Influence of energy and angular spread on stopping-power ratios for electron beams

The influence of energy and angular spread at the phantom surface on the water/air stopping-power ratios for electron beams has been studied using the Monte Carlo method. Calculated sw,air values for beams with most probable energies around 10 MeV and energy and angular spread of different widths as well as low-energy electron contamination have been compared with 10 MeV monoenergetic and plan-parallel beams. Variations in the calculated sw,air values of up to 1.6% at the depth of maximum absorbed dose have been obtained. The energy and angular spreads cover the interval generally occupied by clinical beams. A simple correction factor for the influence of energy and angular spread on the stopping-power ratio is proposed which only requires the dose fall-off gradient in a broad beam as input parameter. Most dosimetry protocols select water/air stopping-power ratios as a function of the mean energy at the phantom surface and the depth of measurement. Results obtained with such a procedure have been compared with Monte Carlo calculated depth-dose curves, and corresponding slowing-down spectra and stopping-power ratios for clinical beams. Difference in sw,air values smaller than 1% for beams with extreme energy and angular spread have been found. For an energy spread of 0.5 MeV and angular spread of 50, which are normal values in clinical electron beams, stopping-power ratios agree within 0.5% over the clinically useful depth interval. (author)