What are the two main types of radiation therapy?

There are two main types of radiation therapy: external beam and internal.

External beam radiation therapy comes from a machine that aims radiation at your cancer. The machine does not touch you but can move around you, sending radiation from many directions. It is a local treatment, meaning it treats a specific part of your body. If the cancer is in your lung, only your chest is treated, not your whole body. The machine is large, and it may be noisy.

Internal radiation therapy places a source of radiation inside your body. The source can be solid or liquid, and that distinction matters more than it sounds.

A solid source is called brachytherapy. Seeds, ribbons, or capsules holding a radiation source are placed in your body, in or near the tumor. Brachytherapy is also a local treatment. The source keeps giving off radiation for a while after it is placed.

A liquid source is a form of systemic therapy. You swallow it, or receive it through an IV line, and it travels in the blood to tissues throughout the body. Two named versions exist. Targeted radionuclide therapy links a radioisotope to a molecule that seeks out cancer cells. Radioimmunotherapy links a radioactive substance to a monoclonal antibody, which carries the radiation to the cancer.

Both types work the same way at the cell level. Radiation damages the DNA inside cells. Cancer cells whose DNA is damaged beyond repair stop dividing or die, and the body clears the debris. This is not instant. It takes days or weeks of treatment before enough DNA damage builds up. Cancer cells then keep dying for weeks or months after radiation therapy ends. That is worth remembering if a scan taken soon after treatment does not look as different as you hoped.

Different cancers get different types. Brachytherapy is used most often for cancers of the head and neck, breast, cervix, prostate, and eye. A version called radioembolization may be used for liver cancer, or for cancer that spread to the liver. Radioactive iodine, also called I-131, is a systemic treatment used most often for certain thyroid cancers. Targeted radionuclide therapy is used in some advanced prostate cancers and some neuroendocrine tumors. Radioimmunotherapy may be used for lymphoma.

The systemic kind comes with a practical detail nobody should learn by surprise. After systemic radiation, your body fluids, including urine, sweat, and saliva, give off radiation for a while. Your team will give you instructions about contact with other people, especially children and anyone pregnant. Ask for those instructions in writing before the day of treatment.

There is also a ceiling. An area of your body can only safely receive so much radiation over a lifetime. If a region has already been treated, you may not be able to have radiation there again. A different area can often still be treated, as long as the two are far enough apart. This is one reason your radiation oncologist will ask carefully about treatment you had years ago.

Which type you have depends on factors such as the type of cancer, the size and location of the tumor, how close it sits to normal tissue that is sensitive to radiation, your general health, and whether you are having other treatments. Your care team will explain the plan chosen for you.