Estimation of Errors of Using Preconfigured Beam Data Using New Epoms from WGTG51 Report 374
Abstract
Purpose
To quantify the systematic uncertainty introduced to pre-configured beam data if the Effective Point of Measurement (EPOM) proposed in AAPM TG-51 Report-374 is implemented.
Methods
Pre-configured beam data for Varian TrueBeams from Varian Advanced Oncology Solutions (AOS) were evaluated. Measurements were performed using an IBA CC13 ionization chamber (inner radius 3 mm) and IBA myQA Accept software. Original depth-dose curves were shifted by 0.6r (1.8 mm) for photons and 0.5r (1.5 mm) for electrons to account for EPOM. Report-374 recommends Monte Carlo based EPOM values of 1.2 mm for photons and 0.7 mm for electrons. As AOS does not plan to update the pre-configured data, dosimetric errors were assessed. First, PDD differences were evaluated assuming Report 374 EPOMs as reference. Second, effects on TG-51 output calibration were examined. Effects on k_Q and ionization-to-dose conversion were ignored due to negligible sensitivity to small EPOM shifts.
Results
For photons, PDD at 10 cm was 0.24–0.36% lower; for electrons, the 90% dose depth was 0.8 mm shallower. Using 0.6r for photons yields output errors of 0.24–0.36% higher, with calculated dose matching the true dose except at d_max. Using Report-374 photon EPOM gives correctly calibrated output, with similar deviations in calculated dose. For electrons, using 0.5r introduces output errors of 0.025–0.39%, whereas adopting Report-374 EPOM gives differences of 0.1–0.4%. The small difference is due to d_ref is close or at the d_max where the gradient is low.
Conclusion
Clinically, uncertainties from the updated EPOMs are small (<1%), within annual QA tolerances. However, MPPG-8b recommends using TPS data as a baseline, and for electrons, the 0.8 mm PDD difference could challenge the 1 mm beam-quality criterion. Ultimately, pre-configured data should be updated to reflect the revised EPOMs.