Simple calculation of the electron-backscatter factor.

The authors have studied the dependence of the electron-backscatter factor (EBF) on mean electron energy and on backscatterer atomic number by using the semiempirical depth-dose code EDMULT. A plane-parallel electron beam is assumed to be normally incident on a polystyrene slab, which is backed with a layer (backscatterer) of different materials of effectively semi-infinite thickness. A small air cavity to measure ionization is embedded in the polystyrene slab at the boundary facing the backscatterer. The EBF is defined as the ratio of the ionization with the backscatterer to the ionization with a full polystyrene medium, and is approximated by the ratio of the doses computed at the depth of the cavity. Values of EBF obtained show trends similar to the experimental data of Klevenhagen et al. [Phys. Med. Biol. 27, 263-373 (1982)], although the former are generally lower than the latter. When the typical energy spread of clinical electron beams is taken into account, the difference between the experimental and the calculated values is reduced. The present results also show the same trend of increase of the backscatter factor with increasing energy as observed by Klevenhagen et al. in some series of measurements for the lead backscatterer at the lowest energies. This is explained by the rapid buildup of the dose with depth for electrons of low initial energies incident on the full polystyrene medium.