Photon absorbed dose standards

In this review the current status of absorbed dose to water standards for high-energy photon beams (60Co - 50 MV nominal accelerating potential) is discussed. The review is focused on calorimeter-based absorbed dose standards for photon radiation therapy calibrations with typical dose rates of a few gray per minute. In addition, two alternative types of absorbed dose standards are also discussed. The overall uncertainty on measured dose to water in static reference fields is nowadays on the order of 0.4% to 0.5%. The components contributing to the uncertainty budgets are discussed. The discussed absorbed dose to water standards are expected to continue to have their place not only in the dissemination of absorbed dose to water but also in the determination of beam quality conversion factors essential in reference dosimetry in high-energy photon beams. (authors)