Stopping-power ratios and their uncertainties for clinical electron beam dosimetry

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Abstract
The influence of energy and angular spread and electron and photon contamination on the water/air stopping-power ratios for 'realistic' electron beams of 10 MeV has been investigated using the Monte Carlo method. Differences smaller than 0.5% have been found in the sW,air value at the depth of maximum absorbed dose compared with sW,air(E0,z) determined according to most dosimetry protocols. The use of broad independent energy and angular distributions to sample the initial state of electrons for Monte Carlo simulations has been analysed. Uncertainties in sSAW,air values, evaluated with a Monte Carlo iterative technique, are approximately 0.5% at dmax. The combination of uncertainties in sSAW,air due to the calculation procedure and to the 'sW,air(E0,z) method' yields an estimated total uncertainty of approximately 1% for the sW,air at dmax in clinical electron beams with energies around 10 MeV, which is smaller than values quoted recently.