Monte Carlo Calculation of Stopping-power Ratios for Clinical Electron Beams Produced by a Linac for Iort

Stopping power ratios sw,air for electron beams produced by the NOVAC7® IORT accelerator were calculated at different depths in water by the SPRRZnrc Monte Carlo code. As input for the SPRRZnrc code a phase-space data file obtained for any beam by the BEAMnrc code was used. The calculated stopping power ratios were compared with those recommended by the IAEA dosimetry protocols TRS-381 and TRS-398. The Monte Carlo calculations in this work show that using the data of the TRS-381 protocol, the sw,air values result to be underestimated at zmax by no more than 0.3%, while using the data of the TRS-398 protocol the sw,air values at zref result to be overestimated up to 0.4%. Only at depths beyond R50 where absorbed dose measurements are not frequent, the differences may be found to be up to within 1.5%. These deviations are likely to be due to different energy spectra existing in a conventional electron beam and in heavily-scattered collimated electron beams as those produced by the NOVAC7® IORT accelerator. For practical dosimetry such differences may be acceptable in most cases.