Absorbed Dose Equations. On the Derivation of a General Absorbed Dose Equation and Equations Valid for Different Kinds of Radiation Equilibrium

The absorbed dose is defined by the ICRU (Report No. 33, International Commission on Radiation Units and Measurements, Washington, DC, 1980). It is a nonstochastic quantity and can, in principle, be calculated. In such calculations, complete knowledge about the radiation field and the details of interaction processes are required provided no kind of radiation equilibrium exists. They can be considerably simplified by restricting them to, in particular, those states of equilibrium existing within a homogeneous medium in which particles are generated uniformly. Absorbed dose equations are derived both for the general case and for various cases of equilibria which exist due to the uniform liberation of different groups of ionizing particles in a homogeneous medium. The concept of equilibrium is throughly analyzed and corresponding evaluations of absorbed dose in the literature are reviewed. The aim of the work is educational: to give a clear exposition of the concepts of absorbed dose and equilibrium, the understanding of which is of vital importance in finding ways to improve numerical calculations of the absorbed dose in, for instance, cavity theories. In particular, the definition of mass energy absorption coefficients for photons and the expression for the absorbed dose in the Spencer-Attix cavity theory are elucidated.