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 observed for dose levels above 1 Gy; however, a region of enhanced sensitivity 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 radiation response with a two-population LQ model suggests that for these data this explanation is unlikely. Another possibility is that the whole cell population is initially hypersensitive, becoming radioresistant as damage is sustained by the cell. Conceivably this radioprotective mechanism could act in one of two ways. The cell could move from a radiation-sensitive to a radiation-resistant state by a continuous function of dose, or alternatively, only after a sufficient accumulation of damage, i.e. a "triggering dose." Both of these possibilities have been explored in the results of fitting two "induced resistance" models.

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