John D. Boice, Sc.D.
From the Radiation Epidemiology Branch, Division of Cancer Etiology,
National Cancer Institute, Bethesda, Maryland
(Original article here...)
Ionizing radiation is a known cause of cancer and other adverse effects. It is one of the most extensively studied human carcinogens and may account for about 3 percent of all cancers (NAS, 1990). Ionizing radiation is able to remove electrons from atoms and to change the molecular structures of cells. It is these cellular changes that may cause cancer to develop. The genetic DNA in the cell nucleus is thought to be the critical target for radiation-induced damage.
Some radiation comes from natural sources, such as that from cosmic rays and radioactive substances in the earth's crust. Each of us is exposed to this "background" radiation at a rate of about 1 to 2 mGy per year. (The gray [Gy] is a unit of measurement for the amount of radiation energy absorbed by body tissues. It is equal to 100 rad and is now the unit of dose.)
Very high doses of radiation (tens of gray) received all at once may be fatal, but if spread out over a period of time, a high dose of radiation may be less damaging to healthy tissues. A single, 5-Gy dose of whole-body irradiation, for example, would cause about half the people exposed to it to die within 30 days. But patients who receive daily radiation therapy treatments of 2 Gy directed to a small area of the body can be exposed to tens of gray over a period of weeks. It is difficult to measure the effects of low-level radiation (less than .25 Gy) from common sources like medical X-rays.
Some of what we now know about the effects of radiation exposure was learned by studying patients treated with radiation in the past. Some of the first quantitative evidence that radiation causes cancer came from studies of patients who received radiation therapy for ankylosing spondylitis, a spinal disorder, before 1954 (Darby et al., 1987). These patients were found to have more leukemia and cancers of the lung, esophagus, bone, and other organs within the irradiated field than would be expected in a healthy population. Second cancers following radiation treatments are rare but do occur. The most extensive survey, conducted among patients with cervical cancer, concluded that, at most, 5 percent of second cancers could be attributed to radiation therapy (Boice et al., 1988).
Between 1935 and 1954, fluoroscopy, an X-ray procedure, was used to monitor treatment of patients with tuberculosis; the women who were thus treated could receive an average dose of 1 Gy to the breast. Ten to 15 years later, these women had a high incidence of breast cancer (Boice et al., 1991).
There is a high risk of thyroid cancer many years after childhood exposure to radiation for therapy of noncancerous conditions of the head and neck. Exposure during childhood can be particularly damaging because rapidly growing cells may be more sensitive than slower-growing cells irradiated later in life. Before 1950, individuals with enlarged thymus glands were treated with intense radiation. An elevated risk of thyroid cancer and leukemia has since been found in this population (Shore et al., 1985). Similar increases have been observed following childhood irradiation for ringworm of the scalp (Ron et al., 1989). Those who may have received radiation treatments for thymus conditions or other childhood problems, such as scalp ringworm or enlarged tonsils, should alert their physicians.
Because the dangers of radiation were not recognized before the 1920s, radiologists who used X-rays extensively--and without shielding--in the early years of the century developed leukemia at excessive rates.
A classic study of women who painted radium dials before 1930 showed high rates of bone sarcomas and head cancers. They had swallowed large quantities of radioactive radium by licking their paint brushes to make fine tips; it has been estimated that the average dose reaching their bone tissues was very high--on the order of 17 Gy.
Much of our information about the effects of radiation comes from studies of atomic bomb survivors in Japan, among whom have been found increased rates of leukemia and cancers of the breast, thyroid, lung, stomach, and other organs (NAS, 1990). Female survivors who received a single dose of radiation from the blast were found to be at the same risk for breast cancer as women with tuberculosis who had repeated fluoroscopy exposures over a 3- to 5-year period. This suggests that in the case of breast cancer--but not necessarily other cancers--repeated small doses over the years may be as hazardous as a single, large dose. The risk, however, seemed to be inversely correlated to the age at exposure to the blast, with no apparent increased risk in women over the age of 40.
Today, many women at risk of developing breast cancer are periodically examined with low-dose breast X-rays known as mammography. For high-risk women, particularly over age 50, the benefits of detecting cancer early far outweigh the small risk of developing cancer from repeated mammograms.
While exposure to low levels of radiation before birth is associated with the development of cancer during childhood, especially leukemia (Bithell and Stewart, 1975), not all researchers are convinced that prenatal irradiation is the cause of childhood cancer. Individuals exposed prenatally during the atomic bomb blasts in Japan do not have higher cancer rates. The current practice is to use ultrasound, rather than X-rays, during pregnancy whenever possible.
There are other environmental and occupational exposures to radiation. Radioactive fallout, for example, is produced during nuclear weapons tests when airborne radioactive particles settle to the ground. One study showed that persons accidentally exposed to very high levels of fallout had an increased risk of thyroid cancer (NAS, 1990).
Uranium miners inhale radioactive radon gas produced underground by the natural decay of uranium and have high rates of respiratory cancer (Lubin et al., 1994). It is possible that an interaction between inhaled radioactive gas and smoking enhances the risk from the radon exposure. Radon is becoming a public health concern in some locations because of its presence in groundwater and building materials. Based on extrapolations from studies among underground miners, it has been estimated that as many as 10 percent of all lung cancer deaths may be related to indoor radon exposures (Lubin and Boice, 1989).
In general, the breast, thyroid, and bone marrow are most sensitive to the effects of ionizing radiation. There may be a minimum lag time after exposure of about two years before leukemia develops, and 10 to 15 years before other cancers develop.
Avoidance of unnecessary medical X-rays is one of the best ways to reduce exposure to ionizing radiation. However, in many instances, the benefits outweigh the risks, as in mammography for some women, as a tool for diagnosis of various diseases or injuries, and as an effective way to treat some cancers.
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