Calculation of high-energy electron dose distributions in tissue-equivalent media. I. Determination of the dose function of point unidirectional sources.

A method for the calculation of high-energy electron dose distributions in tissue-equivalent media is suggested. The total electron source on the exposed surface of the medium is regarded as a superposition of point unidirectional sources. A sufficiently simple analytical function for describing the axial and radial dose distribution produced by such point source is suggested. A method for determining this function, which make direct dose measurements with narrow beams unnecessary, is developed. The parameters of the point source function are derived from the depth dose distribution in a sufficiently broad beam and from the transverse dose profiles for rectangular fields. Equations for computing relative dose distributions in the volume of a tissue-equivalent medium with a flat surface are presented. The method is primarily intended for use in electron beam treatment planning but may find other applications as well.