Cone Beam Computed Tomography For Dose Calculation Quality Assurance For Magnetic Resonance-Only Radiotherapy

Background and purpose: Magnetic Resonance (MR)-only prostate radiotherapy using synthetic Computed Tomography (sCT) algorithms with high dose accuracy has been clinically implemented. MR images can suffer from geometric distortions so Quality Assurance (QA) using an independent, geometrically accurate, image could be required. The first-fraction Cone Beam CT (CBCT) has demonstrated potential but has not been evaluated in a clinical MR-only pathway. This study evaluated the clinical use of CBCT for dose accuracy QA of MR-only radiotherapy.Materials and methods: A total of 49 patients treated with MR-only prostate radiotherapy were divided into two cohorts. Cohort 1 (20 patients) received a back-up CT, whilst Cohort 2 (29 patients) did not. All patients were planned using the sCT and received daily CBCT imaging with MR-CBCT soft-tissue matching. Each CBCT was calibrated using a patient-specific stepwise Hounsfield Units-to-mass density curve. The treatment plan was recalculated on the first-fraction CBCT using the clinically applied soft-tissue match and the doses compared. For Cohort 1 the sCT was rigidly registered to the back-up CT, the plan recalculated and doses compared.Results: Mean sCT-CBCT dose difference across both cohorts was 0.6 +/- 0.1% (standard error of the mean, range -2.3%, 2.3%), with 47/49 patients within [-2%, 1%]. The sCT-CBCT dose difference was systematically lower than the sCT-CT by 0.7 +/- 0.6% (+/- 95% limits of agreement). The mean sCT-CBCT gamma pass rate (2%/2 mm) was 96.1 +/- 0.4% (85.4%, 99.7%).Conclusions: CBCT-based dose accuracy QA for MR-only radiotherapy appears clinically feasible. There was a small systematic sCT-CBCT dose difference implying asymmetric tolerances of [2%, 1%] would be appropriate.