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 AI to 8 mm AI HVT and source-tosurface 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.

Whenever a heterogeneity is present in an electron beam treatment field during radiotherapy, there is the possibility of tissue overdosage at the tissue-heterogeneity interface due to electrons backscattered from the heterogeneity. Measurements of this effect were made in a polystyrene phantom using a purpose-built thin-window parallel-plane ionisation chamber. Materials of various atomic numbers were used as scatterers and the investigations were made over a wide range of electron energies. Electron backscatter factor (EBF), defined as the ratio fo dose at the interface surface with and without the scatterer present, was found to increase with increasing atomic number and decrease with increasing beam energy. Both of these relationships were found to be non-linear. The EBF dependence on the scatterer thickness was also investigated. All data in this work were expressed in relation to the beam energy incident on the scatterer in preference to the nominal beam energy set on the accelerator. This approach enables the dose enhancement at an interface to be predicted from a knowledge of the heterogeneity (atomic number and thickness,), its depth in tissue and the beam energy being used for treatment. The results of this work were compared with the published data and an explanation is offered to account for the difference.

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The backscatter factors computed by Grosswendt (1984) for x-rays generated at voltages between 10 and 100 kV using a Monte Carlo method differ significantly from the corresponding factors published in supplement 17 of the Brirish Journal ofRadiology (1983). The data of Grosswendt were subsequently used in the recently published International Code of Practice for Radiation Dosimetry by the IAEA (1987) in the section dealing with low-energy x-rays. The factors given in supplement 17 were obtained by a collation of experimental data from measurements performed by various investigators and include some extrapolated values. They may be subject to systematic uncertainties and it was therefore thought necessary to undertake a study of the backscatter in a uniform experiment in order to verify these data and the theoretical calculations of Grosswendt (1984).A special 3 mm thin plane-parallel ionisation chamber was constructed for this work and measurements were performed using water as a scattering medium. The experiments involved field sizes of 2.0 to 20 cm beam diameter and x-ray qualities between 0.1 and 4 mm AI for generating voltages between 16 and 140 kV. The results yielded backscatter factors which are in close agreement with those obtained by Monte Carlo calculations. The agreement with the supplement 17 data is only satisfactory at HVLS above 1 mm AI. The possible causes for the disagreement between the various published data are analysed.

Dosimetry of low energy x-rays is performed more and more often with the use of thin window parallel-plate ionization chambers because of their favourable performance characteristics compared with cylindrical chambers. The thin window is particularly attrao live for measuring beam parameters at the skin (phantom) surface but it also introducer dosimetric problems by allowing the secondary photoelectrons produced in the beam collimating system to enter the chamber. This may result in erroneous dosimetric data. This paper deals with some o f the problems caused by the secondary electrons and their effect on absorbed dose and depth dose measurements. To minimize these effects, it is suggested to increase the window thickness of the ionization chamber employed thus making the chamber only x-ray sensitive.