ABSTRACT
This chapter deals with measurements that are absolute as opposed to relative. It describes some discussion of absorbed dose primary standards, but the main concern is the accurate measurement of absorbed dose using a calibrated ion chamber under reference conditions. Reference dosimetry requires a detector that has a calibration coefficient traceable to a standards dosimetry laboratory, which provides the conversion of the detector signal to absorbed dose to water. These reference detectors are generally reserved for the output calibration of radiation therapy machines. International compatibility relies on a measurement infrastructure in which the various national primary standards are regularly compared in a process that is coordinated by the Bureau International des Poids et Mesures. The associated higher dose rates and doses per pulse of flattening filter free beams improve treatment delivery efficiency but may need larger corrections for recombination and polarity behaviour of ion chambers. Special attention is paid to the role of calorimeters in reference dosimetry.
