Experimental Validation of Ultrafast Proton Therapy Using Pin Ridge Filters
Abstract
Purpose
Pin-ridge filters (pRFs) have emerged as a promising hardware-based approach for ultrafast proton beam delivery through upstream proton fluence modulation. While pRF-based planning techniques having been explored computationally, experimental implementation has been limited by challenges in fabricating patient-specific pRFs and validating the resulting highly modulated dose distributions using conventional quality assurance tools. This study presents a comprehensive end-to-end experimental framework for pRF-based ultrafast proton therapy, with emphasis on practical fabrication and dosimetric verification techniques.
Methods
Patient-specific pRF models for two plans were generated in RayStation using our iterative spot-reduction algorithm adapted to a water-phantom geometry for patient-specific QA (PSQA). The pRFs were fabricated using a large-format fused deposition modeling 3D printer with natural PLA filament. Each pRF was positioned on solid water, and its placement was verified using matched kV-kV imaging prior to irradiation. Absolute dose measurements were acquired using a MatrixxPT ionization chamber array at seven depths, while relative dose distributions were assessed using EBT3 Gafchromic film. Agreement between the planned and measured doses was evaluated using gamma analysis with multiple acceptance criteria.
Results
Across all 30 measured pRF fields, gamma pass rates exceeded 90% for all evaluated criteria. Mean pass rates were 98.29 ± 0.46% (3%/3 mm), 97.39 ± 0.54% (3%/2 mm), and 96.21 ± 0.68% (2%/2 mm). Lower pass rates were observed at distal depths, with an average of 92.5 ± 1.57% for the 3%/2 mm criterion. Film measurements yielded pass rates of 96-99% at mid-range depths and 90-94% near the distal edge, consistent with increased film quenching effects.
Conclusion
Our end-to-end pRF planning, fabrication, and measurement workflow demonstrates that standard PSQA techniques can be used to verify pRF-based ultrafast proton therapy plans. These results support the feasibility of pre-clinical and clinical translation of pRF-based ultrafast proton therapy using existing clinical PSQA infrastructure.