Harvard Reports on Cancer Prevention
Volume I: Human Causes of Cancer
Cancer Causes & Control:
An International Journal of Studies of Cancer in Human Populations
Official Journal of the International Association of Cancer Registries
Volume 7 Supplement November 1996 ISSN 0957-5243

Radiation

Introduction
Just 100 years ago, Wilhelm Conrad Roentgen discovered the X-ray and revolutionized the practice of medicine. Fifty years later, World War II was brought to an end after atomic bombs were dropped on Hiroshima and Nagasaki. By 1957, the first commercial nuclear power plant began operating, and now nearly 20 percent of the electricity produced each year in the United States is from nuclear energy. Over the course of the century, radiation has become pervasive in our world, sometimes with deleterious consequences. The Chernobyl (Russian Federation) nuclear reactor accident occurred in 1986 and spewed radioactivity around the world. The trapping of radon gas in our homes might increase the risk of lung cancer. We live in a sea of low-level invisible radiation. Odorless and colorless, a multitude of ionizing radiations continually bombards our bodies throughout life, and it is the release of ionizing energy cells that can cause cancer.

We know much about radiation risks. Comprehensive epidemiologic studies have been conducted with Japanese atomic bomb survivors, patients given radiotherapy for benign and malignant conditions, workers exposed on the job, populations living in areas of enhanced environmental radiation, and others. Both the United Nations and the US National Academy of Sciences periodically publish authoritative volumes on the effects of radiation.^1,2 No other environmental carcinogen, with the possible exception of tobacco, has been studied as extensively; yet, there remains a public mystique about radiation that tends to exaggerate the actual hazard. While radiation is considered a universal carcinogen, it is a relatively weak one, in part because it is such an effective cell killer. The average per capita dose from all sources of radiation is about 3.4 mSv (millisieverts) per year, of which natural sources (such as radon, cosmic rays, uranium, and potassium-40) contribute 2.9 mSv per year, or about 88 percent of the total. The remaining 12 percent is derived primarily from medical exposures (0.5 mSv per year). Based on linear extrapolation from data on people exposed to high doses of radiation (500 to 2,000 mSv), it can be inferred that only a small fraction, about one to three percent, of all cancers might be attributable to radiation arising largely from natural sources, in contrast to the approximate 30 percent attributable to tobacco use. Lowering the risk of radiogenic cancer within the population can come mainly by exposure avoidance, such as reducing environments with high radon levels and minimizing unnecessary medical exposures.

Radon
Based on studies of underground miners exposed to very high radon levels, it is estimated³ that approximately 10 percent of all lung cancer deaths (or 14,400 per year) might be attributable to indoor radon. Because the entire population of the US, over 250 million citizens, breathes in radon with every breath, even a small risk can translate into large numbers of estimated deaths. Since most population exposure to radon is from very low levels, clean-up of all homes above 4 pCi/L (picocurries)—the level at which the US Environmental Protection Agency recommends intervention—would reduce the lung cancer risk attributable to radon from 10 percent to only six percent or eight percent. The main uncertainty in such estimates, however, is whether an underground mine is representative of an above-ground home. Miners are exposed to silica dust, arsenic, diesel fumes, blasting smoke, and other lung irritants or carcinogens that conceivably could exacerbate the effects of radon on lung cancer. Many miners were heavy cigarette smokers and convincing evidence suggests that radon and tobacco smoke interact to cause more lung cancers than expected from the sum of their individual effects.

Indoor radon studies have not been very informative, despite comprehensive measurement assessments. Studies in Sweden have suggested elevated risk while equally rigorous investigations in China, Canada, and Missouri (USA) find no evidence of excess risk. A recent incidence study of lung cancer among nonsmoking women in Missouri estimated that less than two percent of all lung cancers could be attributable to radon levels greater than 4 pCi/L, which was less than the risk attributable to passive smoking (estimated at 6.1 percent) and other factors.⁴ Because the increased risk with exposures of 4 pCi/L is very low - on the order of a 10 percent to 20 percent increase—the published studies, while negative overall, are not powerful enough to reject the possibility that estimates of risk from miner studies are valid.⁵

In terms of risk comparisons, the risk of lung cancer is increased by about five times by smoking from one to nine cigarettes per day, which, based on the miner data, would be equivalent to an exposure to indoor radon of approximately 120 pCi/L on average for 30 years. Average radon levels are about 1 pCi/L indoors and 0.2 pCi/L outdoors. Levels greater than 100 pCi/L are extremely rare and occur in only 0.0002 percent of US homes. Because of the enhanced interaction between radon and tobacco smoke, the best way to reduce your presumed risk of radon-induced lung cancer is to stop smoking and/or reduce the level of environmental tobacco smoke.

