The use of Perspex-based devices in patient specific quality assurance of Acuros XB plans

A transition from the dose-to-water algorithm AAA to the dose-to-medium algorithm Acuros XB (AXB) presents challenges for comparison with measurements made in the dose-to-water calibrated ArcCHECK. This study evaluates the implications of this transition on patient-specific quality assurance (PSQA) measurements. A modified dose-medium calibration for the ArcCHECK to was introduced to address these limitations, by developing a formalism to compare ionisation chamber measurements to water-like plastics such as Perspex to AXB dose-to-medium calculations. A retrospective analysis was conducted on 40 VMAT plans: 5 prostate, 5 cervix, 10 head/neck, 10 lung (5 standard VMAT, 5 SBRT), and 10 breast. Plans were recalculated in AXB using a Perspex-assigned ArcCHECK phantom and compared with dose-to-water and dose-to-medium calibrated ArcCHECK measurements with a γ (3%,2 mm) criteria. Only one of 35 non-SBRT AXB dose-to-medium plans was within clinical tolerance during dose-to-water calibrated ArcCHECK comparisons, compared to 33 of 35 AAA dose-to-water plans. Applying the proposed dose-to-medium calibration improved results, with 30 of 35 AXB plans performing within tolerance. In the SBRT lung cases, no AXB dose-to-medium plans passed ArcCHECK gamma comparisons, compared to four out of five AAA plans. Applying the dose-to-medium calibration to the ArcCHECK did not improve the passing rates, suggesting limitations of AXB calculations on the ArcCHECK device. This study highlights the inadequacy of dose-to-water calibrated PSQA devices for verifying AXB dose-to-medium calculations and demonstrates practical solutions, including applying a dose-to-medium calibration for the ArcCHECK for AXB PSQA applications.