Relative Biological Effectiveness of 241Am Relative to 1921r for Continuous Low-Dose-Rate Irradiation of BA 1 12 Rat Sarcomas

Sealed sources of 24'Am have been developed for intracavitary irradiation of gynecological cancers. Relative to conventional isotopes (that is, 226Ra, 137Cs, '92Ir), 24'Am allows for better shielding of dose-limiting normal tissues in the patient. In addition, the long half-life of 24'Am (432 years) makes it an attractive isotope both for clinical use and for long-term radiobiology studies. Using a previously developed in vivo applicator system, BA1112 sarcomas on WAG/ Rij Y rats were irradiated using 241Am or '92Ir at three different dose rates. Following in vivo treatment of the sarcomas with graded doses of radiation, cell survival curves were determined using an in vitro colony formation assay. The slopes of the resulting cell survival curves were observed to increase significantly as the dose rate increased from 0.30 to 0.60 Gy/h, then to decrease slightly as the dose rate increased from 0.60 to 0.95 Gy/h. The relative biological effectiveness (RBE) of 24 'Am relative to 192Ir was observed to increase linearly with increasing dose rate; the RBEs were 0.96 ? 0.009, 1.09 + 0.12, and 1.17 + 0.11 at dose rates of 0.30, 0.60, and 0.95 Gy/h, respectively. ? 1989 Academic Press, Inc.