An analytic model of neutron ambient dose equivalent and equivalent dose for proton radiotherapy

Abstract

Stray neutrons generated in passively scattered proton therapy are of concern because they increase the risk that a patient will develop a second cancer. Several investigations characterized stray neutrons in proton therapy using experimental measurements and Monte Carlo simulations, but capabilities of analytical methods to predict neutron exposures are less well developed. The goal of this study was to develop a new analytical model to calculate neutron ambient dose equivalent in air and equivalent dose in phantom based on Monte Carlo modeling of a passively scattered proton therapy unit. The accuracy of the new analytical model is superior to a previous analytical model and comparable to the accuracy of typical Monte Carlo simulations and measurements. Predictions from the new analytical model agreed reasonably well with corresponding values predicted by a Monte Carlo code using an anthropomorphic phantom.

Figure 1

Schematic illustration of the PSPT treatment nozzle and the water phantom. The nozzle includes a vacuum window (A), a beam profile monitor (B), a range modulator wheel (C), a second scatter (D), shielding plates (E), a range shifter assembly (F), backup and primary monitors (G), the snout (H) and the final aperture (I). Neutron dose was calculated in 2 cm diameter spherical receptors (open circles) in both axial (z) and lateral (x) directions. In the figure, d indicates the distance from the effective neutron source to the neutron receptor, d′ indicates the distance from the phantom surface to the neutron receptor along the path between the effective neutron source and the receptor, d_iso indicates the distance from the effective neutron source to the isocenter. The figure is not drawn to scale.

Figure 2

Schematic illustration of the PSPT treatment unit and the computational anatomical male phantom modeled using Monte Carlo simulations. The locations of the neutron dose receptors are shown as yellow circles on the phantom. The dash line box represents the imaginary box used to enclose the phantom for calculation convenience. The figure is not drawn to scale. (Rendering of anthropomorphic phantom was provided courtesy of Tom Jordan.)

Figure 3

Results from Monte Carlo simulations and analytical models predictions of ambient neutron dose equivalent per therapeutic dose (H/D) free in air as a function of vertical distance (a) and (b) and lateral distance (c). These values were for a specific condition (an unmodulated 250 MeV proton beam incident upon a medium-sized closed aperture). Analytical model (new) is from this study, analytical model (old) is from Zheng et al (2007).

Figure 4

Results from Monte Carlo simulations and analytical models predictions of neutron equivalent dose per therapeutic dose (H/D) in the water phantom as a function of vertical distance (a) and (b) and lateral distance (c). These values were for a specific condition (an unmodulated 250 MeV proton beam incident upon a medium-sized closed aperture). Analytical model (new) is from this study, analytical model (old) is from Zheng et al (2007).