Radiation therapy for prostate cancer uses radiation to kill cancer cells and shrink the size of the tumor. Radiation is usually applied to the body by exposing the prostate to a beam of external radiation, or by implanting radioactive seeds inside the prostate.

The use of radioactive seed implants is called brachytherapy. Radioactive seeds are placed into the prostate through thin plastic tubes. A transrectal ultrasound is used to guide the placement of the radioactive seeds.