Radiosensitivity is the relative susceptibility of cells, tissues, organs or organisms to the harmful effect of ionizing radiation.
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Cells types affected
Cells are least sensitive when in the S phase, then the G1 phase, then the G2 phase, and 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 observations, 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 because most of their effects are mediated by the free radicals produced by ionizing oxygen.
It has meanwhile been shown that the most sensitive cells are those that are undifferentiated, well nourished, dividing quickly and highly active metabolically. 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.
There also appears to be a genetic basis for the varied vulnerability of cells to ionizing radiation. This has been demonstrated across several cancer types and in normal tissues.
Cell damage classification
The damage to the cell can be lethal (the cell dies) or sublethal (the cell can repair itself). The effects on cells can be, according to the International Commission on Radiological Protection, either deterministic or stochastic.
Deterministic effects
Deterministic effects have a threshold of irradiation under which they do not appear and above which they are the necessary consequence 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 the population dies within 60 days). Above 5 Sv most people die within 60 days, and above 6 to 7 Sv all people die. Of course, these effect depend also on many other factors, like age, sex, and health.
Stochastic effects
Stochastic effects do not have a threshold of irradiation, are coincidental, and cannot be avoided. They can be divided into somatic and genetic effects. 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 effects are those that are caused by free radicals, generated especially in water radiolysis and oxygen radiolysis. The genetic effects confer the predisposition of cancer to the offspring. The process is not well understood.