Relative contribution of bystander and targeted cell killing to the low-dose region of the radiation dose-response curve.

Human keratinocytes show a bystander effect when exposed to low doses of low-LET radiation. In this paper, data are presented showing a method of correcting the overall survival curve to enable analysis of the relative contributions of the bystander effect and the effect attributable to direct interaction of the radiation with the target cell. The technique used is to obtain a standard clonogenic survival curve using the assay of Puck and Marcus and, with a different set of flasks containing cloning densities of unirradiated cells, to assay the cell killing caused by medium harvested from 2 x 10(5) cells irradiated with the same doses. The data show that for this human epithelial cell line, doses of 0.01-0. 5 Gy show clonogenic death by the bystander effect only, if maximum potential bystander killing is assumed. The magnitude of the effect is relatively constant, and it appears to saturate at doses in the range of 0.03-0.05 Gy. After doses greater than 0.5 Gy, the curves for clonogenic death are the result of a dose-dependent non-bystander effect and a dose-independent bystander effect. If these particular dose-response effects occur in epithelial cells in vivo, they may have important consequences for therapy and studies of low-dose risk.