Do X-rays and Gamma Rays Cause Cancer?

What the expert agencies say about radiation and cancer risk

Several national and international agencies study substances in the environment to determine if they might cause cancer. In general, the American Cancer Society (ACS) does not determine whether something should be considered a carcinogen (something that can cause cancer). Instead, ACS looks to other respected organizations for help with this.

Based on studies done on people and in the lab, several expert agencies have evaluated the cancer-causing potential of x-rays and gamma rays.

• The International Agency for Research on Cancer (IARC) classifies x-rays and gamma rays as “carcinogenic to humans.”
• The US National Toxicology Program (NTP) has classified radiation from x-rays and gamma rays as “known to be a human carcinogen.”
• The US Environmental Protection Agency (EPA) sets limits and develops guidelines to help minimize exposure to x-rays and gamma rays. This is in part because it recognizes that this form of radiation can cause cancer.
• The US Food and Drug Administration (FDA) recognizes that x-rays may increase cancer risk over time because they can damage DNA. However, when they are used for medical imaging or cancer treatment, the benefits are usually felt to be greater than the small risk.

For more information on the classification systems used by these agencies, see How Do We Know What Causes Cancer? and Known and Probable Human Carcinogens.

Radiation risk and imaging tests

Imaging tests create pictures of the inside of a person’s body. Some, like x-rays, CT scans, and PET scans, use small amounts of ionizing radiation to do this. Others, like ultrasound or MRI, don’t use radiation at all.

When radiation is used for an imaging test, the dose is kept as low as possible. Even so, some people worry about whether these tests might increase cancer risk. Research has not been able to provide a clear answer.

Most studies of cancer risk look at higher radiation exposures from atomic bombs and nuclear accidents. Because imaging tests use lower doses of radiation, it is hard to compare the risk levels. Studies that have found an increased risk of cancer after imaging tests have involved people who have had many imaging tests or high-dose procedures.

To learn more, see Understanding Radiation Risk from Imaging Tests.

Radiation risk and radiation therapy

The risk of cancer from radiation therapy depends on:

• The dose of radiation and number of treatments
• How much of the body is treated
• The age of the person

To treat benign conditions

In the past, before the risks were known, radiation was used to treat some benign (noncancerous) diseases, such as peptic ulcer disease and some head and neck conditions. However, over time, concerns about a potential link between radiation treatment and certain types of cancer led to a move away from these treatments.

Studies of people treated for these conditions provided information about how radiation might affect cancer risk. They also helped doctors learn ways to provide safer radiation treatment.

Today, lower doses of radiation therapy are sometimes used to treat some benign conditions. These include heterotopic ossification (abnormal bone growth) and plantar fasciitis (inflammation along the bottom of the foot). However, it is not known if these lower levels of radiation treatment increase the risk of cancer later in life, as not enough people have been treated this way yet. Cancer from radiation usually takes many years to develop.

To treat cancer

Studies have linked radiation therapy used to treat cancer with an increased risk of developing another type of cancer, such as leukemia, thyroid cancer, and breast cancer. When this happens, it is called a second cancer. The increase in risk depends on several factors, including:

• The total dose of radiation
• The part of the body being treated and the organs in the treatment field
• The person’s age, with younger people generally at greater risk
• The use of other treatments, such as chemotherapy (chemo), at or around the same time

Other factors might also affect a person’s risk. For example, some genetic conditions can make a person’s cells more vulnerable to radiation damage. Having one of these conditions might raise a person’s risk more than for someone without these gene changes.

Childhood cancer survivors who received radiation therapy are at higher risk for cancer as they get older. The most common second cancers in children are central nervous system cancer, thyroid cancer, bone cancer, skin cancer, and soft tissue sarcoma.

Cancer after radiation therapy doesn’t happen right away

If cancer develops after radiation therapy, it doesn’t happen right away. Most leukemias develop within about 5 to 9 years after exposure. Most other types of cancer are not seen for at least 10 years after radiation therapy. Some are diagnosed more than 15 years later.

For people who need radiation therapy to treat cancer, the benefits generally outweigh the risks. Overall, radiation therapy alone does not appear to be a very strong cause of a second cancer. This is probably because modern radiation therapy allows doctors to focus the radiation more precisely. This way, the cancer cells receive the highest dose of radiation while limiting the exposure of nearby normal cells. Treatment is planned to destroy the cancer while limiting the risk that a second cancer will develop later on.

For more information, see Second Cancers.

Radiation risk and 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, Japan, in 1945. These survivors were exposed to different amounts of radiation, depending on how far they were from the explosions. Over time, it was found that they had higher risks of some types of cancer, including:

• Most types of leukemia (although not chronic lymphocytic leukemia)
• Myelodysplastic syndrome
• Thyroid cancer
• Bladder cancer
• Breast cancer
• Lung cancer
• Uterine cancer
• Colon cancer
• Esophageal cancer
• Stomach cancer
• Liver cancer
• Gallbladder cancer
• Pancreas cancer
• Brain cancer
• Prostate cancer
• Skin cancer (except for melanoma)

For most of these types of cancer, the risk was found to be highest for people exposed as young children. Survivors exposed when they were older had lower risk. Those exposed while still in the womb (in utero) had similar risks to those exposed in early childhood.

Exposure to higher doses of radiation was linked to higher risk of cancer. However, even lower amounts of radiation were linked to an increased risk of getting cancer and dying from it. There was no clear cut-off for a safe level of radiation exposure.

For people who survived the atomic bombs and later developed cancer, it usually did not happen right away. It often took years, and some types of cancer appeared sooner than others.

Leukemias were typically seen first, with cases rising about 2 years after the explosions, peaking at around 6 to 8 years, and then declining. Other cancers, like breast and lung cancer, took longer to appear, with cases starting to rise around 10 years later. For many cancers, higher rates were still seen even 50 years after exposure.

Radiation risk and nuclear plant accidents

Accidents in two nuclear power plants led to the release of high doses of radiation: Chernobyl, Ukraine, in 1986 and Fukushima, Japan, in 2011. Some studies reported an increased risk of thyroid cancer in children and teens living near these areas at the time of the accidents. This increased risk was not seen in adults living in the area at that time. Ongoing studies continue to watch for health effects based on the levels of exposure.

Workers in cleanup operations in Chernobyl from 1986 to 1990 had an increased risk of all types of leukemia. The workers had higher and more prolonged radiation exposures than some of the people living near the plant.

In Japan, health officials continue to monitor Fukushima cleanup workers for health effects.

Radiation risk and nuclear weapons testing

Researchers have studied people who lived near places where nuclear tests were done. These studies suggest an increase in the incidence of some types of cancer. Those most linked to this radiation exposure from testing were thyroid, colon, and stomach cancers, as well as leukemia.

Radioactive particles in the environment can continue to release radiation for a long time. Studies on former nuclear testing sites are still being done.