How robust are secondary dose algorithms for Patient-Specific quality assurance in modulated stereotactic Radiosurgery?

PURPOSE: Accurate dose verification is essential in stereotactic radiosurgery (SRS), where small fields and steep dose gradients may amplify uncertainties, especially with complex techniques like multitarget single-isocenter VMAT. This technical note evaluated the robustness of two secondary dose calculation algorithms for SRS patient-specific quality assurance (PSQA). METHODS: RadCalc Collapsed Cone Convolution Superposition (RC-CCCS) and Monte Carlo (RC-MC) were commissioned on beam-matched VersaHD linacs. Percentage depth doses (PDDs), profiles, and output factors (OFs) of 6 MV FFF beams were compared with measurements to assess small-field modeling (1 × 1-10 × 10 cm2). Thirty VMAT treatments (57 brain metastases) were recalculated with RadCalc and benchmarked against Pinnacle3 TPS. Agreement was evaluated using local gamma analysis (2%/2 mm, 30% threshold), DVH metrics, and end-to-end tests. RESULTS: Both RadCalc algorithms showed strong agreement with measurements and TPS for PDDs, field sizes, and OFs. RC-CCCS enlarged the lateral penumbra, particularly in the in-plane direction (∼1.5 mm). In patient plans, significant differences were observed between algorithms (p < 0.001), with mean gamma passing rates of 96.8% for RC-MC and 94.1% for RC-CCCS. On average, RC-CCCS overestimated the central target dose by ∼ 5%. End-to-end measurements were highly consistent with both TPS and RC-MC (<1% mean deviation, p = 0.74), whereas RC-CCCS showed larger deviations (mean 5.7%,p < 0.001). CONCLUSIONS: RC-MC provides accurate and robust independent dose verification for intracranial SRS. RC-CCCS exhibited relevant inaccuracies in penumbra and 3D dose modeling, particularly in multi-lesion plans, highlighting the need to consider algorithm-specific limitations when implementing secondary dose calculations in PSQA workflows.