ABSTRACT
To convert an ionization measurement to absorbed dose, the TG-21, the IAEA, and the TG-25 protocols for the calibration of high energy electron therapy machines demand an accurate knowledge of the mean restricted collision mass stopping power ratio, (L/rho)medair, and the electron fluence correction factor, Prepl. This paper presents a semi-analytical expression to calculate (L/rho)medair at depths in the range of 0 to 30 g/cm2 and for any energy in the range of 4-60 MeV, for water, polystyrene, and acrylic relative to air, and another expression to calculate Prepl for any cylindrical ionization chamber with an inner diameter in the range of 3 to 7 mm. The published values for (L/rho)medair were fitted to a single analytical expression with 10 coefficients using a nonlinear least square routine. Another expression with 4 coefficients was used to fit the electron fluence correction factor, Prepl, for cylindrical ionization chambers, as a function of chamber diameter and the mean electron energy at depth of measurement. In the electron energy range of 4-60 MeV, and for all published depths, the calculated (L/rho)medair values agree with the published values within +/- 1% in 99% of the cases. The calculated Prepl values agree with the tabulated values within +/- 1% in all of the cases. The proposed equations now allow one to use a computer to calculate the values of (L/rho)medair and Prepl accurately, doing away with the time-consuming procedure of interpolating between values from tables during the routine calibration of high energy electron therapy machines.
