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.