Radiation Exposure and Lifetime Cancer Risk in Middle Age Studied
Lifetime cancer risks stay fairly constant for adults exposed to radiation in middle age, compared to those exposed as younger adults, a detailed analysis shows. The findings are described in a study from Columbia University Medical Center published online in the Journal of the National Cancer Institute. The study found that the estimated cancer risk for a 50-year-old exposed to ionizing radiation is about twice that of earlier estimates, a finding that could have practical implications regarding X-ray diagnostic tests and CT scans, which are predominantly performed on middle-aged adults. The findings should also be of particular importance for people in occupations that involve radiation, where most exposures are generally in middle age, too. “Practically speaking, there could be considerable societal consequences if the excess lifetime cancer risks for radiation exposure in middle age are higher than previously estimated,” say study authors Dr. Igor Shuryak, an associate research scientist at Columbia University Medical Center and David J. Brenner, the Higgins Professor of Radiation Biophysics at College of Physicians & Surgeons of Columbia University. “A common source of radiation exposure in middle age is from diagnostic radiology. Exposure to ionizing radiation from medical sources has increased sixfold in the past three decades in the U.S. population, mostly because of the rapid increase in computerized tomography imaging.” Until now, conventional wisdom has suggested that the older someone is when exposed to radiation, the less chance there is of developing cancer, according to Drs. Shuryak and Brenner. Long-term studies of atomic bomb survivors in Japan, however, provide statistical evidence for the opposite view, which indicates that the risk of developing radiation-caused cancers remains as high for people exposed after age 40 as for people exposed earlier in adulthood. Because these epidemiological findings were inconsistent with standard models of how radiation causes cancer, they had been widely overlooked, they said. To explore this issue, Dr. Brenner and Rainer Sachs at the University of California at Berkeley created a model of cancer risk assuming two different but well established pathways through which radiation exposure can ultimately lead to cancer. The first is initiation of gene mutations that convert normal stem cells to premalignant cells which could eventually lead to cancer; the second is radiation-induced promotion, or “expansion,” of the number of existing premalignant cells in the body. In their model, the initiation effect plays a greater role in children than in adults because cells initiated at an early age have a longer time available to expand in number and progress on the pathway to cancer. The promotion effect, on the other hand, is more important for radiation exposures in middle age, because the adult body already contains larger numbers of premalignant cells. The researchers first applied the model to the Japanese atomic bomb survivor data and were able to reproduce the cancer risk patterns associated with age at radiation exposure that were observed in the Japanese atomic bomb survivors. They then applied the same model to predict cancer risks as a function of age in a U.S. population captured in the latest National Academy of Sciences’ BEIR (Biological Effects of Ionizing Radiation) report. As with the Japanese atomic bomb survivors, the researchers found that the predicted cancer risks do not decrease with increasing age of exposure in the age range from about 30 to 60. “This finding is very different from current ‘best’ radiation risk estimates that predict sharply decreasing risks over this range of exposure ages,” Dr. Brenner says.