A comparison between calculated and experimental kQ photon beam quality correction factors

To validate the calculated values of k(Q) for high-energy photon beams given in the International Code of Practice for radiotherapy dosimetry based on water-absorbed-dose standards, a comparison with experimental values derived in standards laboratories and in clinical beams has been made. The study includes a compilation of experimental values for ionization chambers of the type NE2561/2611, NE2571. PTW30001 and PR06. The energy dependence of the G(Fe3+) ratio of high-energy x-rays to Co-60 gamma-rays by Klassen et al is taken into account for all the Fricke- derived values. For three of the chamber types analysed, the comparison shows that the calculated values are a very good estimate of the average values of k(Q) in the entire range of photon beam qualities available for clinical use. For the NE2571 chamber type a difference which increases with energy between calculated and experimental k(Q) factors has been observed; however, the largest difference with a fit describing the entire set of experimental data is always smaller than 0.4%. It is concluded that if the recommendation of the Code of Practice for an individual calibration of the user's chamber at a range of photon beam qualities is not available, the use of calculated k(Q) factors will yield absorbed dose to water determinations accurate within the uncertainty limits of the majority of experimental data available. The good agreement between calculated and measured values, obtained for practically all the experimental data using TPR20,10 as photon beam quality specifier, is not satisfied in some cases for two high-energy soft beams used at the Canadian NRC. There appears to be no justification for a change to a different photon beam quality specifier solely on the grounds that such a limited set of data is not described by the same distributions as the rest of the experimental data.