X-rays, Gamma Rays, and Cancer Risk

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Do x-rays and gamma rays cause cancer?

Yes. X-rays and gamma rays are known human carcinogens (cancer-causing agents). The evidence for this comes from many different sources, including studies of atomic bomb survivors in Japan, people exposed during the Chernobyl nuclear accident, people treated with high doses of radiation for cancer and other conditions, and people exposed to high levels of radiation at work, such as uranium miners.

Most studies on radiation and cancer risk have looked at people exposed to high doses of radiation in the settings above. It is harder to measure the much smaller increase in cancer risk that might come from much lower levels of radiation exposure. Most studies have not been able to detect an increased risk of cancer among people exposed to low levels of radiation. For example, people living at high altitudes, who are exposed to more natural background radiation from cosmic rays than people living at sea level, do not have noticeably higher cancer rates.

Still, most scientists and regulatory agencies agree that even small doses of gamma and x-radiation increase cancer risk, although by a very small amount. In general, the risk of cancer from radiation exposure increases as the dose of radiation increases. Likewise, the lower the exposure is, the smaller the increase in risk. But there is no threshold below which this kind of radiation is thought to be totally safe.

What do the studies show?

Atomic bomb survivors

Much of what we know about cancer risks from radiation is based on studies of the survivors of the atomic bombs in Nagasaki and Hiroshima. These people had higher risks of some, but not all cancers. Studies have found an increased risk of the following cancers (from higher to lower risk):

  • Most types of leukemia (although not chronic lymphocytic leukemia)
  • Multiple myeloma
  • Thyroid cancer
  • Bladder cancer
  • Breast cancer
  • Lung cancer
  • Ovarian cancer
  • Colon cancer (but not rectal cancer)
  • Esophageal cancer
  • Stomach cancer
  • Liver cancer
  • Lymphoma

For most of these cancers, the risk was highest for those exposed as children, and was lower as the age at exposure increased. Those exposed while still in the womb (in utero) had lower risks than those exposed during childhood.

Higher radiation exposure was linked to higher risk of cancer, but even low amounts of radiation were linked to an increased risk of getting and dying from cancer. There was no clear cut-off for safe radiation exposure.

These cancers took years to develop, but some cancers appeared sooner than others. Deaths from leukemia went up about 2 to 3 years after exposure, with the number of cases peaking after about 10 years and going down after that. Solid tumors took longer to develop. For example, excess deaths from lung cancer began to be seen about 20 years after exposure.

Chernobyl accident

Children and adolescents living near the Chernobyl plant at the time of the accident had an increased risk of thyroid cancer linked to exposure to radioactive iodine. The risk was higher in areas that were iodine deficient. This increased risk was not seen in adults living in the area.

Workers employed in cleanup operations from 1986-1990 had an increased risk of leukemia (all types). These individuals had higher and more prolonged radiation exposures that the population residing around the plant.

Radiation therapy

To treat benign conditions

Although radiation therapy is now mostly used to treat cancer, it was used to treat a number of benign (non-cancerous) diseases before the risks were clearer. Studies of these patients have helped us learn about how radiation affects cancer risk.

Peptic ulcer disease: A large study of people who were treated with high doses of radiation (an average of 15 Gy or 15,000 mSv) for the treatment of peptic ulcers found a higher risk of cancer of the stomach and pancreas.

Ringworm of the scalp: Studies of people who were treated with radiation to treat a fungal infection of the scalp (called scalp ringworm or tinea capitis) have found an increased risk of basal cell skin cancers. The risk was lower in people who were older when treated. This increased risk was seen only in white patients, and the cancers occurred more often in sun-exposed skin of the head and neck (as opposed to the scalp), which implies that ultraviolet (UV) radiation plays a role in these cancers as well.

Ankylosing spondylitis: Studies have looked at the cancer risks of patients with the autoimmune disease ankylosing spondylitis who were injected with a form of radium.

In one study, patients who received a high dose (average bone dose of 31,000 mGy) had an increased risk of bone sarcoma. The risks of some other cancers, such as breast, liver, kidney, bladder, and other sarcomas, may also have been increased. About one-quarter of the patients in this study were younger than 20 years of age when they were treated with radiation.

In another study, patients treated with a lower dose of radium (average bone dose of 6,000 mGy) had a higher risk of leukemia, but not of any other cancers. Most of the patients in this study were adults at the time of treatment.

Other studies: Treatment of the head and neck area with radiation for benign conditions has also been linked to cancers of the salivary gland and brain and spinal cord in adults in some studies. Children treated with radiation to this area also have an increased risk of thyroid cancer.

