Monte Carlo and analytical model predictions of leakage neutron exposures from passively scattered proton therapy

Stray neutron radiation is of concern after radiation therapy, especially in children, because of the high risk it might carry for secondary cancers. Several previous studies predicted the stray neutron exposure from proton therapy, mostly using Monte Carlo simulations. Promising attempts to develop analytical models have also been reported, but these were limited to only a few proton beam energies. The purpose of this study was to develop an analytical model to predict leakage neutron equivalent dose from passively scattered proton beams in the 100-250-MeV interval.

To develop and validate the analytical model, the authors used values of equivalent dose per therapeutic absorbed dose (H∕D) predicted with Monte Carlo simulations. The authors also characterized the behavior of the mean neutron radiation-weighting factor, wR, as a function of depth in a water phantom and distance from the beam central axis.

The simulated and analytical predictions agreed well. On average, the percentage difference between the analytical model and the Monte Carlo simulations was 10% for the energies and positions studied. The authors found that wR was highest at the shallowest depth and decreased with depth until around 10 cm, where it started to increase slowly with depth. This was consistent among all energies.

Simple analytical methods are promising alternatives to complex and slow Monte Carlo simulations to predict H∕D values. The authors' results also provide improved understanding of the behavior of wR which strongly depends on depth, but is nearly independent of lateral distance from the beam central axis.