The influence of energy and angular spread at the phantom surface of the water/air stopping-power ratios for electron beams has been studied using the Monte Carlo method. Calculated s w.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 plane-parallel beams. Variations in the calculated s w.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.
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Andreo et al. (1989) studied this question.
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