MR-guided radiotherapy requires novel QA methods for IMRT treatment plans (TPs). Here, an optimized method for TPs for a 1.5 T MR-linac was developed and implemented clinically. A static solid phantom and an MR-compatible 2D ionization chamber array were used. The array’s response with respect to the incident beam gantry angles was characterized for four different orientations of the array relative to the beam. A lookup table was created identifying the optimum orientation for each gantry angle. For the QA of clinical MR-linac TPs, beams were grouped according to their gantry angles and measured with up to four setups. The method was applied to n=106 clinical TPs of 54 patients for different tumour entities. Reference plans and plans created in the online adaptive workflow were analyzed, using a local 3%/3 mm gamma criterion for dose values larger than 30% of the maximum. Pass rates were averaged over all beam groups. The array’s response strongly depends on the beam incidence angle. Optimum angles typically range from -10° to 80° around the phantom setup angle. Consequently, plan verification required up to four setups. For clinical MR-linac TPs, the overall median pass rate was 98.5% (range 88.6%-100%). Pass rates depended on the tumour entity. Median pass rates were for liver metastases SBRT 99.2%, prostate cancer 99%, pancreatic cancer 98.9%, lymph node metastases 98.7%, partial breast irradiation 98%, head-and-neck cancer 97.7%, rectal cancer 94% and others 96.6%. 85% of plans were accepted straightaway, with pass rates above 95%. A single plan with a pass rate below 90% was subsequently verified with a modified method. Off-axis target volumes, e.g. partial breast irradiations, were verified successfully using a lateral shift of the phantom. The method is suitable to verify reference and online adapted TPs for a 1.5 T MR-linac, including plans for off-axis target volumes.
© 2020 Institute of Physics and Engineering in Medicine.