The MammoSite Radiation Therapy System (RTS) has become the most widely used brachytherapy method used in the treatment of breast cancer, due to its ease of use, short learning curve, and requirement of only one interstitial path through the breast skin. The dosimetry is simple, one source position in the middle of the MammoSite balloon catheter. The data on long-term complications, however are not available, though developing. Trials for DCIS are being developed, as well as a comparison trial to standard external beam radiation as well as other forms of accelerated partial breast irradiation (APBI).