Saying that CT "causes cancer" is not an empty claim. Radiography is a double-edged sword. On the one hand, the image can clearly distinguish the cause of the disease, is an important diagnosis and treatment method; on the other hand, more and more research shows that it is harmful to the human body, abuse may induce cancer. But people don't seem to pay much attention to it.
A new U.S. study warns that the misuse of CT scanning technology by doctors in the treatment process has led to more than 20 million Americans, especially children, being exposed to unnecessary "super-X-ray" radiation, putting them at a higher risk of developing cancer.
CT scan radiation causes cancer at a rate of 2 percent
The study, by Eric Hull and Columbia University medical physicist David Brennen Sodium***, was published in the 29th issue of the New England Journal of Medicine, and was funded by the U.S. government.
Hull estimates that up to 2 percent of cancer cases in the United States in the past few decades may have been caused by radiation from CT scans. While the risk of a single CT scan causing a person to develop cancer is small, he said, "we're very concerned about the cumulative risk to public health over time."
Statistics show that the average number of Americans receiving radioactive scans, most of them CT scans, has doubled since 1980.
CT scans have become more commonly used largely because they are quick, relatively inexpensive and painless for patients. The three-dimensional images it produces are sharp and provide a great deal of help to doctors in quickly diagnosing trauma, abdominal pain, chronic headaches, kidney stones and other conditions. Half of all radiation exposure to Americans now comes from medical radiation, but that proportion used to be only one-sixth, when the primary source of radiation came from the natural environment.
But CT scans are extremely radioactive. A single CT scan of the chest emits 10 to 15 millisieverts of radiation, compared with 0.01 to 0.15 millisieverts for a routine X-ray chest x-ray, 3 millisieverts for a mammogram, and only 0.005 millisieverts for a dental X-ray.
Radiation levels vary from machine to machine and from person to person, with fat people needing more intense radiation than thin people -- and the harm caused by radiation can accumulate throughout a person's life.
Add up to atomic bomb radiation?
The Hull team's previous studies prompted the U.S. Food and Drug Administration to recommend in 2001 that hospitals reduce the number of CT scans children receive. But since then, the use of CT scans has continued unabated.
Some doctors have said that some patients with chronic conditions, such as kidney stones, are likely to receive too many CT scans, and that some patients as young as 30 years old have already received at least 18 CT scans.
The report said the survivors of the atomic bombings in Japan had been exposed to 50 to 150 millisieverts of radiation, and the cumulative amount of radiation from several CT scans had been equal to that. Some experts have questioned this claim, but others have supported it.
The advent of computed tomography (CT) in the 1970s was a major change in diagnostic radiology, and its clinical application has become increasingly widespread over time. It is reported that as many as 62 million CT scans are performed annually in the United States alone, of which at least 4 million are for children. However, recent reports of radiation exposure from CT scans in the foreign press cannot help but provoke deep thought. According to a recent study published in the New England Journal of Medicine (N. Engl. J. Med. 2007, 352(22):2277-2284.), researchers at Columbia University Medical Center in New York estimate that up to 2 percent of all cancers in the United States over the next 20 to 30 years may be caused by radiation from CT scans.
Radiation "justified" or "unjustified"?
David Brenner, a professor of radiation oncology at Columbia University Medical Center in New York, points out in the New England Journal of Medicine that there is no question that radiation from CT scans increases the risk of cancer. While the chances of a single CT scan causing a person to develop cancer are small, the public health hazards of undergoing the test over a long period of time cannot be ignored. Many radiologists support this view with reservations, as there has been a lack of rigorous controlled epidemiologic studies, and the exact magnitude of the risk remains an unanswered question. Carl Schultz, a professor of clinical emergency medicine at the University of California, said, "This may be a bit alarmist, radiation is certainly not good for the human body, but the conclusion that CT scans increase the risk of cancer needs to be backed up by conclusive evidence." In addition, some experts have questioned whether the conclusion is merely a theoretical prediction based on data from victims of the atomic bombings in Japan, and perhaps not credible at all.
Not coincidentally, a study published in the Journal of the American Medical Association (JAMA. 2007, 298:317-323.) concluded that the application of a new, fast CT scan (64-slice CT) to examine the heart or the large blood vessels that surround it increased a patient's risk of developing cancer. The risk is especially pronounced in patients with breast or lung cancer, especially in young women. For example, a 20-year-old female patient undergoing a 64-slice CT scan of the heart and aorta has a 1:114 cancer rate; for an 80-year-old patient, the rate shrinks to 1:3,000.
