Phenol compounds for EPR dosimetry in radiation therapy

Among the various dosimetric techniques used for characterizing the radiation beams used in radiation therapy, the electron paramagnetic resonance (EPR) arouses increasing interest for applications in various therapy procedures [1]. Free radicals are known to be produced when a compound is irradiated with ionizing radiations. The concentration of radiation-induced free radicals is proportional to the absorbed dose and this allows for dosimetric measurements through EPR technique which able to quantitatively determine the radical concentration [2]. Our research group has started an investigation of the EPR response of some phenols compounds for possible EPR dosimetric applications suitable features, such as high efficiency of radiation-matter energy transfer and radical stability at room temperature [3-5]. In this work we report the EPR investigation of IRGANOX 1076 pellets and thin films exposed to various type of radiation beams (clinical photon and electron beams, neutron-photon mixed field). Phenols are compounds possessing a benzene ring attached to a OH group. After irradiation the final product is a stable phenoxy radical. The stability of such radical can be improved by adding other alkyl chains which can be attached to the benzene ring. In particular, the phenol octadecyl-3-(3,5-ditert. butyl-4-hydroxyphenyl)-propionate (IRGANOX 1076) gave interesting results. Moreover, its high molecular weight, the low volatility and the compatibility with the dosimeter binding material (wax) are advantages with respect to lower molecular weight phenols. Photon and electron irradiations at various energies were performed with clinical LINAC. Thermal neutron irradiations were performed at the thermal neutron column at the Triga Mark II reactor of Pavia (Italy). EPR dosimeters were readout by means of a Bruker ECS106 spectrometer equipped with a TE102 rectangular cavity at room temperature. The dosimetric features of these EPR dosimeters were investigated and the results regarding the dependence on microwave power and modulation amplitude are reported. The dependence on beam type and energy, the detection limits for various beam typologies, signal stability after irradiation were analyzed. The dose response was found to be linear for all beams used in the dose range analyzed. The possibility of obtaining depth dose profile was investigated. In conclusion, the phenols show radiometric features that designate it as a new material for EPR.