What is brachytherapy?
It is a way of delivering radiation therapy in a very targeted fashion to a tumor site, minimizing radiation to the surrounding area. It has mainly been used thus far in delivering curative doses of radiation to the prostate gland and the female cervix, when disease is localized to the area.

Traditional delivery of radiation via external sources delivers larger doses to the surrounding areas than is required for curing the primary tumor. In an area like the pelvis, normal tissue in the rectum and bladder is in very close proximity to the target area, and is unduly affected, causing diarrhea and bladder inflammation. Brachytherapy is achieved by insertion of radioactive needles in the tumor bed, and removal after the dose has been delivered a few days later. 
This method of delivering radiation is also being used in limited circumstances in delivering radiation to a breast, after the primary tumor has been resected. Traditionally, a lumpectomy is followed rby whole breast radiation, to decrease local recurrence rates. This is delivered in daily fractions over 6-7 weeks. Brachytherapy would involve implantation of radioactive needles via a balloon implanted into the lumpectomy site, with removal of the balloon with the needles after completion of therapy. The superiority or equivalence of this method is still being established with regard to local recurrence rates, and should really be used in selected circumstances, and in the context of a clinical trial.