Patient neutron dose equivalent exposures outside of the proton therapy treatment field

A large fraction of dose to healthy tissue located outside of the treatment field during proton therapy is attributable to neutrons produced in the beam-delivery apparatus. In this work, the neutron dose equivalent (H) per therapeutic proton absorbed dose (D) was estimated for typical treatment conditions as a function of range modulation width, angle with respect to the incident proton beam, and the distance from the isocentre at the Harvard Cyclotron Laboratory's (Cambridge, MA) passively spread treatment field using Monte Carlo simulations. For a beam with 16 cm penetration (depth) and a 5 x 5 cm2 lateral field size at the patient location along the incident beam direction at 100 cm from the isocentre, the predicted H/D values are 0.35 and 0.60 mSv Gy(-1) from the simulations and measurements, respectively. At all locations, the predicted H/D values are within a factor of 2 and 3 of the measured result for no modulation and 8.2 cm of modulation, respectively.