Factors Affecting Radiosensitivity
As already mentioned, various factors affect the radiation damage in the
cell and hence the survival curve. The dose rate, the LET of the radiation,
the presence of chemical molecules and the stage of the cell cycle all affect
the survival curve.
Dose Rate
The dose rate, that is, the delivery of dose per unit time, is an important
factor in cellular damage. The higher the rate of dose delivery, the greater
will be the cell damage. At low-dose rates, only single-strand breaks of DNA
molecules occur, and so cells have time to repair, whereas at high-dose
rates double-strand breaks occur, and so repair is less likely to occur
because of the shorter time available to the cells between ionizing events.
Figure 15.13 illustrates the effects of two dose rates on the cell survival
curve. The dose-rate effect is very important in radiation therapy, because
unless an appropriate dose rate is prescribed, intended therapeutic effect
may not be achieved. When a total dose is given to a patient in fractions
over a period of time, it should be kept in mind that the interval between
fractional doses should be short enough to keep repair of damage to abnor-
mal cells to a minimum.
Factors Affecting Radiosensitivity 241Fig. 15.13. The cell survival curves indicating the effect of dose rates. At high dose
rates, the shoulder of the curve is reduced, with smaller values of Dq. The opposite
is true at low dose rates.