Comparison of dosimetry recommendations for clinical proton beams

The formalism and data in the two most recent dosimetry recommendations for clinical proton beams, ICRU Report 59 and the forthcoming IAEA Code of Pra ctice, are compared. Chamber calibrations in terms of air kerma and absorbe d dose to water are considered, including five different cylindrical ioniza tion chamber types commonly used in proton beam dosimetry. The methodology for both types of calibration for ionization chambers is described in ICRU Report 59. The procedure based on air kerma calibrations is compared with a n alternative formalism based on IAEA Codes of Practice (TRS-277, TRS-381), modified for proton beams. The new IAEA Code of Practice is exclusively ba sed on calibrations in terms of absorbed dose to water and a direct compari son with ICRU Report 59 recommendations is made. Common to the two formalisms are the fundamental quantities W-air and w(air ) and their atmospheric conditions of applicability. The difference in the recommended values of the ratio w(air)/W-air (protons to Co-60) is as large as 2.3%. The use of W-air and w(air) values for dry air (IAEA) and for amb ient air (ICRU) is a contribution to the discrepancy, and the ICRU usage is questioned. For air kerma based chamber calibrations, ICRU Report 59 does not take into account the effect of different compositions of the build-up cap and chamb er wall on the calibration beam quality. For the chamber types included in the study, this introduces discrepancies of up to 1.1%. Combined with diffe rences in the recommended basic data, discrepancies in absorbed dose determ ination in proton beams of up to 2.1% are found. For the absorbed dose to w ater based formalism, differences in the formalism, notably the omission of perturbation factors for Co-60 in ICRU 59, and data yield discrepancies in calculated k(Q) factors, and in absorbed dose determinations, between -1.5 % and +2.6%, depending on the chamber type and the proton beam quality.