Evaluation of machine log files/ MC ‐based treatment planning and delivery QA as compared to Arc CHECK QA

Purpose A treatment planning/delivery QA tool using linac log files ( LF ) and Monte Carlo ( MC ) dose calculation is investigated as a standalone alternative to phantom‐based patient‐specific QA (Arc CHECK ( AC )). Methods Delivering a variety of fields onto Map CHECK 2 and Arc CHECK , diode sensitivity dependence on dose rate (in‐field) and energy (primarily out‐of‐field) was quantified. AC and LF QA s were analyzed with respect to delivery complexity by delivering 12 × 12 cm static fields/arcs comprised of varying numbers of abutting sub‐fields onto Arc CHECK . About 11 clinical dual‐arc VMAT patients planned using Pinnacle's convolution–superposition ( CS ) were delivered on Arc CHECK and log file dose ( LF ‐ CS and LF ‐ MC ) calculated. To minimize calculation time, reduced LF ‐ CS sampling (1/2/3/4° control point spacing) was investigated. Planned (“Plan”) and LF ‐reconstructed CS and MC doses were compared with each other and AC measurement via statistical [mean ± StdDev( σ )] and gamma analyses to isolate dosimetric uncertainties and quantify the relative accuracies of AC QA and MC ‐based LF QA . Results Calculation and Arc CHECK measurement differed by up to 1.5% in‐field due to variation in dose rate and up to 5% out‐of‐field. For the experimental segment‐varying plans, despite CS calculation deviating by as much as 13% from measurement, Plan‐ MC and LF ‐ MC doses generally matched AC measurement within 3%. Utilizing 1° control point spacing, 2%/2 mm LF ‐ CS vs AC pass rates (97%) were slightly lower than Plan‐ CS vs AC pass rates (97.5%). Utilizing all log file samples, 2%/2 mm LF ‐ MC vs AC pass rates (97.3%) were higher than Plan‐ MC vs AC (96.5%). Phantom‐dependent, calculation algorithm‐dependent ( MC vs CS ), and delivery error‐dependent dose uncertainties were 0.8 ± 1.2%, 0.2 ± 1.1%, and 0.1 ± 0.9% respectively. Conclusion Reconstructing every log file sample with no increase in computational cost, MC ‐based LF QA is faster and more accurate than CS ‐based LF QA . Offering similar dosimetric accuracy compared to AC measurement, MC ‐based log files can be used for treatment planning QA .