Monte Carlo simulations of stray neutron radiation exposures in proton therapy

Proton radiotherapy patients receive unwanted stray neutron radiation generated in the treatment apparatus. The literature on this neutron radiation is sparse and disparate and pertains mainly to unique or antiquated systems. The present work reports on neutron dose equivalent per therapeutic absorbed dose (H/D) in a contemporary proton therapy system. We applied the Monte Carlo method to predict neutron exposure as a function of treatment field size and location in the treatment room. For a 250-MeV proton beam, H/D values at isocenter increased from 13 to 20 mSv Gy−1 as the collimating aperture was decreased from 18 × 18 to 0 × 0 cm2. H/D values generally decreased with distance from isocenter (20 mSv Gy−1 at isocenter versus 0.9 mSv Gy−1 at a distance of 150 cm). Neutron spectral fluence calculations revealed that >50% of the dose equivalent at all locations considered was from neutrons >10 MeV.