A421 Radiationinduced cancer

There is substantive evidence that almost all cancers originate from a single cell. However, single changes in the cell genetic code are usually insufficient to initiate a cancer. Several cell mutations (two to seven) are required in the carcinogenesis process from pre-neoplasia to cancer. Radiation may act at several stages of the process, but it seems to have a major role in the initial conversion of the cell to a pre-neoplastic state. A pre-neoplastic cell is immersed in an environment of normal cells, which tend to suppress and constrain pre-neoplastic properties. Overcoming these constraints results in a cancer.

Cancer may be triggered by many factors such as smoke, chemical agents etc., and it is therefore impossible to determine whether radiation is the cause of a particular type of cancer or not. The only way to ascertain a correlation between radiation and cancer induction is statistical. Epidemiology is the study of the distribution of diseases among people, and it is still an observational rather then experimental science: therefore bias or confounding factors are highly probable. In the present context, the so-called Life Span Study (LSS) is an ad hoc study on survivors of Hiroshima and Nagasaki which has produced a significant amount of data on effects of exposure to radiation on humans. Studies of people partially exposed to radiation due to medical investigations or treatments are another source of data, together with information available from studies of occupational exposures, i.e., in the Mayak facility in Russia, and the Chernobyl accident [UNSCEAR, 2000].

From a general point of view, linear (or linear-quadratic) no-threshold dose response is to be expected, even though for certain cancers and at low doses correlations are less precise about it. Some interesting results are those for solid cancers obtained by the Life Span Study (LSS) where EER (excess relative risk) (Figure A.5) and EAR (excess absolute risk) (Figure A.6) are estimated. EER and EAR represent the increased cancer rate in an exposed group relative to an unexposed group, measured on relative and absolute scales. An EER of 1 corresponds to a doubling of the cancer rate. EAR may be expressed as the number of excess cases of cancers per, for example, 10,000 persons. They can be expressed per unit dose or per a specific dose (i.e., 1 Sv) [UNSCEAR, 2000].

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