Response of the alanine/ESR dosimetry system to MV X-rays relative to (60)Co radiation

The measurand relevant in dosimetry for radiation therapy is the absorbed dose to water, D(W). The Physikalisch-Technische Bundesanstalt (PTB) has established a secondary standard for D(W) for high-energy photon and electron radiation based on electron spin resonance (ESR) of the amino acid alanine. Since the calibration is usually performed using (60)Co radiation while a huge part of the external radiation therapy is done with high-energy x-rays from linear accelerators, determination of the response is an important issue. The results presented in this paper are the most accurate ones available today with uncertainties assigned to the relative response for 8 MV and 16 MV of the order of 0.3%. The experimental results are compared to Monte Carlo simulations using the EGSnrc software package. In the appendix, it is demonstrated how mean values from repetitive irradiations and their uncertainties are obtained in a consistent way using Bayesian statistics, even in the presence of at first sight inconsistent data. It is important to note that the formulae derived to obtain the final results follow from first principles, without recurring to ad hoc solutions or simple recipes and are valid for all kinds of repetitive measurements.