Replacement correction factors for electron measurements with a parallel-plate chamber

The AAPM Task Group dosimetry protocol is ambiguous regarding the replacement correction factors Prepl to be applied to electron measurements with parallel-plate chambers. By intercomparison with a cylindrical chamber whose Prepl values at dmax may be calculated from Task Group 21, the Prepl values for a PTW/Markus parallel-plate chamber have been determined in the range of mean incident energies of 5-11 MeV. The Prepl values for this chamber are found to differ significantly from unity, if one assumes that the cylindrical chamber values are valid.