CT imaging is widely used as an aid in diagnosing diseases such as bleeding in the brain, lung cancer, and appendicitis. During the examination, an X-ray machine rotates to take a cross-sectional image of the body, which is processed by a computer to produce a three-dimensional image. The image quality of CT scans is significantly better than that of conventional X-rays, but the price the patient pays is increased radiation exposure. The researchers cited the example of a CT scan of the abdomen, which was exposed to more than 50 times the radiation of an X-ray of the same part of the body. The significance of this finding is that the more effective the imaging device, the higher the radiation dose to which the patient is exposed. In general, the misuse of screening can lead to excessive radiation exposure, especially in pediatric patients, cautioned Robert Smith, director of the American Cancer Society's Division of Cancer Screening.
Children more vulnerable to radiation?
The results of this study also disclosed that children are at a higher risk of developing cancer after exposure to radiation, and that children are 10 times more sensitive to radiation than adults. Thea McCullough, a radiation physicist at Mayo Medical Center, explained that children's tissues are more sensitive to radiation, and they are younger and therefore more likely to develop cancer compared to adults. But there's no need for parents to worry about this; after all, the odds of developing cancer are very small for an individual. If a CT scan is needed, it should not be delayed. Twenty-five percent of Americans will develop cancer in their lifetime, and the chances increase by only 0.1 percent when a child receives a CT scan, McCullough said.
A report in the 2007 issue of the Journal of Pediatrics concluded that the benefits of CT scans for children outweigh the drawbacks. Ellen Brody, a researcher at Cincinnati Children's Hospital in Cincinnati, Ohio, concluded from a retrospective survey of the relevant literature that the risk of cancer was almost negligible for children who needed to undergo CT scans. They found no direct correlation between CT exams and developing cancer thereafter. There are a variety of clinical tests that can be used, and ultrasound and magnetic **** vibration imaging (MRI) are relatively safer because these two techniques do not expose a person to radiation. Given the unique nature of pediatric patients, Brody said radiologists have a responsibility to consider further improvements in scanning techniques. For pediatricians, they need to consider whether CT scans are necessary, and they need to communicate with parents to get agreement.
What exactly are the risks?
While radiologists agree that CT scans slightly increase the risk of cancer, they disagree on whether the risk is high enough that patients should avoid them. The Columbia University researchers based their estimate on a survey that showed that 25,000 survivors of Hiroshima and Nagasaki were at very high risk of cancer deaths, and that residents living miles away from the atomic bomb sites were exposed to trace amounts of radiation comparable to that received by patients undergoing CT scans, which ranged from 5 to 150 mSv. In addition, a recent large-scale study of 400,000 nuclear workers supports the survey's conclusions. also supports the conclusions hypothesized in that survey. These workers were exposed to radiation doses averaging 20 mSv, which is close to the amount of radiation an adult would receive from a single CT scan of a specific organ. The results also found a high correlation between radiation dose and cancer risk when the subjects were exposed to radiation doses between 5 and 150 mSv. Nonetheless, some experts believe that the actual risk of CT scans cannot be inferred from this alone, as the nature of the radiation is very different. An atomic bomb blast emits gamma rays, while a CT scan utilizes beta rays, which are lower in energy and can be attenuated with protective devices. It's like comparing apples and oranges, Schultz said, and the two are not comparable.
In addition, radiologists and researchers have to admit that because of the ease and speed of CT scanning technology, it has almost reached the point of abuse today, such as the number of CT scans applied in the U.S. has skyrocketed from 3 million in 1980 to 68 million today. Devra Davis, director of the Center for Environmental Oncology at the University of Pittsburgh Cancer Institute, explains that this is mainly due to the fact that clinicians rely too much on instrumental tests for diagnosis and ignore the resulting negative effects. On the other hand, the increasing prominence of medical disputes is forcing clinicians to make thorough and detailed examinations of their patients.
Despite the fact that CT scans are used for diagnostic purposes, researchers say that as many as one-third of these scans could be replaced by other methods or are not necessary at all. That means 20 million adults and 1 million children are innocently exposed to radiation each year in the United States alone.
Davis said CT scans are an important and valuable tool for patients in true emergencies, but it is abusive to perform them in non-emergency situations when other technologies are available. Schultz agreed: "Clinicians prefer CT scans, mostly because the technology can replace a meticulous physical examination. Radiation levels from CT examinations have declined considerably over the past 20 years, but continuing to reduce radiation levels while maintaining image quality is a difficult task. Therefore, both medical professionals and the public must be cautious about applying potentially harmful diagnostic tools."