Calculation of water/air stopping-power ratios using EGS4 with explicit treatment of electron-positron differences.

Using the EGS4 Monte Carlo simulation program, a general purpose code has been written to calculate Bragg-Gray and Spencer-Attix stopping-power ratios for use in radiation dosimetry. The stopping-power ratios can be calculated in any material in any region in a general cylindrical geometry with a large number of source geometries possible. The calculations take into account for the first time the differences between the stopping powers and the inelastic scattering of positrons and electrons. The results show that previous calculations ignoring these effects were accurate. The present results agree, typically within 0.1%, with the Spencer-Attix water-to-air stopping-power ratios for broad parallel beams of electrons given in the AAPM and IAEA protocols except at the surface where the present calculations follow the buildup of secondary electrons in more detail and see a 2% reduction in the stopping-power ratios.