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Absorbed‐dose beam quality conversion factors for cylindrical chambers in high energy photon beams
Author(s) -
Seuntjens J. P.,
Ross C. K.,
Shortt K. R.,
Rogers D. W. O.
Publication year - 2000
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1328081
Subject(s) - absorbed dose , dosimetry , calibration , laser beam quality , beam (structure) , photon , primary standard , calorimeter (particle physics) , ionization chamber , physics , materials science , optics , nuclear medicine , nuclear physics , detector , ion , medicine , quantum mechanics , laser beams , laser , ionization
Recent working groups of the AAPM [Almond et al., Med. Phys. 26 , 1847 (1999)] and the IAEA (Andreo et 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 Qwhich scales an absorbed‐dose calibration factor at the reference quality60 Co 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 Qfor 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 at60 Co[ TPR 10 20= 0.572 ,% dd ( 10 ) x= 58.4 ] , 10 MV [ TPR 10 20= 0.682 ,% dd ( 10 ) x= 69.6 ) , 20 MV ( TPR 10 20= 0.758 ,% dd ( 10 ) x= 80.5 ] and 30 MV [ TPR 10 20= 0.794 ,% dd ( 10 ) x= 88.4 ] . The uncertainty on the calorimetric determination of k Qfor a single chamber is typically 0.36% and the overall 1σ uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured k Qand the TG‐51 protocol value is 0.8%. The overall rms deviation between measurement and the TG‐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 TPR 10 20 , and measurements are compared with k Qvalues 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 k Qvalues are expressed as a function of TPR 10 20 . For the PR06‐C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our k Qmeasurements 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 Qvalues, 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).