Purpose: Measurement-based patient-specific quality assurance (PSQA) is not possible for online adaptive radiotherapy (oART). The Varian Ethos CBCT-guided adaptive system utilises MobiusAdapt for online PSQA. This work compares MobiusAdapt and measurement-based PSQA for original and oART plans.
Methods: Fifty-two clinical IMRT plans (17 original and 35 adapted) were generated with Ethos. A variety of treatment sites were selected to ensure the plan complexity range reflected that of typical clinical practice. MobiusAdapt calculations were performed pre-treatment for original, and pre-beam-on for oART plans. Post-treatment, phantom-based dose measurements were performed for all plans using Delta4+ for 2D dose verification and a homogenous CIRS phantom for point dose verification.Different analysis criteria were evaluated to aid the comparison. For MobiusAdapt, gamma analysis was performed with 95% pass-rate for 3%/2mm and 3%/3mm (department criteria). For Delta4+, gamma analysis was performed with 95% pass-rate for 2%/2mm, and 3%/2mm (department criteria). CIRS point dose deviations (DD) were evaluated with department criteria 3% and compared to MobiusAdapt target structure meanDD, and Delta4+ medianDD. Results were compared for each plan, and the pass-rates relative to plan complexity metric MU/Gy evaluated.
Results: All plans passed department PSQA criteria. Average gamma pass-rates were 98.3% and 99.8% for MobiusAdapt 3%/2mm and 3%/3mm criteria, and 92.4% and 99.9% for Delta4+ 2%/2mm and 3%/2mm criteria. MobiusAdapt 3%/2mm pass-rates were on average 1% higher (range -2% to 3%) compared to Delta4+ 3%/2mm.Both CIRS point dose and Delta4+ medianDD measured on average 1% higher than Ethos, indicating agreement between measurement methods. Conversely, MobiusAdapt target meanDD was on average 1.4% lower than measurement.Greater plan complexity (MU/Gy) resulted in lower pass-rates for MobiusAdapt and Delta4+, and greater medianDD for Delta4+.
Conclusion: Online calculation-based QA and post-treatment measurement-based QA was performed on Ethos IMRT plans of varying complexity. MobiusAdapt gamma pass-rates were comparable to Delta4+.