THE RESPONSE OF A HUMAN TUMOR-CELL LINE TO LOW RADIATION-DOSES - EVIDENCE OF ENHANCED SENSITIVITY

The survival of asynchronous, exponentially growing DU-145 human tumor cells was measured after single doses of X rays in the dose range of 0.05-4 Gy using the cell sorting assay. When the response was modeled with the linear-quadratic (LQ) equation, a good fit to the data was ob served for dose levels above 1 Gy; however, a region of enhanced sensi tivity was observed at doses less than this. One possible explanation of this low-dose substructure is that a small, sensitive subpopulation of cells is selectively killed at low doses. Modeling of the radiatio n response with a two-population LQ model suggests that for these data this explanation is unlikely. Another possibility is that the whole c ell population is initially hypersensitive, becoming radioresistant as damage is sustained by the cell. Conceivably this radioprotective mec hanism could act in one of two ways. The cell could move from a radiat ion-sensitive to a radiation-resistant state by a continuous function of dose, or alternatively, only after a sufficient accumulation of dam age, i.e. a ''triggering dose.'' Both of these possibilities have been explored in the results of fitting two ''induced resistance'' models.