Absorbed-dose beam quality conversion factors for cylindrical chambers in high energy photon beams

Recent working groups of the AAPM [Almond et al., Med. Phys. 26, 1847 (1999)] and the IAEA (Andreo ct al., Draft V.7 of An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water, IAEA, 2000) have described guidelines to base reference dosimetry of high energy photon beams on absorbed dose to water standards. In these protocols use is made of the absorbed-dose beam quality conversion factor, k(Q) which scales an absorbed-dose calibration factor at the reference quality Co-60 to a quality Q, and which is calculated based on state-of-the-art ion chamber theory and data. In this paper we present the measurement and analysis of beam quality conversion factors k(Q) for cylindrical chambers in high-energy photon beams. At least three chambers of six different types were calibrated against the Canadian primary standard for absorbed dose based on a sealed water calorimeter at Co-60 [TPR1020 = 0.572, %dd(10)(x)=58.4], 10 MV [TPR1020=0.682, %dd(10)(x)=69.6), 20 MV (TPR1020=0.758, %dd(10)(x)=80.5] and 30 MV [TPR1020 = 0.794, %dd(10)(x)=88.4]. The uncertainty on the calorimetric determination of k(Q) for a single chamber is typically 0.36% and the overall 1 sigma uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured k(Q) and the TG-51 protocol value is 0.8%. The overall rms deviation between measurement and the TCS-51 values, based on 20 chambers at the three energies, is 0.41%. When the effect of a 1 mm PMMA waterproofing sleeve is taken into account in the calculations, the maximum deviation is 1.1% and the overall rms deviation between measurement and calculation 0.48%. When the beam is specified using TPR1020, and measurements are compared with k(Q) values calculated using the version of TG-21 with corrected formalism and data, differences are up to 1.6% when no sleeve corrections are taken into account. For the NE2571 and the NE2611A chamber types, for which the most literature data are available, using % dd(10)(x), all published data show a spread of 0.4% and 0.6%, respectively, over the entire measurement range, compared to spreads of up to 1.1% for both chambers when the kg values are expressed as a function of TPR1020. For the PROG-C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our k(Q) measurements and calculations with an analysis in terms of air-kerma protocols with the same underlying calculations but expressed in terms of a compound conversion factor C-Q, we observe that a system making use of absorbed-dose calibrations and calculated k(Q) values, is more accurate than a system based on air-kerma calibrations in combination with calculated C-Q (rms deviation of 0.48% versus 0.67%, respectively). (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01512-1].