the center and off-center setup, respectively. The maximum dose to the ArcCheck detector array was approximately 200 cGy and 650 cGy, for the center and off-center setup, respectively. The gamma pass rate was 90.1% and 100.0% for the center and off-center setup, respectively.
Conclusion: The modified off-center setup with a customized QA platform improved patient-specific QA for treatment targets far from the central axis. It is a possible solution to the challenge of patient-specific QA phantom positioning in fixed-gantry linacs.