Comparison of secondary radiation dose between pencil beam scanning and scattered delivery for proton and VHEE radiotherapy

Abstract

Background

Very high-energy electrons (VHEEs) in radiotherapy may offer several potential advantages over conventional electron beams and other techniques, for example, the fact that they can be used at ultra-high dose rates (UHDRs), therefore enabling FLASH radiotherapy. However, the production of secondary particles at high energies (50–200 MeV) has yet to be studied in detail for this technique currently under development.

Purpose

The aim of this work was to examine the secondary dose produced by VHEEs, with particular emphasis on bremsstrahlung photons and neutrons, for two beam delivery systems (double scattering [DS] and pencil beam scanning [PBS]).

Methods

The electron, X-ray, and neutron doses arising from two beam delivery systems (DS or PBS) were computed using Monte Carlo (MC) simulations in the TOPAS (TOol for PArticle Simulation)/Geant4 toolkit, and a preliminary assessment of the secondary dose for a clinical VHEE treatment was performed using a whole-body phantom. An evaluation of the secondary dose produced by this preliminary design of a VHEE nozzle set in a clinical proton facility was performed, taking into account realistic PBS or DS nozzle configurations.

Results

The mean doses received by a patient undergoing DS-VHEE irradiation were found to be up to 5.3-fold and 6.8-fold higher for in-field or out-of-field organs for photons and neutrons, respectively, compared to the PBS-VHEE plan. The results for the secondary neutron dose in intracranial treatments also demonstrate the characteristic of VHEE compared to proton beams for reducing the out-of-field secondary neutron dose. The dose to the public area that could be delivered to meet regulatory limits surrounding a possible treatment room in a proton therapy facility was assessed. A regulatory limit of 0.5 µSv/h would give a restriction of 49 and 83 Gy per patient and per fraction for DS and PBS, respectively.

Conclusions

This work describes a method to simulate and compare secondary radiation doses resulting from scattered, scanned VHEE or proton therapy treatments. The results indicate that a conventionally shielded proton therapy room results in acceptable public doses for a preliminary VHEE design and could be of interest for radiation protection purposes and for similar setups. Other facilities with differing layouts may, however, lead to different conclusions, requiring further studies.