Radiation therapy is a type of cancer treatment that uses beams of intense energy to kill cancer cells. Radiation therapy most often gets its power from X-rays, but the power can also come from protons or other types of energy.

The term “radiation therapy” most often refers to external beam radiation therapy. During this type of radiation, the high-energy beams come from a machine outside of your body that aims the beams at a precise point on your body. During a different type of radiation treatment called brachytherapy (brak-e-THER-uh-pee), radiation is placed inside your body.

Radiation therapy damages cells by destroying the genetic material that controls how cells grow and divide. While both healthy and cancerous cells are damaged by radiation therapy, the goal of radiation therapy is to destroy as few normal, healthy cells as possible.

Radiotherapy can be given in two ways:

External radiotherapy

Radiation beams from a large machine called a linear accelerator are aimed towards the area of the body where the cancer is located. The process is similar to having an x-ray. You will lie on a treatment table underneath the machine, which will remain outside of your body. You will not see or feel the treatment, although the linear accelerator can be noisy.

Internal radiotherapy

A radiation source is placed inside the body, injected into a vein, or taken by mouth. Types of internal radiotherapy include brachytherapy, where a temporary or permanent radiation source is put inside the body into or near the cancer radionuclide therapy, where a radioactive substance is given as a capsule or liquid to swallow or via an injection and selective internal radiation therapy (SIRT), which uses pellets to treat cancer in the liver.