Analytic representation of the backscatter correction factor at the exit of high energy photon beams.

In high energy X-ray beams, the dose calculated near the exit surface under electronic equilibrium conditions is generally overestimated since it is derived from measurements performed in water with large thickness of backscattering material. The resulting error depends on a number of parameters such as beam energy, field dimensions, thickness of overlying and underlying material. We have systematically measured for four different energies and for different combinations of the above parameters, the reduction of dose due to the lack of backscatter. This correction is expressed as a multiplicative factor, called "Backscatter Correction Factor" (BCF). This BCF is larger for lower energies, larger field sizes and larger depths. The BCF has been represented by an analytical expression which involves an exponential function of the backscattering thickness and linear relationships with depth, field size and beam quality index. Using this expression, the BCF can be calculated within 0.5% for any conditions in the energy range investigated.