Relative biological effectiveness of 125I sources in a murine brachytherapy model

The relative biological effectiveness of 125I and 192Ir has been determined in a murine brachytherapy model that uses a clonogenic cell assay as the end point. Removable 125I or 192Ir sources were implanted at right angles to the surface of RIF-1 tumors grown in the flanks of C3H/He mice. After irradiation for 1-5 days, mice were sacrificed and isodosed annuli of irradiated tumor tissue were sampled for the clonogenic cell assay. The slopes and intercepts of the two radiation survival curves for implanted sources with activities of 10 to 50 Gy for 125I (dose rate = 39.8 +/- 4.3 cGy/hr) and 192Ir (dose rate = 42.3 +/- 2.7 cGy/hr) were identical; the relative biological effectiveness was 1.