Reference dosimetry in clinical high-energy photon beams: Comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols

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Abstract
Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) has recently developed a new protocol for the calibration of high-energy photon and electron beams used in radiation therapy. The formalism and the dosimetry procedures recommended in this protocol are based on the use of an ionization chamber calibrated in terms of absorbed dose-to-water in a standards laboratory's 60Co gamma ray beam. This is different from the recommendations given in the AAPM TG-21 protocol, which are based on an exposure calibration factor of an ionization chamber in a 60Co beam. The purpose of this work is to compare the determination of absorbed dose-to-water in reference conditions in high-energy photon beams following the recommendations given in the two dosimetry protocols. This is realized by performing calibrations of photon beams with nominal accelerating potential of 6, 18 and 25 MV, generated by an Elekta MLCi and SL25 series linear accelerator. Two widely used Farmer-type ionization chambers having different composition, PTW 30001 (PMMA wall) and NE 2571 (graphite wall), were used for this study. Ratios of AAPM TG-51 to AAPM TG-21 doses to water are found to be 1.008, 1.007 and 1.009 at 6, 18 and 25 MV, respectively when the PTW chamber is used. The corresponding results for the NE chamber are 1.009, 1.010 and 1.013. The uncertainties for the ratios of the absorbed dose determined by the two protocols are estimated to be about 1.5%. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration. The latter has been found to have a considerable influence on the differences in clinical dosimetry, even larger than the adoption of the new data and recommended procedures, as most intrinsic differences cancel out due to the adoption of the new formalism.