Purpose: The dosimetric accuracy of Eclipse’s Acuros XB (AXB) algorithm in patients relies on proper assignment of physical medium in the planning CT. However, using AXB for phantom-based patient specific quality assurance (PSQA) presents challenges due to differences between phantom material and human tissue. This study evaluates the impact of physical material selection on PSQA results for AXB plans using ArcCheck (Sun Nuclear) phantom with test plans, and verifies the result using clinical cases.
Methods: Single field beam calculation on a vendor-provided digital phantom of ArcCheck tested in AXB plans varying: overwritten physical material, beam energy, field size, MLC blocking, and dose-to-medium reporting. Dose at the incident detector, the exit detector, and the center of phantom, together with the ratio between incident and exit dose, were calculated and compared against ArcCheck measurements with all software corrections applied across multiple TrueBeam machines (Varian). Potentially optimal settings for QA dose calculation were determined by maximizing the agreement between AXB dose calculation and ArcCheck measurements. 10 clinical cases were re-planned using AXB with PSQA verification plans generated using the optimized AXB settings. Gamma indices between PSQA plans and ArcCheck measurements were calculated using Sun Nuclear SNCPatient software (version 8.4.1).
Results: Based on testing plans, the optimal settings for AXB PSQA dose calculation is to have physical material overwritten as PMMA. In addition ArcCheck dose calibration needed to be performed to normalize with AXB calculated dose to medium or water. Systematic uncertainty in AXB PSQA reduced as average Gamma Index passing rate of clinical cases’ PSQA using the optimal setting increased 3.5% for institutional clinical criteria (3% global dose, and 2mm DTA) compared to AXB dose calculated using recommendation settings.
Conclusion: Optimal settings has been determined for ArcCheck phantom based AXB PSQA using testing plans, and validated using clinical PSQA results.
Quality Assurance, Phantoms, Dosimetry