Radiosensitivity

Radiosensitivity is the relative susceptibility of cells, tissues, organs or organisms to the harmful effect of ionizing radiation. Cells are least sensitive when in the S phase, then the G1 phase, then G2 phase and the most sensitive in the M phase of the cell cycle. This is described by the law of Bergonié and Tribondeau, formulated in 1906 :

X-rays are more effective on cells which have a greater reproductive activity.

From their observation, they concluded that quickly dividing tumor cells are generally more sensitive than the majority of body cells. This is not always true. Tumor cells can be hypoxic and therefore less sensitive to X-rays that mediate most of their effects through free radicals produced by ionizing oxygen.

Later it has been shown that the most sensitive cells are those that are undifferentiated, well nourished, divide quickly and are highly metabolically active. Amongst the body cells, the most sensitive are spermatogonia and erythroblasts, epidermal stem cells, gastrointestinal stem cells. The least sensitive are nerve cells and muscle fibers.

Very sensitive cells are also oocytes and lymphocytes, although they are resting cells and do not meet the criteria described above. The reasons for their sensitivity are not clear.

Cell damage classification
The damage of the cell can be lethal' (the cell dies) or sublethal (the cell can repair itself). The effects on cells can be, according to ICRP, deterministic and stochastic.

Deterministic effects
Deterministic effects have a threshold of irradiation under which they do not appear and are the necessary cosequence of irradiation. The damage they cause depends on the dose: they are sublethal from 0,25 to 2 Sv (a less pronounced form of disease), lethal from 2 to 5 Sv (a certain percent of population dies within 60 days), above 5 Sv the majority of people die within 60 days and above 6 to 7 all people die. Of course, these effect depend also on many other factors, like age, sex, health etc.

Stochastic effects
Stochastic effects are coincidental and cannot be avoided. They don't have a threshold. These can be divided into somatic and genetic. Among the somatic effects, secondary cancer is the most important. It develops because radiation causes DNA mutations directly and indirectly. Direct effects are those caused by ionizing particles and rays themselves, while the indirect are those that are caused by free radicals, generated especially in water radiolysis and oxygen radiolysis. The genetic effects confer the predisposition to cancer to the offspring. The process is not well understood.