Purpose: A 6FFF Monte Carlo (MC) dose calculation algorithm was commissioned for spine stereotactic radiosurgery (SRS) planning on the Versa HD. Initial model generation, validation, and ensuing model tuning will be presented. Methods: The model was generated by the manufacturer, Brainlab, using both in-air and in-water commissioning measurements of field sizes between 6mm and 400mm on the Versa HD. Initial model validation was performed by simulating a water tank in the planning system, calculating commissioning fields, and comparing calculations to measured data. Model accuracy was quantified for output factors, PDDs, profile sizes, and profile penumbras. Nine previously treated Spine SRS patients were aggressively re-optimized with the new MC model to lower spinal cord dose through increased modulation. The resulting plans were calculated on the StereoPHAN phantom and subsequently delivered to the microDiamond and SRSMapcheck to verify calculated dose accuracy. Model tuning was performed by varying the model’s light field offset (LO) distance between the physical and radiological positions of the collimating jaws and MLC, with goal of improving the accuracy of the field size and StereoPHAN calculations. Results: Calculated output factors were within 2% of measured for square field sizes down to 10mm. PDDs were within 2% of measured past dmax. Penumbra widths were within 1mm for the MLCs and 0.5mm for Jaws. Field sizes were within 0.5mm for both MLC and Jaws. On average, microdiamond measured point doses were within –1.01%+/- 2.18% and 0.311%+/-1.20% for targets and OARs, respectively. Average per-plan pass rates using a 2%/2mm/10% threshold relative gamma analysis were 99.1%+/-0.89%. Adjusting LOs led to improved open field and patient-specific dosimetric agreement. Conclusion: Initial validation of a Monte Carlo algorithm for simple fields and complex SRS spine deliveries in homogeneous phantoms have been performed. Subsequent validation is needed in heterogeneous phantoms.
Knill C, Sandhu R, Loughery B, Lin L, Halford R, Drake D, et al [Snyder M] Commissioning and validation of a Monte Carlo algorithm for spine stereotactic radiosurgery. Med Phys. 2022 June; 49(6):E456.