Monte-Carlo calculation of beam quality correction factors and comparison with experimental measurements

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine absorbed dose at a user’s beam contains the well known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user’s radiation. Calculated beam quality correction factors can be obtained from dosimetry protocols - directly from tables or using provided formulas and data - for a large variety of ionization chambers and qualities with a relative standard uncertainty of about 1%. With the increasing power of the Monte Carlo method applied to ionization chambers it might be possible to arrive at calculated correction factors with a lower uncertainty. A combined study of calculation and experimental determination of corrections factors was carried out to investigate the reliability and accuracy of the Monte Carlo method. The experimentally obtained dependence of the correction factors on beam quality and chamber type is well modelled by the calculations. However, the Monte Carlo methods still requires an improvement of accuracy.