Studies on biological response to ionizing radiation at low doses have been a hot spot recently since relevant methods have become available that allow accurate measurement of clonogenic survival fraction at very low levels of cell killing. The results of using these methods show that the linear-quadratic (LQ) equation can not describe the cell-survival curve in the low dose region correctly. The surviving data suggest that in human tumor cell lines increased cell killing per unit dose occurs when the doses are blow 1 Gy. This phenomenon is termed low dose hyper-radiosensitivity (HRS), and it is followed by increased radiation resistance (IRR) above a certain dose. The purpose of this study was to examine the low-dose sensitivity of human hepatoma SMMC-7721 cells exposed to low-LET γ-rays.
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