HYPERSENSITIVE RESPONSE OF NORMAL HUMAN LUNG EPITHELIAL-CELLS AT LOW RADIATION-DOSES

The effect of very low single X-ray doses (0.05-4 Gy) was investigated in a human lung epithelial cell line (L132). Cell survival measuremen ts were made using a Dynamic Microscopic Imaging Processing Scanner (D MIPS), which allowed single cells to be located accurately, their posi tions recorded and these positions revisited after an appropriate incu bation period at 37 degrees C; surviving cells were identified by thei r ability to produce a colony greater than or equal to 50 cells. The s urvival data at doses greater than or equal to 2 Gy were well-fitted b y a linear-quadratic (LQ) model. For doses < 1 Gy, increased X-ray eff ectiveness was observed with cell survival below the prediction from t he fit of the LQ model to the higher dose data, extrapolated into the low dose region. This is the first evidence for the existence of a hyp ersensitive survival response to very low doses in normal human cells. The transition between the low dose hypersensitive region and greater resistance at higher doses, could result from induced radioresistance which requires a threshold of radiation-induced damage before being t riggered.