Purpose: The Accuray Radixact system utilizes kV imaging first to build a tracking model and then subsequently on-treatment to continuously verify the model. It is the on-treatment kV-imaging that can be problematic to patient specific QA (PSQA) for devices such as SunNuclear ArcCheck with a limitation to discriminate between kV and MV dose. In this work, we highlight by quantifying the kV imaging dose and then discuss how the effect can be mitigated.
Methods: Our center’s design for PSQA on a moving phantom utilize the CIRS dynamic platform (008PL) with the ArcCheck atop. The ArcCheck ion chamber insert is replaced with an in-house developed lung tumor insert using off the shelf materials. PSQA is developed with lung tracking on the phantom. To quantify the kV imaging dose, we proceeded with a sample PSQA case and then setup and acquired dose from 3 angles, 0, 90, and 270 degrees done for 2 preset kV setups, ThoraxPelvic-XS (kV=100, mAs = 1) and ThoraxPelvic -XL (kV=140, mAs = 4).
Results: The kV imaging dose measured on the ArcCheck, reporting the maximum detector dose was 0.54cGy for the ThoraxPelvic -XL and down a factor of 11 to 0.05cGy for the ThoraxPelvic -XS.
Conclusion: The cumulative effect from several kV-images within a single PSQA may affect the outcome for some patients. The following mitigations may be useful (a) Use the minimum on-treatment kV image per gantry rotation which is 2 (an option to turn off kV imaging will be ideal). (b) Select the preset kV setup with the smallest dose (c) It is not currently possible to perform PSQA in simulation mode and it is not currently possible to perform dose subtraction on the ArcCheck software otherwise one could perform PSQA twice and subtract the dose distribution from simulation mode to compare with the planned.