A proof-of-concept implementation of a patient-specific Geant4 Monte Carlo dose evaluation framework for SFRT

Accurate dose calculation in spatially fractionated radiotherapy (SFRT) is challenging because highly heterogeneous dose distributions and charged-particle disequilibrium can reduce the accuracy of fast analytical algorithms, including the anisotropic analytical algorithm (AAA) used in commercial treatment planning systems (TPSs). Monte Carlo (MC) methods are considered the reference standard for complex dose calculations, but patient-specific end-to-end studies of SFRT under realistic clinical conditions remain scarce. In this proof-of-concept study, we implemented a patient-specific Geant4-based MC framework for SFRT dose calculation and applied it to one clinical treatment plan. Patient geometry was reconstructed from planning computed tomography DICOM data. A Varian-provided 6 MV flattening-filter-free phase-space source upstream of the secondary collimators was used to simulate theTrueBeamlinear accelerator. Secondary collimators, including the jaws and a 120-leaf multileaf collimator (MLC), were modeled explicitly. Clinical volumetric modulated arc therapy control points were sequentially accumulated to enable end-to-end simulation. The three-dimensional MC dose distribution was quantitatively compared with the TPS-AAA calculation. Open-field water-phantom measurements yielded percentage-depth-dose errors within 3% and dose-profile errors within 4% for three standard square fields. No measurements of MLC-shaped fields or patient-specific measurement-based quality assurance (QA) were performed. For the investigated SFRT plan, three-dimensional global gamma passing rates using MC as the reference were 94.62%, 97.82%, and 99.40% under the 2%/2 mm, 3%/2 mm, and 3%/3 mm criteria, respectively, with a 10% dose threshold. The PVDR values were 2.050-2.620 for AAA and 1.884-2.454 for Geant4. Dose-volume histogram analysis showed slight underdosage in the gross tumor volume, while profile comparisons showed case-specific agreement between AAA and MC. These results demonstrate the technical feasibility of the framework and provide a transparent methodological basis for future validation studies. The framework is not intended to replace routine TPS calculations or serve as a commissioned clinical QA system.