Measurements of cell survival at low doses of radiation

The effect of low closes of radiation is of primary importance if we wish to understand the basic mechanisms of radiation damage. In vitro experiments performed at clinically relevant doses can also lead to better understanding of radiotherapy protocols and fractionation regimes. The availability of accurate data at low doses can facilitate the examination of survival models which describe dose-effect relationships. Most cellular radiobiology experiments are performed at high doses (3 to 30 Cray). The errors in these experiments, while acceptable at high doses, are too large to allow determination of radiobiological parameters, such as oxygen enhancement ratio (OER) and relative biological effectiveness (RBE), at low doses (0 to 3 Cray). These experiments are limited in the low dose region because we are measuring only the surviving fraction, in a population of predominantly surviving cells, and because there is an uncertainty of 10 to 15% in the number of cells plated. We have developed a technique to assay cell survival at low doses. The exact number of cells plated is determined microscopically. After incubation, the number of killed cells and the number of surviving cells are both determined, by microscopic examination. While extremely labour intensive, this technique yields survival data, in the low dose region, which is much more accurate than the data obtained using classical methods. This technique can be used to measure many radiobiological parameters. We have chosen to examine the effect of oxygen at low doses. Our results clearly demonstrate that, for asynchronous Chinese Hamster Ovary (CHO) cells, the radiosensitizing effect of oxygen is reduced at lower doses. A Picker X-ray source (280 kVp, HVL 1.7 mm Cu) was used for these experiments. The choice of a survival model has important implications in the low dose region. The predictions of three different survival models regarding the effect of oxygen at low doses are discussed. This technique can be used to complement the classical "high dose assay" to obtain data that encompasses a large dose range. This will be especially valuable, for example, when attempting to fully describe radiobiological parameters, or when examining survival models.Science, Faculty ofPhysics and Astronomy, Department ofGraduat