(A) study on secondary neutron dose assessment and reduction at double scattering proton beam therapeutic process

Abstract

Proton therapy is prospective and expanding field worldwide with advanced medical merit. But additional neutron generation during therapeutic process is giving potential health risk to human. Considering that ICRP’s radiation protection threshold is 100mSv, several hundred of secondary neutron dose should be treated as an interest of health physics. This study have a purpose of neutron dose assessment and its reduction with above backgrounds. Main idea for neutron dose assessment is MCNP6 monte carlo simulation. Through the proton beam nozzle manufacture’s CAD drawings and architectural plan review, accurate modeling input was built. Before main simulation, the validity of standard nozzle modeling was checked with proton beam output range comparison to NIST data and it had a good accordance within 0.9%. Then, main simulation was conducted. Five common beam settings (190MeV to 230MeV) were considered, 3D neutron dose equivalent profile base on ICRP 74 (Ambient dose equivalent dose conversion factor) and energy spectrums were acquired. Detailed data was compared among beam settings and dose & energy distribution features were analyzed. Additionally, photon contribution was checked and it was very low (Max. 3.6% at target, Max. 1.9% except for target). To support validity of simulation data, He-3 embedded neutron detector measurement was conducted in six points around the nozzle and measurement was maximum 13% lower than simulation. Finally, simulation data was compared to former researches. Neutron doses on the axis from the edge of the proton beam to 150cm was extracted with a unit of milisievert per unit proton absorbed dose. Doses varied from 3.361 mSv/Gy to 3.971 mSv/Gy at the edge of the proton beam field depending on the beam setting. And these results were comparable to other researches. In latter part, various options were tried to reduce neutron doses. Through the energy distribution analysis, it was found that high energy neutrons near 500keV, 2MeV and 50MeV were dominantly contribute to total dose and main sources were first scatterer, range modulator, second scatterer, snout and aperture. On the basis of above knowledge, three dose reduction options were applied. In nozzle shielding cases, 5 cm uniform polyethylene option was the practical (average 58% reduction). In Patient shielding cases, 5cm polyethylene shielding was practical (average 40% reduction). And in snot material substitution case, aluminum alloy substitution showed average 32% reduction. Additionally, combination of above cases was considered. The average dose reduction rate was 69%. The most conservative additional neutron dose was 3.971 mSv per proton absorbed dose at the edge of the proton beam field. This means if someone was administrated 72Gy proton therapy plan, the maximum possible dose at just beside of target organ is 285.9 mSv in total. And this dose is able to be reduced to 88.6 mSv with proper reduction plan (combination option: 69%). Major achievements in this study are neutron dose/flux profile in the treatment room and shielding plan to protect patient. Neutron dose/flux profile would be a reference data to understand neutron field at proton therapy and shielding plan would be applicable to facility directly. Additionally, mid step output such as MCNP modeling would give advantages for future researches. Secondary neutron study is required certainly for radiation safety. And, as facilities are increasing, technologies are improving and patient safety is strengthen, demands will grow. Achievements in this study are expected to play a role of providing useful technical aspect and reference data for these demands.