Radiation damage evaluation of materials for radiotherapy quality assurance devices under high dose and ultra-high dose rate electron and proton beams.

Abstract

Objective.FLASH radiotherapy is a promising technique based on the delivery of ultra-high dose rates (UHDR) to spare healthy tissue. Robust quality assurance (QA) is required to ensure a safe delivery of the treatment. QA devices such as phantoms and ionization chambers (ICs) are typically made out of polymeric materials to ensure water equivalence. However, radiation exposure, particularly at UHDR where achieving high doses is easier, may cause irreversible changes to the structure of these materials. Such extreme conditions can, for example, induce microstructural defects that may lead to mechanical failure or alter electrical properties such as the dielectric permittivity or conductivity, potentially compromising the calibration and reliability of critical QA devices, such as ICs. This study aims to characterize radiation damage in common materials used for QA devices and identify which of them can withstand these challenging conditions.Approach.A variety of materials commonly used in phantoms and ICs, with diverse characteristics (transparent, opaque, conductive and non-conductive) were irradiated using 68 MeV proton and 20 MeV electron beams, reaching doses up to 1 MGy under UHDR conditions (average dose rates of 100-500 Gy s^-1). Material damage was evaluated through optical, chemical, mechanical and electrical tests to quantify change in color, structural integrity, and relevant electrical properties such as conductivity and dielectric constant that can affect detector response.Main Results.External appearance of some materials significantly changed after irradiation. All transparent materials exhibited change in color post-irradiation. Chemical analyses conducted after irradiation and repeated two years later indicated partial recovery in some materials. No significant difference in damage was observed between proton and electron irradiation, suggesting comparable radiation damage mechanisms. Dose rate alone did not exacerbate damage beyond total dose effects; however, extended irradiation at high dose rates resulted in thermal damage under some conditions. Mechanical testing revealed increased fragility, changes in hardness and dimensions, while some materials experienced significant changes in dielectric constant and conductivity.Significance.Materials such as Polymethyl Methacrylate (PMMA), PC, Polyoxymethylene (POM), and its conductive variant POM ELS, are unsuitable for prolonged irradiations at UHDR due to significant structural degradation observed. For see-through phantom construction, CPS offers a more durable alternative to PMMA. For detector construction, PEEK (for non-conductive parts) and graphite (for conductive parts) demonstrated promising durability, making them preferable choices under high dose and UHDR conditions. The higher stability of these materials can be attributed to the presence of aromatic rings in their chemical structure, which enhances radiation resistance.