Relation Between Kerma and Absorbed Dose in Photon Beams

The relation between kerma and absorbed dose has been calculated using a diffusion equation approximation for the electron transport in photon beams and by equating the depth integral of the collision kerma with that of the absorbed dose. The results show that the absorbed dose in the transient equilibrium region practically coincides with the total kerma in water both for 60Co and 21 MV roentgen rays. The ratio between the absorbed dose at maximum build-up and the collision kerma at the surface (katt) was calculated. The results show that the dependence of katt on the atomic number is very small. For 60Co katt was determined for water and air to 0.992 and 0.991, respectively.