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Radiation in medicine: A double-edged sword

APR 2009

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X-rays, CT scans, and other procedures should be used judiciously.

Diagnosing heart disease without opening the chest and looking at the heart is like trying to tell what's wrong with a car's engine without opening the hood. Treating heart disease without "opening the hood" is even more problematic.

Yet doctors have figured out a variety of ways to examine the heart from the outside. They can open clogged arteries without opening the chest and will someday fix faulty valves that way. Radiation is the key to these advances. But wherever radiation has been put to use, it has turned out to be a razor-sharp double-edged sword. Medicine is no exception. Radiation offers extraordinary benefits for the diagnosis of a wide range of diseases and ailments, from broken bones to heart disease. It is a mainstay for treating some types of cancer. Yet exposure to radiation can also damage DNA, the operating manual of a cell. This damage can lead to uncontrolled cell division, the hallmark of cancer. The larger the dose, the greater the risk of developing cancer. This delicate balance between benefit and risk demands the judicious and appropriate use of radiation for diagnosing and treating disease.

Rapid growth

Each of us is exposed to small amounts of radiation from cosmic rays, naturally occurring radon gas, and radioactive substances in the Earth. The average dose from this so-called background radiation is3 millisieverts (mSv) a year. This is a tiny amount of radiation that has little effect on health.

Up until the end of the 19th century, background radiation was the only kind around. That changed with the discovery of x-rays and radioactive minerals and their use in medicine, power generation, warfare, and a host of other applications. By the early 1980s, radiation from medical procedures, nuclear power plants, and the fallout from nuclear tests, along with small amounts from smoke detectors, television sets and computer monitors, airport security scanners, and other uses had added another 0.5 mSv per year. Since then, the use of radiation in medicine has grown so much that it now rivals background radiation, adding an average of 3 mSv per person each year, says the National Council on Radiation Protection and Measurement. Much of the increase comes from the rapid growth in the use of computed tomography (CT) scans, which deliver far more radiation than ordinary x-rays (Health Physics, November 2008).

Radiation from tests or procedures

Test or procedure

Effective radiation dose in millisieverts* (mSv)

Dental x-ray

0.005

Chest x-ray

0.02

Mammogram

0.7

Coronary calcium scan

1–3

Background radiation over a year

3

Abdominal CT

10

Cardiac CT

64-slice

7–23

320-slice

10–18

Angioplasty

7–57

Technetium stress test

6–15

Thallium stress test

17

Dual isotope stress test

18–38

Angiogram

2–23

Echocardiography

Magnetic resonance imaging (MRI)

*The sievert reflects the biological effects of radiation on tissues.

Sources: American College of Radiology; Health Physics Society; research and review articles

Exposure varies

The amount of radiation you might get from a medical test or procedure varies widely. A chest x-ray is at the low end of the scale, at 0.02 mSv. It goes up from there: 1 to 3 mSv for a coronary artery calcium scan, 2 to 23 mSv for an angiogram, 7 to 23 mSv for a cardiac CT scan, 6 to 38 mSv for a nuclear stress test, and 7 to 57 mSv for angioplasty.

Why are there such big ranges? An angiogram requires repeated x-rays so the doctor performing it can see the catheter (wire) as he or she gently maneuvers it from a blood vessel in the wrist or groin into the heart. Some people have more convoluted arteries than others, and it takes more time (and more x-rays) to guide the catheter into the heart. Artery-opening angioplasty needs even more x-rays to enable the doctor to guide the balloon into the correct spot in a blocked coronary artery, to make sure the balloon has cleared the blockage, and to make certain the stent is firmly in place. Nuclear stress tests vary based on the radioactive element being used and whether it is administered during exercise, rest, or both.

Cardiac CT (sometimes called cardiac CT angiography) is the newcomer in this field. It is being used to check for calcium in arteries and is being tested as an alternative to angiograms to detect blockages in coronary arteries. Cardiac CT is also being marketed as a "heart scan" to people worried about heart disease.

The amount of radiation from cardiac CT scans varies widely. An international study of the procedure at 50 medical centers showed that the dose varied from one CT scanner model to another, by how the machine was operated, and by whether radiation-reducing techniques were used (Journal of the American Medical Association, Feb. 4, 2009).

Balancing act

In general, the cancer risk from a single medical test or procedure is low. The National Academy of Sciences Committee on the Biological Effects of Ionizing Radiation estimates that for every 1,000 people exposed to 10 mSv (the amount from an abdominal CT scan), the radiation would add one extra case of cancer to the 420 "natural" cases expected as those people go through life.

But the skyrocketing growth in the use of CT scans — from eight million in 1990 to 62 million today — suggests that medical imaging may be adding to the cancer burden. A report from the Center for Radiological Research at Columbia University Medical Center estimates that radiation from CT scans now accounts for 1.5% of all cancers in the United States.

Age is a big factor. It usually takes 10 to 20 years before DNA damaged by a low dose of radiation leads to cancer. The older you are, then, the lower the chances that radiation poses a threat. On the flip side, the hazards of radiation are greater for children and young people.

Protecting yourself

Most tests for heart disease are important, even essential. A stress test using thallium or technetium can tell how the heart functions when it needs to work harder. An angiogram can reveal severely narrowed or blocked arteries that must be opened.

Some tests, though, may not be worth the radiation received. The value of coronary calcium scans and cardiac CT scans hasn't yet been established, especially when they are used for healthy people "just to see" what shape the heart's arteries are in.

You would never agree to surgery unless you needed it. A similar principle should apply to medical testing that involves radiation — don't agree to it, or ask for it, unless it will give you and your doctor important information about your health or your body. And even then, see if it's possible to get the lowest dose of radiation possible.