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Do CT scans cause cancer?

For older men the risk from diagnostic CT scans is relatively small.

By one estimate, Americans have more than 70 million CT scans every year. This raises a concern: The scans expose people to x-rays, and this so-called ionizing radiation can damage cells and lead to cancer down the road. But for older men, CT scanning does not present a great risk compared with the benefits of diagnostic scans.

“Radiation-related risk in anyone above 65 years of age from a diagnostic CT of the chest or abdomen is negligible to none,” says Dr. Dushyant V. Sahani, associate professor at Harvard Medical School and director of CT imaging at Massachusetts General Hospital. “The doses are very low and the time it takes cancer to develop can be 20 years or more.”

Still, you should have only the scans that are necessary, and choose alternatives that don’t require radiation exposure when possible.

CT radiation and cancer

Our understanding of the hazards of radiation comes from studies of the survivors of the atomic bombings of Japan. Scientists calculate the risk of cancer based on the amount of radiation that survivors were exposed to and how many of them later developed cancer.

The dose of radiation received per diagnostic scan is measured in millisieverts (mSv). We are all exposed to some amount of natural radiation from the sun, from the earth, and even from some natural chemicals in our body. The average natural background radiation in the United States is 3.7 mSv per year. A simple chest x-ray (two views) exposes a person to an average of 0.01 mSv, or roughly the amount of radiation you get in a day from the natural background.

CT doses are higher. The exposure from a standard chest CT is 7 mSv. However, Dr. Sahani notes, with smart use of modern scanners to minimize the dose, the actual exposure can be closer to 4 mSv. A 15-second, low-dose chest CT to check for undiagnosed lung cancer delivers as little as 1.5 mSv.

How much is too much?

The more scans you have, the higher your lifetime exposure and therefore the higher your risk. The American College of Radiology recommends limiting lifetime diagnostic radiation exposure to 100 mSv. That is equal to 10,000 chest x-rays, or up to 25 chest CTs.

In the course of treatment for various chronic diseases, including cancer, you could accumulate enough CTs to approach the 100 mSv limit. That’s because the care may require repeated follow-up scans to determine how well treatment is working and to watch for cancer recurrence. In this case, the threat of the existing cancer outweighs the risk posed by the CT scans.

What is your risk?

The body regions where CT-related cancer is most likely to occur are the chest, abdomen, and pelvis, where faster-growing cells are more vulnerable to radiation. The lifetime risk posed by a single abdominal CT of 8 mSv is calculated to be 0.05%, or a one in 2,000 chance of developing cancer. Now consider that the lifetime risk of dying from cancer, no matter what the cause, is about one in four and rises with age. The added CT risk is a blip, barely above the background rate.

The reason is that if you have your CTs in middle age and later in life, it can take decades for the radiation exposure to end in cancer. In contrast, a child or young adult has a long enough lifespan ahead that the radiation from repeated CTs is more likely to lead to cancer.

Even though the benefits of CTs in adult men, particularly those older than 50, may outweigh the risks, no one—even a man in his 70s or 80s—should have CTs without a good reason. “We always need to be cautious with ionizing radiation,” Dr. Sahani says. “If you don’t need more, why get it?” Have only the scans you need, in a modern diagnostic imaging facility that delivers only the dose required.

Lifetime risk of death per 1,000 people

Cause of death

Deaths per 1,000 people

Cancer not related to radiation exposure

228

Motor vehicle accident

12

Living with a smoker

10

Exposure to average level of radon gas in U.S. home

3

Drowning

0.9

One abdominal CT scan

0.5 (one in 2,000)

Common radiation exposure sources

Exposure

Dose

Dental x-ray

0.005 mSv

Chest x-ray

0.01 mSv

Head CT

2 mSv

U.S. average background radiation exposure (annual)

3.7 mSv

Chest CT

4-7 mSv

Abdominal CT

8 mSv

Posted by: Dr.Health

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