A system used to deliver internal radiation therapy to breast cancer patients after surgery to remove their cancer. MammoSite targets only the part of the breast where the cancer was found. After a patient has had a lumpectomy to remove the cancer, a small balloon on the end of a catheter (thin tube) is inserted into the empty space left by the surgery. The balloon is then filled with liquid and left in place. Using the catheter, radioactive seeds are put into the balloon twice a day for five days and removed each time. Once treatment has ended, the catheter and balloon are removed. MammoSite is a type of intracavitary brachytherapy and partial breast irradiation therapy (PBRT). Also called balloon catheter radiation.