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 NOVAC7R 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 resulted 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 resulted 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 NOVAC7R IORT accelerator. For practical dosimetry such differences may be acceptable in most cases. (authors)