Radiation therapies can be internal or external. Doctors may recommend internal radiation, also called brachytherapy, when the goal is a high dose of radiation delivered internally to a small area, as in the treatment of breast cancer. The radiation is implanted or injected in the form of wires, "seeds," or capsules, then left in place for a period of time, either until it's removed or until it decays and becomes inactive.

Brachytherapy is usually done as an outpatient procedure, though it's also common to have a radioactive source implanted at the end of surgery. The root word brachy means "short distance," and brachytherapy can be thought of as concentrated radiation from a short distance. Intracavity brachytherapy indicates that the doctor places the radiation into an existing cavity, while interstitial brachytherapy is the term for placing the radiation directly in tissue. MammoSite brachytherapy is a relatively new procedure used to treat breast cancer in which radiation is delivered in the form of a balloon catheter. The balloon is surgically placed where the tumor was removed, and seeds of radiation are delivered through the catheter into the balloon.

Intraoperative radiation therapy, or IORT, refers to radiation that's delivered to the area where a tumor was removed immediately following surgery. This treatment method is common following lumpectomy. Doing this allows the radiation source to be positioned exactly where it will do the most harm to cancerous tissue, while nearby organs and healthy tissue are protected with shields. New portable radiation units now make it possible for IORT to be completed in the operating room at the end of a surgical procedure. Brachytherapy is the most common form of IORT, but for some types of cancer, doctors use external electron beam radiation immediately after surgery.