A comparison between calculated and experimental kQ photon beamquality correction factors

Abstract

To validate the calculated values of kQ for high-energy photonbeams given in the International Code of Practice for radiotherapy dosimetrybased on water-absorbed-dose standards, a comparison with experimental valuesderived in standards laboratories and in clinical beams has been made. Thestudy includes a compilation of experimental values for ionization chambers ofthe type NE2561/2611, NE2571, PTW30001 and PR06. The energy dependence of theG(Fe3+) ratio of high-energy x-rays to 60Co gamma-rays by Klassenet al is taken into account for all the Fricke-derived values. For three ofthe chamber types analysed, the comparison shows that the calculated valuesare a very good estimate of the average values of kQ in the entire range ofphoton beam qualities available for clinical use. For the NE2571 chamber typea difference which increases with energy between calculated and experimentalkQ factors has been observed; however, the largest difference with a fitdescribing 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 anindividual calibration of the user's chamber at a range of photon beamqualities is not available, the use of calculated kQ factors will yieldabsorbed dose to water determinations accurate within the uncertainty limitsof the majority of experimental data available. The good agreement betweencalculated and measured values, obtained for practically all the experimentaldata using TPR20,10 as photon beam quality specifier, is not satisfied insome cases for two high-energy soft beams used at the Canadian NRC. Thereappears to be no justification for a change to a different photon beam qualityspecifier solely on the grounds that such a limited set of data is notdescribed by the same distributions as the rest of the experimental data.