Okay, all this theory and molecular biology is great—at least I think it is—but what you really want to know is, what are the chances you will get cancer if you are exposed to radiation, right? And how do we know what the chances are?

Scientists have developed methods to study the transformation of normal cells growing in culture into cells that have a malignant pattern of cell growth— known as transformed cells—but the problem with these experiments is that they depend on mouse cell lines. This is because it is very hard to transform human cells into cancerous cells. These kinds of experiments can be used to compare the relative ability of different physical and chemical agents to transform cells, however. Certain polycyclic aromatic hydrocarbons,3 for example, turn out to be much better than radiation at transforming cells (26). Even so, the results of these transformation experiments cannot be used to determine the quantitative, dose-dependent carcinogenic potential of radiation for humans. Mice are also useful to study certain aspects of carcinogenesis, but mouse studies are also not capable of determining the risk of a certain dose of radiation for humans.

The most reliable animal studies for determining human risk of cancer are on dogs. Scientists at Colorado State University did a life-span study of beagle dogs to determine the risk of cancer when dogs were irradiated in utero at various stages of development or at different ages after birth. Relatively large doses of 16 or 83 rads of у rays were given to the dogs, and they were followed through their life­times. Young dogs developed a variety of cancers after irradiation in late fetal and neonatal stages. Dogs irradiated as juveniles also developed thyroid cancers (27, 28). While these studies show that dogs are a good model for certain human cancers, they are not sufficient to establish the risk of cancer to humans from a certain dose or radiation.

So how do we know the risk? The only way to know is to actually study human populations, but of course it is unethical to irradiate humans to see if they get cancer. However, various groups of people have been exposed to radiation for different reasons: people who have been treated with radiation for various condi­tions for a long period; many Japanese who survived the bombing with atomic weapons in World War II; people who were in the vicinity of atmospheric testing of nuclear weapons in the 1950s and 1960s; people who were exposed from the Chernobyl nuclear accident; and uranium miners. There are many other groups exposed either accidentally or during the course of their work. The studies of can­cer incidence in these populations have been reviewed recently in a report by the National Academy of Sciences National Research Council (BEIR VII)(29).

In the 1930s and 1940s, thousands of patients in Great Britain with ankylos­ing spondylitis, a painful disease of the spine, were given radiation to reduce the pain, and some of them later developed leukemia. Thousands of women in the United States and Canada had high doses of radiation to the chest from fluoros­copy procedures, and later some of them developed breast cancer. Up until the 1950s, children emigrating from North Africa to Israel were treated for ringworm by irradiating the scalp, and some of these children later developed leukemia or thyroid cancer. Uranium miners breathing in radon—along with smoking, a con­founding factor—developed lung cancer. Some Marshall Islanders developed thy­roid cancer after exposure to 131I from thermonuclear bomb testing in 1954, and thousands of kids developed thyroid cancer after the Chernobyl nuclear power accident in 1986 (7). But by far the most useful data come from an extremely careful long-term epidemiological study of the survivors of the atomic bombs dropped on Hiroshima and Nagasaki, Japan, in 1945.