Monte Carlo simulation of shielding designs for a cabinet form factor preclinical MV‐energy photon FLASH radiotherapy system

Abstract

Purpose A preclinical MV‐energy photon FLASH radiotherapy system is being designed at Stanford and SLAC National Accelerator Laboratory. Because of the higher energy and dose rate compared to conventional kV‐energy photon laboratory‐scale irradiators, adequate shielding in a stand‐alone cabinet form factor is more challenging to achieve. We present a Monte Carlo simulation of multilayered shielding for a compact self‐shielding system without the need for a radiation therapy vault. Methods A multilayered shielding approach using multiple alternating layers of high‐Z and low‐Z materials is applied to the self‐shielded cabinet to effectively mitigate the secondary radiation produced and to allow the device to be housed in a Controlled Radiation Area outside of a radiation vault. The multilayered shielding approach takes advantage of the properties of high‐Z and low‐Z radiation shielding materials such as density, cross‐section, atomic number of the shielding elements, and products of radiation interactions within each layer. The Monte Carlo radiation transport code, FLUKA, is used to simulate the total effective dose produced by the operation. Results The multilayered shielding designs proposed and simulated produced effective dose rates significantly lower than monolayer designs with the same total material thickness at the regulatory boundary; this is accomplished through the manipulation of the locations where secondary radiation is produced and reactions due to material properties such as neutron back reflection in hydrogen. Borated polyethylene at 5 wt% significantly increased the shielding performance as compared to regular polyethylene, with the magnitude of the reduction depending upon the order of the shielding material. Conclusions The multilayered shielding provides a path for shielding preclinical FLASH systems that deliver MV‐energy bremsstrahlung photons. This approach promises to be more efficient with respect to the shielding material mass and space claim as compared to shielded vaults typically required for clinical radiation therapy with MV photons.

Shielding Analysis of a Preclinical Bremsstrahlung X-ray FLASH Radiotherapy System within a Clinical Radiation Therapy Vault

“…In all cases, the multilayered design resulted in lower effective dose rates by ~20% when compared to the single-layer design while occupying the same volume. This agrees with previous results reported by Rosenstrom et al (2023) in which the multilayered shielding design proved to be more efficient.…”

“…The effect of the multilayered shielding methodology (Rosenstrom et al 2023) was confirmed in simulations where it was applied to the treatment head shielding with a 19% reduction in the effective dose rate in the Control Room, as well as a reduction in the effective dose rate at all points of interest. Additionally, mass and volume reductions can be performed with the multilayered design resulting in small and lighter treatment head shielding.…”

“…The single layer design, approximately 350 kg, stems from information documented in NCRP report 79 (NCRP 1984). The multilayered treatment head shielding design, approximately 390 kg, uses the same shielding methodology as reported by Rosenstrom et al (2023).…”

“…This work describes a novel application of the multilayered shielding method (Rosenstrom et al 2023; Muhammad et al 2019; Hadad et al 2016; Cai et al 2018) in the local treatment head shielding enabling the FLASH-EXACT device to be efficiently shielded such that it may be safely housed in a clinical vault in the Stanford Cancer Center, originally designed for a 6 MV CyberKnife machine (referred to as CK-1). We examine two shielding solutions, one using methods reportedly used in some commercial machines (NCRP 1984) and a new application of the multilayered shielding methodology that uses alternating layers of high-Z and low-Z materials in the treatment head shielding to mitigate radiation more efficiently (Rosenstrom et al 2023).…”

FLASH Radiotherapy and the Use of Radiation Dosimeters

“…The UHDR of FLASH therapy presents new challenges, such as the need for a new shielding system. MC simulation can provide a solution for simulating such a shielding system, as explored in a study [27]. Another investigation focused on ionizing radiation acoustic imaging through simulation and its potential as a dosimetric tool for FLASH RT.…”