Encapsulated iodine-125 in radiation oncology. I. Study of the relative biological effectiveness (RBE) using low dose rate irradiation of mammalian cell cultures.

The use of encapsulated iodine-125 seeds has increased considerably since 1965, due largely to their physical characteristics. The 28 keV x-ray emission offers improved radiation protection and rapid fall-off of dose outside the treatment volume. The Relative Biological Effectiveness (RBE) of the low energy I-125 x-rays has not been adequately assessed. The limited studies have found a wide range of values, but most are between 1.2-1.5 relative to hard x-rays. We used C3H/10T1/2 mouse embryo cells in contact inhibited plateau phase to assess the RBE of I-125 seed x-rays relative to Cs-137 gamma rays (660 keV) using low dose rate continuous irradiation. Replicate experiments found the RBE to be 1.2. This did not vary with dose rate over the range of 10-76 cGy/hour. Calculations made from our cell survival data suggest that, in the case of permanent I-125 implants, where the dose is administered over a considerable period of time, the resultant surviving fraction of tumor cells is dictated largely by the length of the cell cycle. It is suggested that, for this reason, permanent I-125 implants may be less suitable for rapidly growing tumors, such as glioblastomas, than temporary I-125 implants, which are calculated to be virtually independent of the cell cycle duration.