Medical Imaging Radiation and Cancer

Medical imaging such as x-rays and CT scans uses small amounts of ionizing radiation to see inside the body. That radiation carries a very small added cancer risk. Still, a scan your doctor recommends usually brings benefits that far outweigh the risk. Some tests use no ionizing radiation at all, including MRI and ultrasound.

X-rays and CT scans pass ionizing radiation through the body to make images. Doses are generally low, though CT delivers more than a plain x-ray. MRI and ultrasound do not use ionizing radiation. Doctors follow the principle of using the lowest reasonable dose.

Exposure comes almost entirely from medical tests:

• CT scans, x-rays, and some nuclear-medicine tests.
• Cumulative exposure builds up with many repeated scans.
• Children are more sensitive to radiation than adults.

Ionizing radiation from imaging carries a small added lifetime risk of cancer. Leukemia and solid tumors are the main examples. The risk rises with dose and with the number of scans.

IARC places medical imaging radiation in Group 1, carcinogenic to humans. That is its strongest evidence category. It means there is enough evidence that this radiation can cause cancer in people. IARC is the cancer research agency of the World Health Organization. In the United States, the National Toxicology Program lists X-radiation and gamma radiation, the kinds used in medical imaging, in its Report on Carcinogens as known to be human carcinogens.

Calling imaging radiation a carcinogen speaks to hazard. Hazard is whether something can cause cancer under some conditions. That is not the same as your personal risk. Your risk depends on how much you are exposed to, for how long, and other factors. Group 1 covers agents whose everyday risks are nothing alike. The label says a thing can cause cancer. It does not say how likely that is for you.

• Ask if a scan is needed, and whether MRI or ultrasound could work instead.
• Use facilities that follow child-appropriate, low-dose protocols.
• Keep a record of past imaging so tests are not repeated for no reason.
• Do not skip scans your doctor considers important out of fear.

To see where medical imaging radiation sits among all the things that affect cancer risk, read our overview of cancer prevention and what raises cancer risk.

Medical imaging radiation is a known human carcinogen (IARC Group 1). The National Cancer Institute says the cancer risk from these tests is very small, and the benefit is almost always greater than the risk. The useful questions are whether a scan is needed and whether a test without radiation would work.