To treat cancer

Studies have linked radiation therapy to treat cancer with an increased risk of leukemia, thyroid cancer, early-onset breast cancer, and some other cancers. The risk of cancer depends on a number of factors, include the dose of radiation, the part of the body being treated, the age of the person getting it (younger people are generally at greater risk), and the use of other treatments such as chemotherapy.

For example, people who get pelvic radiation therapy would not be expected to have higher rates of cancers in the head and neck because these areas weren’t exposed to the radiation from the treatment. Other factors might also play a role in how likely a person exposed to radiation is to develop cancer. For example, some genetic conditions can mean that a person’s cells are more vulnerable to radiation damage, which might in turn raise their risk more than in someone without these gene changes.

If cancer does develop after radiation therapy, it does not happen right away. For leukemias, most cases develop within 5 to 9 years after exposure. In contrast, other cancers often take much longer to develop. Most of these cancers are not seen for at least 10 years after radiation therapy, and some are diagnosed even more than 15 years later.

When considering radiation exposure from radiation therapy treatment for cancer, the benefits generally outweigh the risks. Overall, radiation therapy alone does not appear to be a very strong cause of second cancers. This is probably due to the fact that doctors try to focus the radiation on the cancer cells as much as possible, which means few normal cells are exposed to radiation. However, some combinations of radiation therapy and chemotherapy are more risky than others. Doctors do their best to ensure the treatment that is given destroys the cancer while limiting the risk that a secondary cancer will develop later on.

For more information, see our document Second Cancers Caused by Cancer Treatment.

Imaging tests

Some studies have estimated the risk of radiation exposure from imaging tests based on the risks from similar amounts of radiation exposure in the studies of the atomic bomb survivors. Based on these studies, the US Food and Drug Administration (FDA) estimates that exposure to 10 mSv from an imaging test would be expected to increase the risk of death from cancer by about 1 chance in 2000.

It can be difficult to study cancer risks from imaging studies that use radiation. In order to see small risks (such as 1 in 2000), a study would have to look at 10s or 100s of thousands of people. Information about other exposures that could be cancer risk factors would be needed, to see if it was likely that the cancer came from the radiation exposure. Since cancers from radiation take years to develop, the study would need to follow the patients for many years.

Often, scientists use questionnaire studies to look for possible causes of cancer. These studies compare exposures among people who have a certain cancer to those who don’t. They may instead compare people who had a certain exposure (like to radiation) to those who didn’t. However, this is difficult to do for diagnostic radiation exposure since many people cannot accurately recall information about things that happened many years before (such as in childhood) and information about all the imaging tests that were done is often not available. There is also a concern that people with cancer tend to over report exposures that they worry may have caused their cancers.

Although most studies haven’t found an increased risk of cancer after imaging tests that use x-rays, a few have:

Fluoroscopy

Studies of women who had been imaged many times with fluoroscopy as a teenager or young woman during treatment for tuberculosis have found an increased risk of breast cancer years later.

Spine x-rays

Teenagers and young women who had many x-rays of the spine to monitor scoliosis have been found to have an increased risk of breast cancer later on.

Dental x-rays

A study compared a group of people with meningioma (a brain tumor that is most often benign) with a group without the tumors. It found that the people who had the tumors were more likely to have had a type of dental x-ray called a bite-wing, and to have had bite-wing or Panorex x-rays every year.

CT scans

A study in England of exposure to radiation from CT scans found that children who received a dose of at least 30 mGy (the same as 30 mSv) to the bone marrow had 3 times the risk of leukemia compared to those who received a dose of 5 mGy or less. For brain tumors, a dose of 50 mGy or more to the brain was linked to more than 3 times the risk.

What do expert agencies say?

Several agencies (national and international) study different substances in the environment to determine if they can cause cancer. (A substance that causes cancer or helps cancer grow is called a carcinogen.) The American Cancer Society looks to these organizations to evaluate the risks based on evidence from laboratory, animal, and human research studies.

Based on animal and human evidence, several expert agencies have evaluated the cancer-causing nature of x-rays and gamma rays.

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. Based on the data available, IARC classifies x- and gamma radiation as a “known human carcinogen”.

The National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP has classified x- and gamma radiation as “known to be a human carcinogen”.

The US Environmental Protection Agency (EPA) sets limits for exposure to x-rays and gamma rays in part because it recognizes that this form of radiation can cause cancer.

For more information on the classification systems used by these agencies, see our document Known and Probable Human Carcinogens.


Last Medical Review: 05/02/2013
Last Revised: 04/18/2014