Session Item

Output factor (OF) corrections based on IAEA TRS-483 Code of Practice (TRS-483) may be several percent for certain detector and beam types and sizes, and the implementation of the corrections may cause the need to adjust other parameters affecting the calculated dose distributions. The aim of this study was to evaluate the effect of TRS-483 based OF corrections on dose calculation accuracy of 6 MV FFF VMAT and conformal arc (CARC) stereotactic treatment plans of small lesions.

Nine treatment plans (6 VMAT, 3 CARC) were re-calculated using corrected OFs and varying values for dosimetric leaf gap (DLG). A TrueBeam STx linear accelerator equipped with high-definition MLC, and configured in Varian Eclipse treatment planning system (TPS) (version 15.6) (Varian Medical Systems Inc, Palo Alto, CA), was used for the dose calculation. Effective target spot size (SS) was 1.5 and 0.0 mm in X and Y directions, respectively. Objects covered three brain (planning target volumes (PTVs) of 0.5-1.7 cc) and three lung targets (PTVs of 5.2-30.0 cc). The plans were calculated using Acuros XB dose calculation algorithm (version 15.6.04). TPS dose distributions were compared to EGSnrc-based full Monte Carlo (MC) simulations and measurements using Delta4 phantom (ScandiDos, Uppsala, Sweden).

Difference in clinical target volume (CTV) mean dose between TPS and MC for VMAT and CARC plans and different OF and DLG values are presented in fig 1. Median dose difference between measured and TPS calculated plans are shown in fig 2. Compared to MC simulations and measurements, TRS-483 based OF correction seems to overestimate the resulting dose, which is seen especially in CARC plans, for which the adjustment of DLG does not fully compensate for the increased output.

Implementation of TRS-483 based OF corrections without adjusting the DLG may cause notable changes in dose calculation accuracy. Differences between VMAT and CARC plans require further investigation of the accuracy of the OFs and optimal DLG and SS values. Overall, the implementation of TRS-483 based OF corrections should be verified with utmost care.