A simple analytical expression to calculate the backscatter factor for low energy X-ray beams

A simple analytical expression aiming to calculate the backscatter factor used in dosimetry protocols to determine the absorbed dose in water for low energy X-rays beams is presented. This expression is based on the linear dependence of the backscatter factor with the generating potential, for fixed values of the half-value layer. The results of a recent work in which 74 X-ray beams with different spectroscopic characteristics, generated with the code xcomp5r and transported with the Monte Carlo code penelope have been used. The expression derived permits to calculate the backscatter factor within 5% accuracy. The predictive power of this expression has been tested for 20 X-ray beams generated with potentials from 50 to 250 kV, for which half-value layers and backscatter factors have been experimentally determined.