Several years after the blasts, researchers began tracking rates of disease and death among more than 120,000 survivors. The results demonstrated, for the first time, that the cancer risk from radiation depends on the dose and that even very small doses can up the odds. Based on such data, a 2006 report from the National Research Council has estimated that exposure to 10 mSv—the approximate dose from a CT scan of the abdomen—increases the lifetime risk of developing any cancer by 0.1 percent. Using the same basic information, the U.S. Food and Drug Administration concluded that 10 mSv increases the risk of a fatal cancer by 0.05 percent. Because these risks are tiny compared with the natural incidence of cancer in the general population, they do not seem alarming. Any one person in the U.S. has a 20 percent chance of dying from cancer. Therefore, a single CT scan increases the average patient's risk of developing a fatal tumor from 20 to 20.05 percent.
All these estimates share a serious flaw. Among survivors exposed to 100 mSv of radiation or less—including the doses typical for CT scans—the numbers of cancer cases and deaths are so small that it becomes virtually impossible to be certain that they are significantly higher than the rate of cancer in the general population. To compensate, the National Research Council and others based their estimates primarily on data from survivors who were exposed to levels of radiation in the range of 100 mSv to 2 Sv. The fundamental assumption is that cancer risk and radiation dose have a similar relationship in high and low ranges—but that is not necessarily true.
Another complicating factor is that the atomic bombs exposed people's entire body to one large blast of gamma rays, whereas many patients receive multiple CT scans that concentrate several x-rays on one region of their body, making accurate comparisons tricky. Compounding this issue, the atomic bomb survivors typically had much poorer nutrition and less access to medical care compared with today's general U.S. population. Thus, the same level of radiation might correspond to greater illness in an atomic bomb survivor than in an otherwise healthy person from today.
Dialing Down the Dose
To conclusively determine the risk of low radiation doses and set new safety standards for CT radiation, researchers are beginning to abandon the atomic bomb survivor data and directly investigate the number of cancers among people who have received CT scans. About a dozen such studies from different countries examining rates of various cancers following CT scans will be published in the next few years.
In the meantime, some researchers have started testing whether good images can be produced with radiation doses lower than those generated in typical CT scans. Sarabjeet Singh, a radiologist at Mass General, and his fellow radiologist Mannudeep Kalra have an unusual way of conducting such investigations. Rather than recruiting living, breathing human volunteers for their studies, they work with cadavers. In that way, they can scan bodies many times without worrying about making people sick and can perform an autopsy to check whether the scan has correctly identified a medical problem.
So far the researchers have discovered that they can diagnose certain abnormal growths in the lungs and perform routine chest exams with about 75 percent less radiation than usual—a strategy Mass General has since adopted. Singh and Kalra are now sharing their methods with radiologists and technologists from hospitals and scanning centers across the U.S. and around the globe.
Medical associations are stepping in to help as well. Because the FDA does not regulate how CT scanners are used or set dose limits, different centers end up using an array of radiation doses—some of which seem unnecessarily high. In the past year the American Association of Physicists in Medicine has rolled out standardized procedures for adult CT exams that should rein in some of these outlier centers, Singh says. Furthermore, an increasing number of CT facilities across the U.S. receive accreditation from the American College of Radiology, which sets limits for radiation doses and evaluates image quality. In 2012 accreditation became mandatory for outpatient clinics that accept Medicare Part B if the facilities want to get reimbursed for scans.