The risks are real—but usually outweighed by the benefits.
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One irony of modern medicine is that, the more effective a treatment or technology becomes, the more we take it for granted and focus on its downsides—real or imagined.
The most glaring example is vaccines, which have saved millions of lives, but which some people—based on zero credible science—continue to blame for autism and other developmental issues in children. The cancer concerns associated with X-ray radiation aren’t nearly so overblown. But before digging into those concerns, it’s important to recognize how much CT scans and other forms of X-ray imaging have revolutionized health care.
Search for lists of the medical innovations that most improved patient care during the last 50 years, and computer tomography—the X-ray technology used to perform CT scans—will always land at or near the top of the list. “It changed the practice of medicine,” says Sanjay Saini, a professor of radiology at Harvard Medical School. Whether isolating a tumor and mapping its growth, or revealing diseases of the bowels or internal organs, a CT scan is much more likely to save your life than imperil it.
That said, there are legitimate cancer risks associated with CT imaging. But those risks are “very small,” says David Brenner, director of the Center for Radiological Research at Columbia University Medical Center.
According to a 2007 study Brenner published in the New England Journal of Medicine, if you’re under 30 at the time of a CT, your risk for cancer may jump anywhere from .01% to .14%. (If you’re older, your cancer risks from CT are lower.) But Brenner says that, thanks to advances in CT technology, his 2007 risk estimates are probably too high. “I would characterize our best estimates of the risks of cancer associated with a CT today in the range of one chance in 2,000 to one chance in 20,000,” he says.
He adds that the cancer risks associated with CT are “not on the same scale” as the risks associated with smoking or other well-established carcinogens.
Researchers single out CT because this form of imaging exposes you to levels of radiation “orders of magnitude” higher than traditional X-ray, Saini explains. While old-school X-rays are designed to grab a single snapshot of your insides, CT scans work by taking a series of 2D images from many different angles, which are then assembled to form a 3D image. “Five hundred chest X-rays are equal to one CT scan,” Saini says. (When it comes to the dentist office X-rays of your teeth—the ones where they drape you with that discomfiting lead vest—the radiation exposures are in the realm of chest X-rays, he adds.)
Saini points out that the whole planet is mildly radioactive, and you’re exposed to more of that radiation when you spend time at high altitudes. “If you take an airplane from Boston to the West Coast, the amount of [radiation] exposure is about what you’d get from a chest X-ray,” he says. There’s no evidence that pilots or flight attendants experience elevated cancer rates due to increased radiation exposure. So as far as basic X-rays are concerned, your cancer risks are minute—if not zero, he says.
Also important to note: Not all medical imaging exposes you to radiation. MRI, for example, uses magnetic resonance—not electromagnetic radiation—to peer inside your body. MRIs are not associated with any cancer risks whatsoever, Brenner explains.
So should you freak out about CT scans? Both Brenner and Siani say radiation exposure and cancer have a linear relationship—meaning the more you’re exposed, the greater your risks. Some researchers dispute this. But a large-scale study from Australia found having a single CT scan before age 20 raises a person’s relative lifetime risk for cancer by 24%. The higher the radiation exposure and the younger the person, the greater the risk.
Yes, that 24 percent figure is frightening. But the actual number of study participants who developed cancer was quite small. Out of the more than 680,000 young people who had a CT scan, the Aussie researchers turned up just 3,150 total cancer cases among them.
Taking into account the recent advancements in CT technology—advancements that allow for lower doses of radiation—the odds that you’ll develop cancer from a CT scan are extremely low, especially if you’re an adult, Brenner says.
Still, he worries about the overuse of CT. “I think there are many situations where people are prescribed CT when not needing it,” he says. “The percentage [of needless CT scans] may be as high as 20 or 25 percent, and given that there are 80 million CT scans performed every year, that’s a big number.”
He says young people who undergo multiple CT scans are most at risk. “People who get kidney stones tend to get them again and again,” he says. The first time a patient has a kidney stone, a CT is needed to rule out other health issues. “But the third and fourth and fifth time these patients have a kidney stone, they don’t need a CT scan to confirm the diagnosis,” he says.
Saini says young people and pregnant women also need to be cautious. “We know that cells replicating are more likely to be injured [by radiation exposure] than those that are not,” he says. This cell injury can lead to the formation of tumors or cancer cells. And because cell reproduction is more common in fetuses and young people, CT scans should not be performed on these patient populations if it can be helped, he says.
So what are you supposed to do if your doctor recommends a CT scan? “There’s no reason you shouldn’t ask your physician if CT is medically justified,” Brenner says. He mentions the appropriateness criteria established by the American College of Radiologists, which are basically a set of evidence-based rules for when to perform a CT, and when it’s not necessary. If your doctor’s justification doesn’t convince you, it may be worth seeking a second opinion before undergoing the CT (assuming it’s not an emergency situation).
“The bottom line is, when you need a CT scan, you absolutely should have one because the benefits are huge and the risks are small,” Brenner says. “But if you don’t need a CT scan, the benefits are zero, and so even small risks outweigh the benefits.”
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