EFFICACY OF A SILVER FILTER IN REDUCING THE SURFACE DOSE IN CO-60 TELETHERAPY

One of the advantages of 60Co radiations in the treatment of malignant diseases is its skin sparing effect. Theoretically, the dose on the skin in 60Co teletherapy is too small to give any observable skin reactions for routine therapeutic dose but the published reports indicate that in practice the skin reactions are much more than expected. The increased skin reactions are due to the increased dose on the skin because the gamma beam from 60Co gets contaminated by scattered electrons. Use of electron filters and increased distance between the source and the patient are two methods in use to reduce the electron contamination of the beam. Earlier studies with electron filters indicate that medium atomic number materials are best for this purpose. A new filter material - silver - has beem tried by us and its efficacy in reducing the surface dose has been studied. Loss of skin sparing of 60Co beam because of oblique incidence has also been reported. This arises from the geometric considerations of the direction of the scattered electrons in the tissue. This problem has also been studied and the role of a silver filter in such circumstances has also been investigated. The studies lead us to conclude that the medium Z materials, such as silver, are best under all treatment conditions. However, if the treatment field is small (10×10 cm2 or less) or the distance between the filter and the skin is more (25 cm or more), the filter materials in a wide range of atomic numbers are equally effective. The silver filter is also effective in reducing the increased surface dose due to oblique incidence of the beam. © 1979.