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

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Abstract
Little information is available on the influence of the thickness of underlying material on the magnitude of the backscatter factor and investigations were therefore undertaken. Radiation beam qualities between 1 mm Al to 8 mm Al half-value thickness (HVT) and source-to-surface distances (SSD) 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.

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