Does Ionizing Radiation Cause Cancer?

Ionizing radiation has been shown to induce (cause) cancer in many different species of animals and in almost all parts of the body. It is one of the few scientifically proven carcinogens (cancer-causing agents) in human beings, although it appears to be a relatively weak carcinogen compared to many chemical agents. Many years may elapse between the radiation exposure and the appearance of the cancer.

The types of cancer that can be caused by radiation can also occur naturally (without increased exposure to radiation), but some occur more frequently as a result of radiation. For example, a higher percentage of small cell lung cancers occur in uranium miners as a result of exposure to alpha radiation (a form of highly ionizing radiation).

Organs differ in their sensitivity to the effects of radiation. The thyroid gland and bone marrow are most sensitive to radiation, while the kidney, bladder, and ovary seem to be least affected. Some forms of leukemia, a type of cancer that arises in the bone marrow, appear to be the most common radiation-induced cancers.

Evidence that ionizing radiation causes cancer comes from studies of atomic bomb survivors in Japan, persons exposed to large amounts of x-rays, and from certain occupational exposures, such as workers with lung exposure to alpha radiation. These studies, however, generally involved relatively high-dose exposure - greater than 10 centigray. (A centigray (cGy) is a standard unit of radiation dose.) Therefore, the risk estimates for lower doses of radiation have to be estimated from the high-dose data, and may not be accurate.

Dose-Related Radiation Effects

Ionizing radiation is probably the only carcinogen with evidence that its effects are related to dose exposure. The probability that cancer will result from radiation exposure increases as the dose increases. However, there is no evidence to suggest that the grade (tendency to grow and spread) of the resulting cancer is affected by the dose. In other words, higher doses of radiation do not cause more aggressive cancers.

Low-Dose Radiation Exposure

A number of studies over the past 20 years have looked at the impact of environmental radiation exposure in the dose range of 10 cGy or less. Careful analysis of this research revealed no significant increase in the incidence of all cancers combined, or of cancers in specific parts of the body. Research in this area is continuing.