Reduction of dose enhancement from backscattered radiation at tissue-metal interfaces irradiated with 6MeV electrons

Due to Electron Back Scatter (EBS), electron irradiation of tissue having under lying lead shielding results in an increase in dose to the tissue on the entrance side of the lead. In these situations dose increases as high as 80% have been reported in the literature. Saunders (British Journal of Radiology, 47, 467-470) noted that dose enhancement is dependent on atomic number of the under lying material approximately as Z0.5, and it increases at lower incident electron energies. In our clinic we use 2mm of lead shielding to protect under lying normal tissue when 6MeV electrons are used to treat lips and ears. The object of this study was to find the thinnest combination of materials to reduce the total dose to an acceptable level, with the provisos that; the patient does not come into contact with the lead or other metals, the finished shield could comfortabley be placed between the patient's lip and teeth, and that the materials are sufficietly malleable to work into custom shields. Various combinations of dental wax and aluminium were trialed. That which proved to give the best compromise between reduction of EBS and total shielding thickness was, 1mm of aluminim on the beam side of the lead with 1mm of dental wax to completely enclose the shield. In practice the manufactured shields are approximately 6 mm thick, and are usually not uncomfortable for the patient. (author)