Evaluation of neutron-induced activation and gamma-ray dose from out-of-field gold markers in proton therapy

• Experimental validation of neutron-induced activation of gold in proton beams. • Monte Carlo simulations accurately reproduce activation yields of irradiated gold. • Out-of-field gold markers emit minimal gamma-ray dose after proton irradiation. • Validated approach supports activation assessments for implanted materials in proton fields. In proton therapy (PT), secondary neutrons can activate implanted materials, potentially generating additional radiation exposure. Gold markers, commonly used in PT, can be activated by secondary neutrons when placed out-of-field; however, the potential impact of such induced activation on patients has not been clearly evaluated. We first validated Monte Carlo simulations using the Particle and Heavy Ion Transport code System (PHITS) by comparing the calculated induced activity of gold with experimental activation measurements, showing agreement within 8%. After validation, PHITS was used to estimate the gamma-ray doses emitted from out-of-field gold markers activated by secondary neutrons in a representative clinical liver treatment. The calculated gamma-ray doses ranged from 0.901 to 40.62 μGy. PHITS demonstrated reliable accuracy in calculating neutron-induced activation of gold in PT. Gamma-ray doses from activated gold markers placed out-of-field were found to be minimal, indicating that the potential impact on patients is negligible.