Concept of depth dose functions for radiotherapy beams: definition and values for different energy beams

Depth dose functions are defined and based on empirically observed interrelation of percent depth doses for different energy beams. By using this function, if the depth dose for a given source-surface distance (SSD), field size and depth are known for a standard beam energy, the corresponding depth dose for another beam energy can be derived. The results obtained are accurate to within +- 1% in a considerable range of SSD, field size and depths used in routine radiotherapy. The formulation can be extended to hold at depths between the surface and depth of maximum but is somewhat less accurate than +- 1% for depths up to two times the depth of maximum. This method is of considerable usefulness in computer dosimetry algorithms to save memory space and/or to increase speed by avoiding multiple table look up procedures. It is the particular importance in irregular field dose calcuations for high energy beams where specific measured data for separation of primary and scatter components of dose are not available