What Are X-rays and Gamma Rays?
Many types of radiation exist. When it comes to cancer risk, people are most often concerned about x-rays and gamma rays.
X-rays and gamma rays can come from natural sources, such as radon gas, radioactive elements in the earth, and cosmic rays that hit the earth from outer space. X-rays and gamma rays can also come from human-made sources, like medical imaging tests, cancer treatment, and nuclear power plants.
X-rays and gamma rays are in packets of energy known as photons. Because x-rays and gamma rays have the same properties and health effects, they are grouped together here.
Are X-rays and gamma rays ionizing radiation?
Both x-rays and gamma rays are forms of high-frequency (high-energy) ionizing radiation. This means they have enough energy to remove an electron from (ionize) an atom or molecule.
Ionizing radiation can damage the DNA (genes) inside a cell. This damage might increase the risk of cancer later on.
In addition to gamma rays and x-rays, other types of ionizing radiation include radon and some higher-energy ultraviolet (UV) rays.
Ionizing radiation can exist as rays and in particle form, such as protons, neutrons, and alpha and beta particles.
Doses of radiation
Radiation exposure can be expressed in different ways.
- Radioactivity is the rate at which a material releases ionizing radiation. It is measured in curies (Ci) or in becquerels (Bq). The higher the number, the more radiation is being released.
- Absorbed dose is the amount of energy deposited in an object or a person’s body per unit of mass. Most often this is measured in rads or grays (Gy). A milligray (mGy), which is 1/1000th of a Gy, may also be used.
- Effective dose is the absorbed dose multiplied by a conversion factor based on the medical effects of the type of radiation. It is often expressed in rems and sieverts (Sv) or millisieverts (mSv), which are 1/1000th of a Sv.
For x-rays and gamma rays (and beta particles), the equivalent dose in Sv is the same as the absorbed dose in Gy.
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- References
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.
Lee CI, Elmore JG. Radiation-related risks of imaging. UpToDate. 2026. Accessed at https://www.uptodate.com/contents/radiation-related-risks-of-imaging on June 22, 2026.
National Toxicology Program. 15th Report on Carcinogens. 2021. Accessed at https://ntp.niehs.nih.gov/research/assessments/cancer/roc on June 22, 2026.
US Environmental Protection Agency. Radiation Basics. Accessed at https://www.epa.gov/radiation/radiation-basics on June 22, 2026.
Last Revised: June 29, 2026
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