Warning: Radiation risks from medical imaging

Ever since the Fukushima nuclear reactor disaster in Japan in March, there has been a lot of interest in the media about the health effects of exposure to radiation  including those from medical imaging such as X-rays and CT (computed tomography) scans. Many patients want to know if radiation from mammograms, bone density tests, nuclear imaging, PET scans, and so forth will increase their risk of developing cancer.

The ability to peer inside the body is nothing short of miraculous and has saved countless lives since X-rays were discovered more than a century ago. Today, the array of imaging tests available is bewildering, the technology intimidating, and circumstances and possible outcomes (Is it cancer? A stroke? A heriniated disc?) often terrifying. For most people, there’s very little risk from routine X-rays such as chest X-rays or dental X-rays. But experts have become increasingly concerned about the overuse of many types of scans, not only because of the huge and growing expense, but even more of the potential risk posed by the cumulative exposure to radiation. They are used to diagnose diseases, trauma, and abnormalities, as well as to guide and monitor treatment.

Radiation On The Rise

The radiation you get from X-ray, CT, and nuclear imaging is ionizing radiation — high-energy wavelengths or particles that penetrate tissue to reveal the body’s internal organs and structures. Ionizing radiation can damage DNA, and although your cells repair most of the damage, they sometimes do the job imperfectly, leaving small areas of “misrepair.” The result is DNA mutations that may contribute to cancer years down the road.

Exposure to ionizing radiation from natural or background sources hasn’t changed for the past 30 years, but in many industrialized countries, total per capita radiation has nearly doubled, and experts believe the main reason is increased exposure that comes from medical sources which has grown from 15 percent in the early 1980s to 50 percent today. CT alone accounts for 24 percent of all radiation exposures.

Most of what we know about the risks of ionizing radiation comes from long-term studies of people who survived the 1945 atomic bomb blasts at Hiroshima and Nagasaki. These studies show a slightly but significantly increased risk of cancer in those exposed to the blasts, including a group of 25,000 Hiroshima survivors who received less than 50 mSv of radiation  an amount you might get from two or three CT scans.

CT Scans

Because they’re so commonly used today, CT scans are the biggest source of radiation of all imaging technologies. A special type of X-ray that produces cross-sectional images, or “slices,” of the body, CT scans are used to diagnose everything, from heart diseases and cancer to brain tumors, kidney stones, and injuries. Abdominal CT scans tend to produce the most radiation — averaging about 500 times more radiation than a simple chest X-ray, and 1,000 times more than a dental X-ray or bone mineral density test (DEXA). Other types of imaging that use radiation include nuclear diagnostic tests, such as PET scans, as well as fluoroscopy. MRI and ultrasound, in contrast, do not use radiation.

The risk from a single CT scan, when appropriately done, is miniscule, but radiation exposures add up over a lifetime. A study in the Archives of Internal Medicine in 2009 estimated that 72 million scans were done in the US in 2007. After excluding scans done following a diagnosis of cancer and those performed during the last five years of a person’s life, the researchers projected that CT scans in the US would cause about 29,000 extra cases of cancer in the future and about 14,5000 deaths. That’s about two percent of all annual cancers.

More Is Always Better

People today get far more medical scans than in the past. Why do we do so many imaging tests?

• Doctors may order tests out of habit, wanting to “leave no stone unturned,” and may not focus on radiation doses or the cancer risks of medical imaging. They may also order scans because of fear of failing to diagnose something and being sued for malpractice.

• Patients may pressure doctors for scans, feeling that high-tech testing is optimal care, “just to be safe.” They may be influenced by ads and websites that promote the benefits of diagnostic imaging but underplay the risks and costs.

• There’s an increased supply of machines, and doctors who own them or have a financial stake in the facilities have an incentive to order more scans.

• As scanning machines become faster and more powerful, providing greater resolution (and often using more radiation), more “incidental findings” are detected. These findings are often clinically insignificant, but lead to more testing and possibly overtreatment.

• Certain conditions, such as cancer and some cardiac problems, may need periodic monitoring.

• CT scans are also being used more often for screening of healthy people, without evidence of benefit. Some examples: CT scans of smokers and former smokers for lung cancer, and “full body” scans that are promoted as a way to detect a variety of cancers, cardiovascular diseases, and other disorders.

Variations

There are no uniform rules on radiation doses from medical scans — except for mammograms, which use low-to-moderate doses of radiation and contribute much less than one percent of total radiation from imaging, according to a paper in the Journal of the American Medical Association in July 2010. Earlier last year, the US FDA announced a broad initiative to reduce unnecessary radiation exposure from medical imaging by, for instance, regulating the devices, recommending appropriate justification for various procedures, and setting optimal radiation doses.

Several studies have found that radiation doses from CT scans tend to be higher than the amounts generally cited and, for a given procedure, vary as much as 13-fold within and among facilities. Moreover, the radiation dose from most CT scans could be reduced by half without affecting diagnostic accuracy, according to Rebecca Smith-Bindman, MD, of the University of California, San Francisco, writing last year in the New England Journal of Medicine.

What To Do

Medical and other expert groups advise that imaging tests be done only if there is a clear benefit that outweighs the risks — and that the minimal level of radiation be used. Among the questions you should ask before undergoing a diagnostic scan are:

• Is this test really necessary, and will it really improve my health care?

• Is there a non-radiation alternative, such as ultrasound or MRI, that’s equally good? Keep in mind, though, that CT is better for certain things (such as broken bones, damaged vertebrae, cancer, pneumonia, bleeding in the brain, organ injuries), while MRIs are better for others (tendons, ligaments, spinal cord, brain tumors, most strokes); sometimes both are used. MRIs are more expensive.

• Will the test use the lowest amount of radiation and fewest scans for adequate imaging? For children, it’s important to make sure the radiation level is appropriate for their smaller size.

• Will the scan be limited to the indicated area and nearby area be properly shielded?

Bottom line: Medical imaging tests are invaluable in the diagnosis and treatment of many disorders. You shouldn’t avoid necessary exams and procedures out of concern about radiation. And if the test doesn’t find anything, that doesn’t mean it was unnecessary. But unless your doctor suggests it, don’t go to those widely advertised clinics offering full-body or coronary screening scans for the “worried well.” The best advice: Before having any imaging scan, discuss the pros and cons with your doctor.