A mathematical model resolving normal human blood lymphocyte population x-ray survival curves into six components: radiosensitivity, death rate and size of two responding subpopulations.

The analysis was based on observations of survival decrease as a function of dose (range 0-5 Gy (= 500 rad) and time after irradiation in vitro. Since lymphocyte survival is also sensitive to culture conditions the effects of radiation were examined daily up to 3 days only, while survival of control cells remained ca. 90 per cent. The time-dependent changes were resolved as the death rates (first-order governed) of lethally-hit cells (apparent survivors), so rendering these distinguishable from the morphologically identical, true (ultimate) survivors. For 12 blood donors the estimated dose permitting 37 per cent ultimate survival (D37 value) averaged 0.72 +/- 0.18 (SD) Gy for the more radiosensitive lymphocyte fraction and 2.50 +/- 0.67 Gy for the less radiosensitive, each fraction proving homogeneously radiosensitive and the latter identifying substantially in kind with T-type (E-rosetting lymphocytes). The half-life of lethally-hit members of either fraction varied widely among the donors (ranges, 25-104 hours and 11-40 hours, respectively). Survival curves reconstructed by summating the numerical estimates of the six parameters according to the theoretical model closely matched those observed experimentally (range in multiple correlation coefficient, 0.9709-0.9994 for all donors). This signified the absence of any additional, totally radioresistant cell fraction.