Experimental Evaluation of Proton Therapy Transmission Spots Using Hybrid Gamma Imaging
Abstract
Purpose
Proton therapy shielding design typically assumes that protons stop within the patient. However, modern treatment planning optimization strategies may introduce high-energy proton transmission (“shoot-through”) spots that exit the patient and deposit energy in treatment room structures. This work experimentally evaluates the downstream dose and room-level radiation associated with proton transmission spots.
Methods
A head & neck treatment plan was created on an anthropomorphic phantom using two otherwise identical plans: one allowing proton transmission spots and a control plan with all transmission spots removed. Downstream dose was measured using a ion chamber array positioned beneath the phantom. Room radiation was evaluated using a hybrid gamma imaging system using coded-aperture and Compton scatter imaging to spatially localize radiation. Energy spectra were recorded to characterize radiation origin. Measurements were obtained within the phantom and after patient treatments.
Results
Ion chamber array measurements demonstrated downstream dose associated with proton transmission spots, with a maximum measured dose of 50.9 cGy compared to 18.5 cGy for the control plan. Hybrid gamma imaging revealed elevated radiation localized to multiple treatment room components during delivery of the transmission-spot plan. The measured energy spectrum was dominated by a 511 keV peak, consistent with positron annihilation from activation. Ambient background levels of 6–7 µR/hr increased to 55–80 µR/hr within the phantom during beam delivery. Floor-level measurements increased from 16 µR/hr without transmission spots to 36 µR/hr when transmission spots were present.
Conclusion
This study provides experimental evidence that proton transmission spots can produce measurable downstream dose and detectable activation in treatment room structures. Hybrid gamma imaging offers a powerful method for visualizing and characterizing these effects in situ. These findings suggest that shielding analyses should explicitly consider the potential impact of proton transmission spots, particularly for tangential beam geometries and robustly optimized plans.