Imaging overload: How many tests are too many?

When your doctor orders an occasional x-ray or computed tomography (CT) scan, you probably focus on the findings, not the amount of radiation you’ll be exposed to. But if you need numerous tests, you may wonder if you’re getting too much radiation exposure and how it might affect you. The concern about radiation exposure is an increased risk for developing cancer later in life. Ionizing radiation from tests such as x-rays or CT scans has the potential to damage tissue in the body, including cell DNA.

“As DNA is damaged, this causes mutations. Some of these are repaired by our cells, but others escape repair. In rare circumstances, these mutations may cause cells to divide rapidly without control,” explains Dr. Mark Hammer, a radiologist with Harvard-affiliated Brigham and Women’s Hospital. “If cancer were to develop, it could take 10 to 20 years for it to become apparent.”

How big is the risk?

We have very limited data on the link between diagnostic testing and future cancer development. Some older observational studies estimated that 1.5% to 2% of cancers in the United States might be attributable to CT scans (about 29,000 new cases per year and 15,000 deaths).

But this estimate was based on the amount of radiation from older diagnostic imaging machines. “We’ve been able to decrease radiation doses for every type of scan we do in the last 10 to 15 years because of advances in technology, cameras, and computer processing,” Dr. Hammer notes. That reduces the already small risk of the diagnostic imaging causing a cancer.

For example, the amount of radiation in an x-ray of your hand or foot is about the same as three hours of exposure to normal background radiation from the environment (see “Radiation exposure”).

Other factors affect risk, too, such as these:

Varying targets. Beams of radiation aimed at different parts of the body over the years don’t add up to one cumulative risk. “The concern is repeated exposure to the same cells, not exposure to the wrist for one x-ray, the head for one CT scan, or the spine and hip for one bone density scan,” Dr. Hammer points out.

Age. Radiation is most dangerous to children and young people, who have more cell division occurring than older people and a longer life span in which to develop cancer. “But even into your 60s, there is time for cancer to develop. Above age 70, the risk of getting cancer from testing is very small. I wouldn’t be concerned about it,” Dr. Hammer says.

Radiation exposureWe are exposed to natural radiation every day. It’s in the air, rocks, soil, water, and even the food and drinks we consume. In a year’s time, the average person is exposed to about 3 millisieverts (mSv) of this “background” radiation.Radiation that travels through the air and penetrates tissue (ionizing radiation) is used for medical purposes: examples are x-rays, bone density tests, dental x-rays, computed tomography (CT), and positron emission tomography–computed tomography (PET/CT).The doses of ionizing radiation in the tests vary. For example, there are0.001 mSv in a bone density test0.005 mSv in a dental x-ray0.4 mSv in a mammogram0.1 mSv in a chest x-ray1.5 mSv in a lung cancer CT screening2.1 mSv in a head CT scan12 mSv in CT angiography16 mSv in abdomen and pelvis CT with contrast dye25 mSv in a PET/CT scan.Doses may be lower, depending on the center that performs the scan.

For people 70 or younger

Getting a diagnostic test once in a blue moon is likely to provide crucial information to guide treatment, and it isn’t believed to pose much cancer risk. But frequent diagnostic testing may slightly increase cancer risk and should be weighed carefully.

In some cases, repeated tests are worth the risk. For example: “There is a clear benefit to doing CT in a person with cancer every few months to know if chemotherapy is working, or screening a former smoker annually for lung cancer,” Dr. Hammer says.

But unnecessary diagnostic tests — which may account for about a third of all CT scans — should be avoided.

“The consensus of experts is that there is no safe threshold of radiation, and that increasing amounts of radiation lead to increasing risks of cancer,” Dr. Hammer says. “Any time your doctor orders a test, you should ask what the test is expected to show, what the doctor will do with that information and whether it will affect treatment, what can be done to reduce the radiation dose, and whether any alternative tests are available.”

Bible verses for today’s meditation and inspiration: Matthew E. McLaren

 “Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable–if anything is excellent or praiseworthy–think about such things.” – Philippians 4:8

 “Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours.” – Mark 11:24

 “Anxiety weighs down the heart, but a kind word cheers it up.” – Proverbs 12:25

 “Who of you by worrying can add a single hour to your life?” – Luke 12:25

 “For I know the plans I have for you, declares the Lord, plans for welfare and not for evil, to give you a future and a hope.” – Jeremiah 29:11

“If you, then, though you are evil, know how to give good gifts to your children, how much more will your Father in heaven give good gifts to those who ask him!” – Matthew 7:11

 “A cheerful heart is good medicine, but a crushed spirit dries up the bones.” – Proverbs 17:22

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