Comparison of the IAEA TRS-398 and AAPM TG-51 absorbed dose to water protocols in the dosimetry of high-energy photon and electron beams

The International Atomic Energy Agency (IAEA TRS-398) and the American Asso ciation of Physicists in Medicine (AAPM TG-51) have published new protocols for the calibration of radiotherapy beams. These protocols are based on th e use of an ionization chamber calibrated in terms of absorbed dose to wate r in a standards laboratory's reference quality beam. This paper compares t he recommendations of the two protocols in two ways: (i) by analysing in de tail the differences in the basic data included in the two protocols for ph oton and electron beam dosimetry and (ii) by performing measurements in cli nical photon and electron beams and determining the absorbed dose to water following the recommendations of the two protocols. Measurements were made with two Farmer-type ionization chambers and three plane-parallel ionizatio n chamber types in 6, 18 and 25 MV photon beams and 6, 8, 10, 12 15 and 18 MeV electron beams. The Farmer-type chambers used were NE 2571 and PTW 3000 1, and the plane-parallel chambers were a Scanditronix-Wellhofer NACP and R oos, and a PTW Markus chamber. For photon beams, the measured ratios TG-51/ TRS-398 of absorbed dose to water D-w ranged between 0.997 and 1.001, with a mean value of 0.999. The ratios for the beam quality correction factors k (Q) were found to agree to within about +/-0.2% despite significant differe nces in the method of beam quality specification for photon beams and in th e basic data entering into kQ. For electron beams, dose measurements were m ade using direct N-D,N-w calibrations of cylindrical and plane-parallel cha mbers in a Co-60 gamma-ray beam, as well as cross-calibrations of plane-par allel chambers in a high-energy electron beam. For the direct N-D,N-w calib rations the ratios TG-51/TRS-398 of absorbed dose to water D-w were found t o lie between 0.994 and 1.018 depending upon the chamber and electron beam energy used, with mean values of 0.996, 1.006, and 1.017, respectively, for the cylindrical, well-guarded and not well-guarded plane-parallel chambers . The D-w ratios measured for the cross-calibration procedures varied betwe en 0.993 and 0.997. The largest discrepancies for electron beams between th e two protocols arise from the use of different data for the perturbation c orrection factors p(wall) and P-dis of cylindrical and plane-parallel chamb ers, all in Co-60. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and th e quantities in the two protocols.