Medical X-rays
In 1977, the US National Cancer Institute held its first conference intended to reach consensus on recommendations on mammography screening of asymptomatic women for the early detection of breast cancer. Since that time, there have been notable improvements in the imaging capabilities of the X-ray units and an appreciable lowering of radiation dose to breast tissue. The controversy today is not whether the radiation exposures are hazardous, but whether younger women, under the age of 50 years, benefit from the exposures. A reduction in breast cancer mortality of 30 percent has been demonstrated convincingly in randomized trials of women over age 50 who received screening examinations that included mammographic X-rays. The benefit for younger women is less clear, but apparently lower. The possible hazard from mammographic X-rays is very low and should not be a factor in individual decisions to undergo this procedure.

The same is true for most diagnostic X-ray procedures. Nearly all studies of women exposed to radiation find that risk of induced breast cancer decreases appreciably with age at the time of exposure, and most studies fail to find a significant increase when exposures occur past the menopausal age.^6-8 Despite recent claims that medical radiation is responsible for up to 75 percent of all breast cancers in the US,⁹ a more reasonable estimate is that less than one percent of all breast cancers might be attributed to medical uses of radiation. For an individual woman, her lifetime risk might increase from 9.09 percent to perhaps 9.18 percent.^10,11 It seems likely that this small adverse effect is more than offset by the benefit of the diagnostic procedure. Nonetheless, unnecessary radiation exposures should be avoided, and continued vigilance is required to ensure that the benefits associated with specific procedures outweigh the future risks.

Nuclear power plants, Chernobyl, and Chelyabinsk
Comprehensive surveys in several countries, including the US, have failed to find increases in cancer risk associated with living in areas with nuclear power plants or facilities. This would be as expected if radiation releases during normal operations were as low as those reported, that is, much lower than natural background exposures.¹² The reactor accident at Chernobyl, however, was a major disaster. Radiation contaminated the environment surrounding the plant, and hundreds of thousands of workers came to clean up the radioactive debris. The accident resulted from serious mistakes made by the operators; the release of radioactive gas was massive because of the absence of a containment vessel, a common safety feature in the West. It is yet too early to expect much increase in tumorous cancer rates within exposed populations. While the latent period for leukemia is usually shorter, there have been no reported increases in childhood or adult leukemia among any exposed group.¹³ The only possible radiation effect to date appears to be a remarkable increase in childhood thyroid cancer in adjacent Belarus. The association with radioactive I-131 remains uncertain, however, because dose-response relationships have yet to be reported, and the increase occurs much earlier than expected based on current understanding of radiogenic thyroid cancer. It is also possible that increased surveillance may have contributed to detecting and reporting tumors.¹⁴

Recently, it was disclosed that high levels of radioactive waste were dumped into the Techa river from the Chelyabinsk nuclear facility in the former Soviet Union.¹⁵ Over 80 villages were evacuated but only after the population ingested or inhaled large amounts of radioactive substances. In the haste to develop a nuclear capability, radiation workers at the Mayak reactor plant also received large cumulative exposures. Ongoing studies of workers and the surrounding population in Chelyabinsk, Russia, might contribute new information on the effects of chronic exposures to low levels of radiation that accumulate to large doses over time.

Genetic predisposition
Except in rare instances, it is not known whether an individual's genetic profile places him or her at unusually high (or low) risk of developing radiogenic cancers.^16,17 One exception is heritable germline retinoblastoma (RB), a rare childhood cancer of the eye. An affected child inherits a defective gene in one allele of the RB tumor suppressor gene and then acquires a new mutation in the opposite allele. Such individuals are at high risk of developing bone cancer, which is further increased following radiotherapy.¹⁸ Ataxia telangiectasia (AT) is another rare disorder associated with a high incidence of lymph cancers. Nonaffected gene carriers appear to be at increased risk for breast cancer and possibly for radiation-induced breast cancer. The recent cloning of the AT gene¹⁹ should spur on research to identify nonaffected gene carriers in the general population and to evaluate their radiogenic cancer risk from mammographic screening and other factors. The question of genetic predisposition is an area of active research, and it could have significant impact on our understanding of radiation risk.

Conclusion
Radiation can cause cancer, but the level of risk is much lower than perceived by the general public.²⁰ Most people exposed to radiation do not develop cancer. Even among the 100,000 Japanese atomic bomb survivors, only 400 to 500 cancer deaths (about one percent) of the over 40,000 deaths from all causes could be attributed to radiation. Reducing high levels of radon makes sense and unnecessary X-rays should be avoided. On the other hand, some radiation control measures are extremely costly and reduce risk only slightly.²¹ Understanding radiation risks and comparing them with other hazards of life should help society and individuals make informed decisions about medical and technological uses.

Major cancers related to radiation

Suggestions

Suggested Further Reading

References