Feasibility studies of a passive scatter proton therapy nozzle without a range modulator wheel

Abstract

The purpose of this work was to determine the feasibility of producing a spread out Bragg peak (SOBP) without a range modulation wheel (RMW) using the passive scattering beam delivery technique. For this study, a comprehensive Monte Carlo model of a passive scattering treatment nozzle was used. The RMW was removed from the model leaving only the initial fixed scatterer (RMW-free configuration). Range modulation was achieved by directly changing the energy of the proton beam entering the nozzle. To produce a uniform SOBP, the number of protons injected into the nozzle at each beam energy was "dose weighted." To do so, the effective number of protons was calculated for the individual initial energies using an analytical dose-weighting function, and the resulting weighted Bragg curves were summed together to produce an SOBP of the desired width. We found that SOBPs calculated using the RMW-free nozzle configuration were in very good agreement to those calculated with the standard nozzle configuration containing the RMW for the 250, 180, and 100 MeV maximum beam energies. The depth of the distal 90% dose and the 80%-20% distal dose falloff of SOBPs calculated with the two different nozzle configurations agreed to within a millimeter for the three beam energy options considered in this study. In addition, the 80%-20% lateral penumbra for the cross-field dose profiles calculated with the RMW-free delivery method agreed with results calculated using the standard RMW technique to less than one millimeter. For an equal number of protons injected into the nozzle, an increase of up to 10% in the delivered dose and a significant reduction in both the in-air secondary neutron fluence and dose equivalent (H/D) were observed at the isocenter by removing the RMW from the treatment nozzle and modulating the initial proton beam energy. However, increases in delivery time of up to 70% were also estimated with this method. Our results suggest that it is feasible to deliver a passively scattered dose distribution with an RMW-free nozzle configuration with clinical characteristics comparable to those using standard methods.

Prompt gamma-ray emission from biological tissues during proton irradiation: a preliminary study

“…A simple Monte Carlo model was developed for this study using the MCNPX Monte Carlo code (version 2.5.0 (Waters et al 2005)). MCNPX has been extensively validated (Fontenot et al 2005, Herault et al 2005, Polf et al 2007b and used in proton therapy for a wide variety of clinical (Fontenot et al 2007, Herault et al 2007, Koch and Newhauser 2005 and research (Harvey et al 2008, Polf et al 2007a, Taddei et al 2008 applications. Our model consisted of a parallel proton beam incident on a phantom as shown in figure 1.…”

Treatment planning aspects and Monte Carlo methods in proton therapy

“…There are several advantages of this approach such as reduced scattering of protons in the treatment head, which in turn decreases the lateral penumbra and distal fall-off, and a reduction in neutron production in the treatment head. 39 However, this approach is not applied in currently available scattering systems due to typically long energy switching times and thus potential interplay effects of dose delivery and organ motion. 39 Alternatively, the range modulation is achieved by using a range modulation wheel, a ridge filter or a binary number of plastic plates.…”

“…39 However, this approach is not applied in currently available scattering systems due to typically long energy switching times and thus potential interplay effects of dose delivery and organ motion. 39 Alternatively, the range modulation is achieved by using a range modulation wheel, a ridge filter or a binary number of plastic plates. The range modulation wheel consists of angle segments of varying thicknesses using different materials ranging from plastic to aluminum.…”

Impact of air gap, range shifter, and delivery technique on skin dose in proton therapy

“…They provide the least amount of scatter when shifting the range 13 . By using an RS, protons are scattered due to the interaction with the range shifting material and increase the lateral penumbra, defined as the 80–20% dose fall‐off 14 . In any delivery technique the angular variance transforms to a spatial broadening when traversing the air gap between the most downstream beam modifier and the patient.…”