The relative biological effectiveness of ytterbium-169 for low dose rate irradiation of cultured mammalian cells.

Purpose: An important step in the development of 169Yb as a new brachytherapy 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 169Yb, with respect to 60Co, for a range of low dose rates.

Method And Materials: The relative biological effectiveness of photon radiation from encapsulated 169Yb was determined by exposing Chinese hamster ovary cells, in exponential growth, to graded doses of radiation from either 169Yb or 60Co. Clonogenic cell survival was determined for continuous low dose rates ranging from 6.5 cGy/hr to 52 cGy/hr.

Results: The relative biological effectiveness of 169Yb, with respect to 60Co, was determined to be 1.2 +/- 0.3 and did not vary significantly over the dose-rate range from 13 cGy/hr to 50 cGy/hr. An inverse dose-rate effect was observed, but only for 60Co irradiation at 8.9 cGy/hr. Therefore, relative biological effectiveness values could not be determined reliably for dose rates less than 13 cGy/hr.

Conclusions: We have established that 169Yb is approximately 20% more effective than 60Co in vitro. It is hoped that this study will guide the introduction of 169Yb into clinical brachytherapy practice.