ductive activity in undifferentiated cells. It should be noted that thousands
of grays are needed to kill differentiated cells, whereas only hundreds of
grays are needed for undifferentiated cells.
Typical cell survival curves are shown in Figure 15.11. For high-LET radi-
ations such as a-particles and low-energy neutrons, the survival curves are
nearly a straight line starting from the lowest doses. In contrast, for low-
LET radiations (e.g., x- and g-radiations), the survival curve exhibits an
initial shoulder, followed by a straight line. This straight line portion on the
semilog plot is an exponential curve on a linear plot. This curve based on a
multitarget modelis characterized by three parameters:D 0 (dose at which
37% of cells survive), the extrapolation number n, and the quasithreshold
dose Dq, and they are related by the expression
logen=Dq/D 0 (15.1)
The quasithreshold dose,Dq, is the dose given by the width of the shoul-
der of the curve. The Dqindicates that, at low doses, almost all cells repair
Cell Survival Curves 239Fig. 15.11. Typical cell survival curves. The cell survival curve for low linear energy
transfer (LET) radiations shows a shoulder of width Dq, which is called the qua-
sithreshold dose. After Dq, the plot becomes linear on a semilog scale, indicating an
exponential dose–response relationship. The extrapolation number nis obtained by
extrapolating the linear portion of the curve back to the ordinate.D 0 is the dose
obtained from the slope of the linear portion of the curve, at which 37% of the cells
survive. The survival curve for high-LET radiations shows no or little shoulder, indi-
cating Dqto be zero and nto be unity.