Radiation therapy is a cancer treatment. It uses high doses of radiation to kill cancer cells and stop them from spreading. About half of all cancer patients receive it. The radiation may be external, from special machines, or internal, from radioactive substances that a doctor places inside your body. The type of radiation therapy you receive depends on many factors, including

• The type of cancer
• The size of the cancer
• The cancer's location in the body
• How close the cancer is to normal tissues that are sensitive to radiation
• How far into the body the radiation needs to travel
• Your general health and medical history
• Whether you will have other types of cancer treatment
• Other factors, such as your age and other medical conditions

Radiation therapy can damage normal cells as well as cancer cells. Treatment must be carefully planned to minimize side effects. Common side effects include skin changes and fatigue. Other side effects depend on the part of your body being treated.

Sometimes radiation is used with other treatments, like surgery or chemotherapy.

NIH: National Cancer Institute