Electron paramagnetic resonance dosimetry with alanine pellets for radiation therapy applications: Calibration methods

Abstract

Introduction Dosimetry using Electron Paramagnetic Resonance (EPR) of alanine pellets is commonly used in the industry to evaluate higher doses of radiation. The purpose of this study is to perform a calibration for a dose range used in radiotherapy, comparing various calibration methods. Material and methods Alanine pellets were irradiated with 6 MV X Rays using an accelerator to a dose range from 0 to 10 Gy, with 7 dose points and 8 dosimeters for each dose point. Dosimeters were readout using an EPR spectrometer giving the derivative of the free radical spectrum for each irradiated dosimeter. Several calibration methods, i.e. dosimeter response assessment, have been conducted to find a robust and reliable method to quantify low doses: – Study of the peak to peak amplitude of the spectrum main peak. – Study of the slope and intercept of the central part of the spectrum. – Study of the dosimeter spectrum as compared to a known unnoisy reference spectrum. Method developed by the National Physical Laboratory (United-Kingdom). – Double integration of the signal which response is proportional to the free radical number induced by irradiation. Results All tested methods present a calibration curve with a correlation coefficient R 2 > 0:99 and an RMS% (standard deviation of all residuals) lower than 3%. However, repeated verifications of the calibration do not give repeatable results for lower doses: significant differences between the theoretical and measured doses greater than the RMS were measured were observed for doses below 2 Gy, whatever the calibration method. Several investigation and optimization routes have been launched in order to overcome this issue. Conclusion This study enabled to obtain several calibration methods with a good correlation coefficient for the use of alanine pellets for dosimetry in radiotherapy. However, further work must be realized to obtain reliable dosimetry method, allowing to determine low doses with good repeatability. This work will have to overcome dosimeter response instabilities which can be due to measurement conditions, pellets imperfections, instabilities of the spectrometer. Several approaches such as the implementation of a dosimeter standard or accounting for the baseline signal of the dosimeters are being evaluated.