A comparison between calculated and experimental k(Q) photon beam quality correction factors

To validate the calculated values of k(Q) for high-energy photon beams give n in the International Code of Practice for radiotherapy dosimetry based on water-absorbed-dose standards, a comparison with experimental values deriv ed in standards laboratories and in clinical beams has been made. The study includes a compilation of experimental values for ionization chambers of t he type NE2561/2611, NE2571. PTW30001 and PR06. The energy dependence of th e 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 ty pe a difference which increases with energy between calculated and experime ntal 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 fo r 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 limit s of the majority of experimental data available. The good agreement betwee n calculated and measured values, obtained for practically all the experime ntal 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. The re appears to be no justification for a change to a different photon beam q uality specifier solely on the grounds that such a limited set of data is n ot described by the same distributions as the rest of the experimental data .