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 phaseand the most sensitive in the M phaseof the cell cycle. This is described by the law of Bergonieand Tribondeau, formulated in 1906:

"X-raysare more effective on cells which have a greater reproductive activity."

From their observation, they concluded that quickly dividing tumor cellsare generally more sensitive than the majority of body cells. This is not always true. Tumor cells can be hypoxicand therefore less sensitive to X-rays that mediate most of their effects through free radicalsproduced 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 metabolicallyactive. Amongst the body cells, the most sensitive are spermatogoniaand erythroblasts, epidermal stem cells, gastrointestinal stem cells. The least sensitive are nerve cellsand muscle fibers.

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

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 stohastic.

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 in 60 days), above 5 Sv the majority of people die in 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 are coincidental and cannot be avoided. They don't have a threshold. These can be divided into somatic and genetic. Among the somatic ones, the canceris the most important. It develops because radiation causes DNAmutationsdirectly 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 waterand oxygen radiolysis. The genetic confer the predisposition to cancer to the offspring. They are not very well understood.

See also

• LNT model, Linear no-threshold response model for ionizing radiation

Reference

• Bergonie J. and Tribondeau L. (1906). De quelques resultats de la radiotherapie et essai de fixation d'une technique rationnelle. Comptes Rendus des Seances de l'Academie des Sciences 143, 983-985.

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It uses material from the http://en.wikipedia.org/wiki/Radiosensitivity Wikipedia article Radiosensitivity.