internal radiation treatment that is given by putting a radioactive source right into the tumor or close to it. There are 2 main types of brachytherapy. In intracavitary treatment, the source is placed into a space near the cancer. In interstitial treatment the source is placed directly into the tissues. Brachytherapy is sometimes used along with external beam radiation therapy. See also high-dose rate brachytherapy, low-dose rate brachytherapy, external beam radiation therapy, ionizing radiation.