Purpose: This work details our initial experience transitioning from ArcCheck to Varian Portal Dosimetry (PD) as our primary method for intensity-modulated radiation therapy quality assurance (IMRT QA).
Methods: A total of 347 IMRT plans (866 beams) were measured in the initial months using Varian PD across four TrueBeams. Gamma analysis results using a 10%, 5%, and 1% threshold, with a dose difference/distance to agreement (DD/DTA) criteria of 3%/3mm and 2%/2mm were recorded for both composite and individual fields as well as mean and max calibrated unit (CU) differences. Development of departmental IMRT QA policies included a detailed troubleshooting flow chart with goal- and acceptable-level passing rates. PD plans failing to meet the action limit tolerance of Γ≥95% or Γ≥90-95% with all individual fields passing Γ<95% at 3%/3mm (IMRT) or 2%/2mm (SBRT/SRS) would be deemed acceptable if ArcCheck passed Γ≥90% with 3%/3mm.
Results: The number of plans with a gamma passing rate Γ≥90% and Γ≥95% was 97.1% and 91.6% using 3%/3mm and 80.1% and 66.0% using 2%/2mm (10% threshold). Min, mean(stdev), and max average CU differences were 0.5%, 2%(1.3%), and 6.9%, respectively. For PD plans with Γ<95% at 2%/2mm, the mean DD/DTA needed for Γ≥95% was 2.8%/2.8mm. Out of the 17 plans that failed to meet the PD action limit and were measured on the ArcCheck, 2 failed on the ArcCheck and required re-planning. Qualitatively, some of the observed benefits of PD IMRT QA include a notably reduced physics time burden, ability to perform composite and field-by-field analysis, and ability to overlay MLC motions on gamma map. Common workflow challenges encountered include increased imaging panel calibration drift for new linacs, large fields not fitting on the EPID without a collimator rotation, and incorrect panel distances selected.
Conclusion: Varian PD has been an effective, practical, and reliable means of IMRT QA.