Experimental verification of clinical dose and LET maps for LET-informed proton therapy

Objective:Linear energy transfer (LET)-informed proton therapy has the potential to improve biological treatment planning beyond the conventional, constant-RBE assumption. Its clinical translation, however, requires experimental tools capable of verifying spatially resolved dose and dose-averaged LET (LET_D) distributions under clinically relevant conditions. We recently introduced an optically stimulated luminescence (OSL) film-based detector system that simultaneously measures two-dimensional (2D) absorbed dose and LET_D. This work provides a first assessment of the system for verifying dose and LET_Ddistributions from clinical proton therapy plans. Approach:The system was tested at two proton therapy centers, ranging from simple uniform fields to more complex geometries, including Bragg curve measurements and a clinically realistic anthropomorphic head phantom treatment plan. Measured dose and LET_Ddistributions were compared against commercial treatment planning system (TPS) calculations and, for selected fields, to external Monte Carlo particle transport simulations due to the absence of traceability of LET_D. For non-uniform irradiations, agreement was assessed using gamma analysis with 3%/2 mm and 5%/2 mm criteria for dose and LET_D, respectively. Main results:For uniform fields, measured dose and LET_Ddistributions generally agreed with TPS and independent Monte Carlo simulations across a range of clinically relevant irradiation conditions. In the Bragg curve measurement, gamma pass rates were 96.6% for dose and 99.7% for LET_D, with the largest dose discrepancy observed near the Bragg peak, consistent with a current over-correction of detector quenching in high-LET regions to be improved in future work. In the anthropomorphic phantom, gamma pass rates ranged from 75% to 100% for dose and from 45% to 95% for LET_D, with a single layer showing a sub-50% pass rate for LET_D. Failing regions were confined to high-dose superior layers for dose and distal-edge, high-LET regions for LET_D. Significance:The presented results support the detector system's use as an experimental benchmarking tool for TPS-calculated dose and LET_Dmaps. Given this and the system's ability to simultaneously measure 2D dose and LET_Dmaps with high spatial resolution and minimal field perturbation, the system shows promise for future commissioning workflows for LET-informed proton therapy.