Comparison of dosimetry recommendations for clinical proton beams

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The formalism and data in the two most recent dosimetry recommendations forclinical proton beams, ICRU Report 59 and the forthcoming IAEA Code ofPractice, are compared. Chamber calibrations in terms of air kerma andabsorbed dose to water are considered, including five different cylindricalionization chamber types commonly used in proton beam dosimetry. Themethodology for both types of calibration for ionization chambers is describedin ICRU Report 59. The procedure based on air kerma calibrations is comparedwith an alternative formalism based on IAEA Codes of Practice (TRS-277,TRS-381), modified for proton beams. The new IAEA Code of Practice isexclusively based on calibrations in terms of absorbed dose to water and adirect comparison with ICRU Report 59 recommendations is made.Common to the two formalisms are the fundamental quantities Wair andwair and their atmospheric conditions of applicability. Thedifference in the recommended values of the ratio wair/Wair(protons to 60Co) is as large as 2.3%. The use of Wair andwair values for dry air (IAEA) and for ambient air (ICRU) is acontribution to the discrepancy, and the ICRU usage is questioned.For air kerma based chamber calibrations, ICRU Report 59 does not take intoaccount the effect of different compositions of the build-up cap and chamberwall on the calibration beam quality. For the chamber types included in thestudy, this introduces discrepancies of up to 1.1%. Combined with differencesin the recommended basic data, discrepancies in absorbed dose determination inproton beams of up to 2.1% are found. For the absorbed dose to water basedformalism, differences in the formalism, notably the omission of perturbationfactors for 60Co in ICRU 59, and data yield discrepancies in calculatedkQ factors, and in absorbed dose determinations, between -1.5% and+2.6%, depending on the chamber type and the proton beam quality.