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

Task Group 51 (TG-51) of the Radiation Therapy Committee of the American As sociation of Physicists in Medicine (AAPM) has recently developed a new pro tocol for the calibration of high-energy photon and electron beams used in radiation therapy. The formalism and the dosimetry procedures recommended i n this protocol are based on the use of an ionization chamber calibrated in terms of absorbed dose-to-water in a standards laboratory's Co-60 gamma ra y beam. This is different from the recommendations given in the AAPM TG-21 protocol, which are based on an exposure calibration factor of an ionizatio n chamber in a Co-60 beam. The purpose of this work is to compare the deter mination of absorbed dose-to-water in reference conditions in high-energy p hoton beams following the recommendations given in the two dosimetry protoc ols. This is realized by performing calibrations of photon beams with nomin al accelerating potential of 6, 18 and 25 MV, generated by an Elekta MLCi a nd SL25 series linear accelerator. Two widely used Farmer-type ionization c hambers having different composition, PTW 30001 (PMMA wall) and NE 2571 (gr aphite 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, r espectively 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 abou t 1.5%. A detailed analysis of the reasons for the discrepancies is made wh ich includes comparing the formalisms, correction factors and quantities in the two protocols, as well as the influence of the implementation of the d ifferent standards for chamber calibration. The latter has been found to ha ve a considerable influence on the differences in clinical dosimetry, even larger than the adoption of the new data and recommended procedures, as mos t intrinsic differences cancel out due to the adoption of the new formalism . (C) 2001 American Association of Physicists in Medicine.