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 Prac tice for Dosimetry based on Standards of Absorbed Dose to Water," IAEA, 200 0) have described guidelines to base reference dosimetry of high energy pho ton beams on absorbed dose to water standards. In these protocols use is ma de of the absorbed-dose beam quality conversion factor, k(Q) which scales a n absorbed-dose calibration factor at the reference quality Co-60 to a qual ity 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 qu ality conversion factors k(Q) for cylindrical chambers in high-energy photo n beams. At least three chambers of six different types were calibrated aga inst the Canadian primary standard for absorbed dose based on a sealed wate r 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 [T PR1020 = 0.794, %dd(10)(x)=88.4]. The uncertainty on the calorimetric deter mination 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 valu es, based on 20 chambers at the three energies, is 0.41%. When the effect o f a 1 mm PMMA waterproofing sleeve is taken into account in the calculation s, the maximum deviation is 1.1% and the overall rms deviation between meas urement and calculation 0.48%. When the beam is specified using TPR1020, an d measurements are compared with k(Q) values calculated using the version o f 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 NE2611 A 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 b oth 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 cal culations with an analysis in terms of air-kerma protocols with the same un derlying calculations but expressed in terms of a compound conversion facto r C-Q, we observe that a system making use of absorbed-dose calibrations an d 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% ve rsus 0.67%, respectively). (C) 2000 American Association of Physicists in M edicine. [S0094-2405(00)01512-1].