Clinical reference dosimetry: Comparison between AAPM TG-21 and TG-51 protocols

We compare the results of absorbed dose determined at reference conditions according to the AAPM TG-21 dose calibration protocol and the new AAPM TG-5 1 protocol. The AAPM TG-21 protocol for absorbed dose calibration is based on ionization chambers having exposure calibration factors for Co-60 gamma rays, N-x. The new AAPM TG-51 dosimetry protocol for absorbed dose calibrat ion is based on ionization chambers having Co-60 absorbed dose-to-water cal ibration factor, N-D,w(60Co). This study shows that the dose changes are wi thin 1% for a cobalt beam, 0.5% for photon energies of 6 and is MV, and 2%- 3% for electron beams with energies of 6 to 20 MeV. The chamber primary cal ibration factors, N-x and N-D,w(60Co), traceable to the Canadian primary st andards laboratory (NRCC). We also present estimated dose changes between t he two protocols when calibration factors are traceable to NIST in the Unit ed States. (C) 2000 American Association of Physicists in Medicine, [S0094- 2405(00)00406-5].