THE RELATIVE BIOLOGICAL EFFECTIVENESS OF YB-169 FOR LOW-DOSE RATE IRRADIATION OF CULTURED-MAMMALIAN-CELLS

Purpose: An important step in the development of Yb-169 as a new brach ytherapy source is to determine its biological effectiveness relative to other commonly used radioisotopes. The purpose of this paper is to determine the relative biological effectiveness of Yb-169, with respec t to Co-60, for a range of low dose rates. Method and Materials: The r elative biological effectiveness of photon radiation from encapsulated Yb-169 was determined by exposing Chinese hamster ovary cells, in exp onential growth, to graded doses of radiation from either Yb-169 or Co -60. Clonogenic cell survival was determined for continuous low dose r ates ranging from 6.5 cGy/hr to 52 cGy/hr. Results: The relative biolo gical effectiveness of Yb-169, with respect to Co-60, was determined t o be 1.2 +/- 0.3 and did not vary significantly over the dose-rate ran ge from 13 cGy/hr to 50 cGy/hr. An inverse dose-rate effect was observ ed, but only for Co-60 irradiation at 8.9 cGy/hr. Therefore, relative biological effectiveness values could not be determined reliably for d ose rates less than 13 cGy/hr. Conclusions: We have established that Y b-169 is approximately 20% more effective than Co-60 in vitro. It is h oped that this study will guide the introduction of Yb-169 into clinic al brachytherapy practice.