Configuration of a Monte Carlo-Based Secondary Plan Verification Tool: An Analytical Approach to User-Specific...

Quality assurance in radiotherapy traditionally relies on pre-treatment measurement-based verification using homogeneous phantoms, which may not fully account for patient-specific heterogeneities. Independent dose calculation (IDC) methods have emerged as an alternative for secondary plan verification, but their accuracy depends on proper modelling of multileaf collimator (MLC) parameters. This work focuses on the efficient and systematic tuning of these parameters. In particular, the MC module of RadCalc® (RCMC, version 7.4) was commissioned for multiple photon beam energies (6X, 6FFF, 10X, 10FFF) for two Varian TrueBeam® linear accelerators, namely TB1 and TB2. An analytical workflow was developed to determine the optimal Additional Radiation to Light Field Offset (ARLFO) value from a linear interpolation of the differences between dose measurements at different MLC-gaps and RCMC calculations, combined with weighting factors reflecting dosimetric sensitivity and clinical MLC-apertures frequencies. The method was validated on an independent clinical set of plans using the Gamma Passing Rate (GPR) evaluation (2%/2 mm local dose criterion, 10% dose threshold), and statistical significance of differences between optimal and default values of ARLFO was assessed via Wilcoxon signed-rank test. The obtained optimal ARLFO values differed between the two linacs, being positive for TB1 and negative for TB2 across all beam energies. The use of the optimized ARLFO led to a systematic increase in median GPRs: for TB1, improvements ranged from +3.8 to +8.4 percentage points (p ≤ 0.028); for TB2, increases ranged from +8.8 to +17 percentage points (p ≤ 0.0020), with no significant change observed for 10X (p = 0.31). The proposed analytical approach enables simple, efficient, and reproducible user-specific parameter tuning in Monte Carlo-based secondary dose verification and is expected to be applicable across different clinical settings. .