The build-up of backscatter in the energy range 1mm Al to 8mm Al HVT

Little information is available on the influence of the thickness of underlying material on the magnitude of the backscatter factor and investigations were therefore under-taken. Radiation beam qualities between 1mm Al to 8mm Al HVT and source-to-surface distances applicable to contemporary superficial radiotherapy were used. Measurements of the backscatter were made in a polystyrene phantom using a purpose-designed parallel-plate ionisation chamber built into the phantom surface. The effect of the thickness of underlying material on the build-up was investigated as a function of field size, beam quality, and SSD. The backscatter build-up curves were found to rise more slowly with increasing field size, increasing HVT and longer SSD; and the experimental data were fitted by an exponential function. A set of equations is proposed correlating the parameters influencing the backscatter behaviour and allowing the fractional backscatter factor to be calculated for a given field size, HVT, SSD and thickness of underlying material. (author)