The calibration of therapy level electron beam ionization chambers in terms of absorbed dose to water.

During 1998, NPL plans to introduce the world's first absorbed dose calibration service for electron beam radiotherapy. The service will be based on the primary standard graphite calorimeter, and will enable the direct calibration of electron ionization chambers, without reference to air kerma standards. This calibration is a two-step process. Firstly, a set of NACP-designed parallel-plate reference chambers have been calibrated against the calorimeter over the last few years. These chambers are then used to calibrate user chambers by direct comparison in a water phantom under standard conditions. This paper describes the calibration of the reference chambers against the calorimeter and the derivation of absorbed dose to water calibration factors (with an estimated uncertainty in this calibration of +/-1.50% at the 95% confidence level).