Semianalytical expressions for (L/rho)(air)med and P(repl) for electron beams

The tables of the mean restricted collision mass stopping power ratios for water, polystyrene and acrylic relative to air given in the AAPM TG-21 protocol have been fitted to an expression with 20 coefficients using the depth in the phantom and the mean incident electron energy as two independent variables. Using these expressions, the calculated values agree with the tabulated values within +0.5% in 85% of the cases and within +/- 1.0% in 95% of the cases. For each of the four cylindrical chamber inner diameters, given in the protocol, the electron fluence correction Prepl has been fitted to an expression with four coefficients using the mean electron energy at depth z as an independent variable.