SU-E-T-740: TG-51A: Part 2: Factors Affecting Alanine Response and Overall Uncertainty of Dose Measurements with Alanine under Conditions Suitable for Calibrations of Photon Beams of Medical Linacs

Abstract

Purpose: Alanine dosimetry now appears suitable not only for measuring actual doses in QA, but also for calibrations of clinical linacs. Like with ionization chambers, alanine calibrations are traceable to calorimetry national standards, but photon energy dependence of alanine response is much weaker, which makes uncertainty related to differences in beam quality smaller. To compare performance of ionometry and alanine dosimetry objectively, it is necessary to carefully analyze factors affecting the alanine response and the overall uncertainty of dose measurements with alanine under conditions appropriate for linac calibrations. Method and Materials: Several Bruker EPR spectrometers equipped with standard reference samples were used to measure responses of alanine under various experimental conditions. Their calibrations were directly traceable to NIST. Alanine dosimeters from GammaService (Germany) and Harwell (UK) were used. Co‐60, Varian Trilogy and Elekta Synergy were used for irradiations. Results: An analysis of all known factors that could possibly affect accuracy of alanine dosimetry will be presented using previously published and newly obtained data. Modality and energy of radiation, temperature and humidity at irradiation and post‐irradiation storage, time between the irradiation and signal measurement, exposure to bright sunlight and some others will be characterized quantitatively in the ranges of variation typical for linac calibration conditions. Based on this analysis, a calculated overall uncertainty of calibration dose measurements with alanine will be presented. Conclusion: Alanine dosimetry now provides accuracy of linac calibration similar to accuracy typical for measurements with ionization chambers from secondary standard laboratories. The very weak energy dependence of the alanine response has been reliably characterized at several national laboratories, which makes this way of calibration free of the theoretical assumptions and uncertainties associated with kQ in ionometry. It appears that alanine dosimetry, based on entirely different physical phenomenon than ionometry, can successfully compliment ionometry in calibrations of clinical linacs.