Low-dose rate irradiation.

There are a number of rather obvious conclusions from these experimental data that have implications in clinical radiotherapy: (1) Survival curves for cells of human origin, in general, have a smaller initial shoulder and exhibit greater sensitivity to ionizing radiation than cells of rodent origin. (2) Nevertheless, the response of human cells to gamma-rays, in a fractionated or low-dose schedule, is dominated by repair. (3) Radiosensitivity, as well as the repair of sublethal damage as evidenced by the dose rate effect, is very similar for a range of normal human cells, including skin and lung fibroblasts, as well as endothelial cells from umbilical cord veins. (4) Human tumor cells are much more variable than normal cells in radiosensitivity, and in the repair of sublethal damage, evident in the dose rate effect. Some show a smaller, and some show a larger dose rate effect than normal cells. (5) Consequently, the use of a large number of fractions, as for example in hyperfractionation regimens, is contraindicated in those tumors that show repair factors larger than for normal tissues; though of course it is a substantial advantage in other cases.