Purpose: To quantify out-of-field doses of neutrons generated by pencil-beam-scanning (PBS) protons irradiating newly developed proton-tissue-equivalent materials (ProTEM).
Methods: Three ProTEM materials; lung, 10 year-old pediatric bone, and soft tissue, were developed to match the corresponding human tissue with the same CT-HU and proton relative stopping power. Planned doses to irradiate a patient phantom constructed using ProTEM can be measured; especially the penetration depths of protons. In parallel, a low out-of-field dose of neutrons can be accurately measured by using ProTEM block-like phantoms to simulate human tissues better than any other solid material. Cubic and cylindrical blocks were made to measure doses at lateral distances away from the field edge and depths deeper than penetration depths to avoid any primary/secondary protons. The CR-39 neutron dosimeter that includes thermal and fast components was used to measure neutron doses. Irradiations of ProTEM block phantom were performed at the UFHPTI. Measured neutron out-of-field doses were scaled to proton dose at 2 cm for each irradiation. Monte Carlo (MC) simulations using the PHITS code with same geometry of irradiated block were performed to extract neutron fluence/dose at locations of measured neutron doses.
Results: Measured and simulated out-of-field neutron doses decrease largely as lateral distance increases. At 2 cm from the field edge, measured neutron doses present higher gradient than farther out locations. Primary/secondary protons near the field edge introduce large gradients due to minor sensitivity of neutron dosimeters to scattered protons. The large gradient was also observed in the MC simulations. However, large neutron doses were seen at depths beyond the distal penetration depth of protons. The gradient as a function of lateral distance has minor differences between depths beyond and within proton penetration.
Conclusion: ProTEMs generate neutrons similarly to the actual human tissues and are now suitable for clinical applications in PT.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by a grant from the University of Florida Proton Therapy Institute
Not Applicable / None Entered.