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.
A study of the quantitative aspects of backscattering of electrons has been published recently by Klevenhagen et al. (ibid., vol.27, p.363-73, 1982). To assess the full effect that the backscatter electrons have on the dose distribution in the region close to the scatterer-tissue interface, it is necessary to know the penetration of these electrons in the medium. This aspect has been studied in the work reported.
In photon beam therapy, the geometric penumbra width is determined by the source-size and the collimator design. The width of the physical (i.e. dosimetric) penumbra involves an additional contribution from secondary electron spread. Using a suitably defined measure of penumbra width, the separate widths due to photons and secondary electrons are additive in quadrature. Secondary electron spread functions were measured using photographic dosimetry for 60Co gamma rays and for 4, 8 and 16 MV X rays. The results suggest that, under typical treatment conditions, secondary electron spread may be the predominant contributor to the penumbra at effective generating voltages exceeding 10 MV. The implications for selection of beam energy in precision small-field radiotherapy are discussed briefly.
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