Results of photon absorbed-dose measurements using the AAPM TG-21 protocol for accelerating potentials up to 26 MV.

The AAPM Task Group 21 protocol for the calibration of high-energy photon and electron beams was produced to accomplish essentially two goals: (1) incorporate the latest physical data available for calculating absorbed dose from ionization measurements and (2) to eliminate inconsistencies in absorbed dose measurements made with various ion chamber and phantom combinations. The ability of the protocol was assessed to consistently determine x-ray absorbed dose from measurements made with four Farmer-type chambers and one parallel-plate chamber in water, polystyrene, and acrylic phantoms. The measurements were performed using seven high-energy x-ray beams from 60Co to 26-MV nominal accelerating potential. The absorbed dose to water calculated from measurements made with the various chamber and phantom combinations were found to be consistent. The doses calculated for the two most common phantom materials, water and polystyrene, were found to be in excellent agreement. This resolved a 1.6% discrepancy in the absorbed dose determined from the two phantoms using the SCRAD protocol. The doses for acrylic phantoms were found to be approximately 1.2%, low for nominal accelerating potentials less than 8.8 MV. For accelerating potentials of 8.8 MV or greater the agreement was considerably better. The mean dose determined for the parallel-plate chamber from measurements in polystyrene was found to be within 0.7% of the mean dose determined using Farmer-type ion chambers in all phantom materials.