The treatment of ophthalmic tumours with low-energy sources

Abstract

One of the features which detract from the advantages of insertion or implantation of radioactive sources in the treatment of tumours is the difficulty in screening sensitive structures within the treatment volume. In addition, where very large doses are administered with electro-magnetic radiation, for instance in the treatment of intra-ocular tumours with radioactive plaques, the doses administered to surrounding structures may be considerable. It seems that these difficulties might be largely overcome by the use of low-energy isotope sources and suitable integral shielding. For example, the use of 125I seeds has been considered, which were first described by Henschke and Lawrence (1965). The isotope emits 27 to 35 keV X rays which permit highly effective screening by thin lead foil (0·25 mm of lead reduces primary radiation from 125I by 99·9 per cent, and absorbs scattered radiation and secondary electrons almost entirely). Suitable seeds for this purpose, consisting of radioactive sodium iodide absorbed onto ceramic beads, may be obtained from the Lawrence Soft Ray Corporation, California. Each titanium-encapsulated seed contains two such beads separated by a ladioopaque gold marker. The seeds are 4·5 mm long and 0·75 mm in diameter. Seeds of 4 mCi activity are now available, which are suitable for the building of applicators and, after a reasonable period of decay, for implantation. Their repeated use makes treatment with this isotope relatively cheap.