Twenty-five years of MTPR--a theoretical and experimental analysis

The system of modified tissue phantom ratio (called tissue standard factor, or TSF, in this paper) was introduced to facilitate the calculation of monitor units in isocentric configurations. Twenty-five years later, TSFS have been measured at 6 MV on different linear accelerators, and are in agreement with the original values, highlighting the property of machine independence. The utility of TSF is shown by the simplicity of the equation for calculating dose per monitor unit at any depth, field size and distance from source. New equations are given for relating isocentric systems to fixed-SSD systems, and the table of TSFS is extended to include a symbolic zero-area column, which is shown to be the primary-dose component of TSF in electronic equilibrium.