An analysis of the revised HPA dosimetry protocols

Full derivations using an internationally accepted formalism for ion chamber dosimetry are given for the air kerma to absorbed-dose-to-water conversion factors C lambda K and C e for photon and electron beams respectively, as defined in the HPA revised codes of dosimetry practice. In common with the recent Dutch and IAEA protocols, consistent use is made throughout the dosimetric chain of the electron stopping-power data recently revised by the ICRU. Particular attention has been paid to the effect of the aluminium central electrodes in the two NE chambers. The resulting C lambda K values, given as a function of TPR 10 20 , are generally between 1 and 1.5% lower than the HPA values, though the comparison is complicated by the HPA's use of nominal MV as a measure of beam quality. The HPA C e values show a markedly different depth dependence for beams with E 0 >10 MeV compared with the new values, though the two sets of values agree well at small depths. The difficulties of predicting the response of the Vinten model 631 flat chamber are also considered.