2938 - Monte Carlo - Based Secondary Dose Check Platform for World's First Ultra Compact Proton Therapy System

Purpose/Objective(s): Upright proton therapy is rapidly transforming the field — of the 12 proton therapy systems currently under construction in the United States, 7 are ultra-compact upright units, with many more planned globally. The system under investigation represents the world's first such platform: a synchrocyclotron-based compact accelerator with a fixed horizontal beam, nested x-y scanning coils achieving 6 mm/ms scan speed, boron carbide energy shifter plates enabling 50 ms energy switching, a dynamic adaptive aperture (AA), a six-degree-of-freedom upright patient positioner, and a diagnostic-quality upright fan-beam CT. Critically, the range shifter and AA design of this system is shared with its predecessor compact platform, of which 15 units are currently in clinical operation worldwide with 10 more under construction. Despite this combined and rapidly growing installed base, no commercial secondary dose check algorithm exists for either system, creating an urgent, broadly felt unmet clinical need. This work presents the first Monte Carlo-based independent dose verification framework for ultra-compact proton therapy, with direct applicability across both platforms.

Materials/Methods: The treatment head of this upright pencil beam scanning system was modeled in TOPAS, incorporating the full nozzle geometry including a 12-plate energy boron carbide modulation system and a 7-pair nickel collimator AA. Integrated depth dose profiles in a water phantom and in-air spot profiles were generated across 11 energy modulations and benchmarked against clinical measurements and RayStation TPS calculations.

Results: Bragg peak fidelity was assessed via distal R80, pristine peak width (80%-80%), and distal falloff (80%-20%), with all deviations within 1 mm, within clinical accuracy thresholds. Spot profile FWHM agreement in both x and y directions fell within 10% tolerance across all energy modulations, consistent with commissioning acceptance criteria. Absolute dose validation for mono-energetic layers and SOBP fields stayed within 1.5% and 2.7%, respectively. Patient specific quality assurance gamma pass rates (3%/2mm) ranged from 90.5–96.8% and 88.3–94.7% (2%/1mm).

Conclusion: This work establishes the first validated Monte Carlo-based secondary dose check framework for an ultra-compact upright proton therapy system. Because the range shifter and AA architecture is shared with the predecessor compact platform (currently representing 15 operational and 10 additional systems under construction worldwide), the proposed framework extends naturally to that broader installed base as well. With no commercial independent verification tool available for either system, this open-source Monte Carlo approach offers the global proton therapy community a clinically accurate, cost-free solution for treatment plan verification at a meaningful scale.