Dose fractionation

Experiments in radiation biology have found that as the absorbed dose of radiation increases, the number of cells which survive decreases. They have also found that if the radiation is fractionated into smaller doses, with one or more rest periods in between, fewer cells die. This is because of self-repair mechanisms which repair the damage to DNA and other biomolecules such as proteins. These mechanisms can be over expressed in cancer cells, so caution should be used in using results for a cancer cell line to make predictions for healthy cells if the cancer cell line is known to be resistant to cytotoxic drugs such as cis-platin. The DNA self repair processes in some organisms is exceptionally good; for instance, the bacterium Deinococcus radiodurans can tolerate a 15 KGy (1.5 MRad) dose.

In the graph below, called a Cell survival curve, the dose vs. surviving fraction have been drawn for a hypothetical group of cells with and without a rest time for the cells to recover. Other than the recovery time partway through the irradiation, the cells would have been treated identically.

The human body contains many types of cells, and the human can be killed by the loss of a single type of cells in a vital organ. For many short-term radiation deaths due to what is commonly known as radiation sickness (3 days to 30 days after exposure), it is the loss of bone marrow cells (which produce blood cells), and the loss of other cells in the wall of the intestines, which is fatal.