A comparison between calculated and experimentalk_Qphoton beam quality correction factors

Abstract

To validate the calculated values of kQ 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 60Co 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 kQ 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 kQ 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 kQ 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 TPR(20,10) 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.

NCS Report 18: Code of practice for the absorbed dose determination in high energy photon and electron beams

“…In this report a Code of Practice is presented for the dosimetry in high energy photon and electron beams based on absorbed dose to water standards for 60 Co reference beams. The Important features are the simple basic concepts to determine absorbed dose to water under reference conditions in a water phantom and the introduction of a single, chamber dependent correction factor taking into account all effects dependent of the radiation beam quality.…”

“…k Q Beam quality correction factor which corrects for the difference in the absorbed dose to water calibration coefficient of a 60 Co beam to that of a radiation beam of quality Q. The subscript Q 0 is omitted when the reference beam quality is 60 Co.…”

“…The absorbed dose to water calibration coefficient for an ionisation chamber (or for the dosimeter assembly: ionisation chamber and electrometer) at the reference beam quality 60 Co.…”

“…A set of nine ionisation chambers was calibrated both in terms of air kerma and in terms of absorbed dose to water in 60 Co.…”

“…This concept was based on one absorbed dose to water calibration coefficient in a 60 Co beam and a beam quality correction factor 0 Q Q k , , accounting for the high energy photon beam quality Q and has further been developed by Andreo [19] and Rogers [20]. The beam quality correction factor is defined as the ratio of absorbed dose to water calibration coefficients in the clinical beam quality Q, N D,w,Q and the reference (calibration) beam quality…”