Determination of electron beam mean incident energy from d50 (ionization) values.

Depth-ionization measurements have been obtained with an air-filled Nordic Association of Clinical Physicists (NACP) design parallel-plate ionization chamber in a water phantom for ten foil-scattered electron beams from two different machines with nominal energies between 6 and 20 MeV and field sizes from 6 X 6 to 25 X 25 cm2. Depths of 50% ionization and practical range have been determined from least-squares fits to both the raw data and values corrected to parallel-beam geometry using measured virtual source distances. Depths of 50% dose have also been obtained from fits to divergence-corrected depth-dose measurements performed under identical conditions using, a p-type silicon diode detector. Utilizing accepted conversion factors between mean incident energy (E0) and depth of 50% dose for parallel incident beams, and taking advantage of the fact that p-type silicon diode detector readings are nearly directly indicative of relative dose, conversion factors between E0 and depth of 50% ionization for divergence-corrected and raw, uncorrected finite source-surface distance depth-ionization data are empirically determined. Those values, obtained using the results of both ETRAN and EGS4 dose calculations as base lines, are compared to values currently recommended for use in clinical dosimetry.