Study of secondary neutrons from uniform scanning proton beams by means of experiment and simulation

Proton radiotherapy is becoming popular as an effective modality to treat cancer. However the advantages of the proton radiotherapy could be offset due to the effect of secondary neutrons. Secondary neutrons are an undesired byproduct in proton radiotherapy. It is important to quantify the dose equivalent due to secondary neutrons since they could lead to secondary cancer later in the patient's life. In this study, our aim was to investigate the off-axis dose equivalent due to secondary neutrons from a uniform scanning proton radiotherapy system at the ProCure Proton Therapy Center, Oklahoma City, OK. Both experiments and simulations were carried out for the purpose of this study. CR-39 plastic nuclear track detectors were used to measure dose equivalent inside a phantom and in air at various depths and angles with respect to the primary beam axis for four different experimental configurations. Three different proton beam energies, 78 MeV, 162 MeV and 226 MeV, all using a 4 cm modulation width, a 5 cm diameter brass aperture, and a small snout located 38 cm from isocenter were used for the experiments. The Monte Carlo radiation transport code FLUKA was used to simulate the experiments for a simplified snout configuration. The measured ratio of secondary neutron dose equivalent to therapeutic primary proton dose (Hn/Dp) ranged from 0.3 mSv/Gy to 50 mSv/Gy. Both experiment and simulation showed a similar decreasing trend in dose equivalent with distance from beam isocenter and the magnitude varied by a factor of about 4 in most of the locations. An overall higher Hn/Dp in air than inside the phantom was observed and this suggests that the production of secondary neutrons in the beam delivery device is significantly higher than inside the body. Comparison of Hn/Dp with other studies suggests that the neutron exposure to patients from uniform scanning systems is similar to that of passive scattering systems