The protocols and codes of practice used for the determination of absorbed dose in megavoltage photon and electron beams

Abstract

Air kerma-to-dose and exposure-to-dose conversion factors for megavoltage photon beams and electron beams are derived for a variety of commonly used ionization chambers, using the various dosimetry protocols and codes of practice currently in use. It is found that differences in the resultant values are present for both photon and electron beams. For example, for a NE2571 graphite-walled thimble ionization chamber, the maximum variation in the air kerma-to-dose conversion factor for photons, Cw,lambda, occurs at a "nominal" energy of 25 MV, when the difference between the maximum and minimum value of Cw,lambda is 5.4%, whereas the maximum difference between different protocols in the air kerma-to-dose conversion factor for electrons, Cw,e, occurs at Eo = 9 MeV, when the spread between maximum and minimum Cw,e values is 2.7%. An analysis of the conversion factors is undertaken to examine if the numerical or theoretical data are the cause of the discrepancies, and it is found to be a combination of both, although the major contribution to this difference arises from the use of different sets of stopping power ratios in the different protocols.