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

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusion: 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.

Figure 1

Simulated water phantom with 100 spherical detecting volumes placed along the CAX and at 10 cm OAX, 40 cm OAX, and 80 cm OAX at the same depths. d represents the distance from the neutron effective source to the detecting volumes. d^′ is the distance from the surface of the phantom to the detecting volumes. d_iso is the distance from the effective neutron source to isocenter. z is the depth in the phantom. Figure modified from Zhang et al. (Ref. 25).

Figure 2

Normalized neutron energy spectra produced by 120-, 160-, 200-, and 250-MeV proton beams at a depth of 10 cm at CAX.

Figure 3

Monte Carlo calculation and analytical model prediction of neutron equivalent dose per therapeutic dose (H/D) values along the CAX at 100-, 160-, 200-, and 250-MeV proton energies.

Figure 4

Monte Carlo calculation and analytical model prediction of neutron equivalent dose per therapeutic dose (H/D) values at 10 cm OAX at 100-, 160-, 200-, and 250-MeV proton energies.

Figure 5

Monte Carlo calculation and analytical model prediction for neutron equivalent dose per therapeutic dose (H/D) values at 40 cm OAX at 100-, 160-, 200-, and 250-MeV proton energies.

Figure 6

Monte Carlo calculation and analytical model prediction for neutron equivalent dose per therapeutic dose (H/D) values at 80 cm OAX at 100-, 160-, 200-, and 250-MeV proton energies.

Figure 7

Monte Carlo calculation and analytical model prediction of the neutron equivalent dose per therapeutic dose (H/D) values at various OAX positions at 100-, 160-, 200-, and 250-MeV proton energies at a depth of 22 cm in the phantom.

Figure 8

$\overline{w_R}$ values for each detecting volume position along CAX and lateral to CAX for 100-, 160-, 200-, and 250-MeV proton energies. $\overline{w_R}$ was calculated using the formalism from ICRP Publication 92 (Ref. 32) and the neutron spectral fluence calculated in our work.

Figure 9

Fit for $\overline{w_R}$. The fit is presented as a function of depth at positions along CAX and at various OAX positions. Each type of symbol in the plot represents an energy. The plot includes all the energies at CAX and 10 40, and 80 cm OAX.