Brachytherapy treatment consists of the insertion of radioactive sources into tissue to deliver radiotherapy directly to the tumor. Early treatment utilizing this method required intra-operative placement of 15–20 catheters by the radiation oncologist, a highly specialized procedure which limited its widespread use despite encouraging results from the first clinical trial and the benefits to the patient of receiving radiation treatment to only 2 cm of tissue beyond the borders of the lumpectomy cavity and the ability to complete the radiotherapy component of breast conserving treatment in 4–5 days, instead of the usual 4–6 weeks. The MammoSite® Radiation Therapy System allows either a radiation oncologist or a surgeon to implant a much simplified brachytherapy device, thus increasing the number of patients who can receive this treatment. The device, which received FDA approval in May 2002, works by creating a cavity inside the lumpectomy site via a balloon. A high dose-rate brachytherapy source is inserted into this cavity, evenly irradiating the tissue at highest risk of containing residual cancer cells. Data collected since its approval show that MammoSite offers tumor control equal to traditional radiotherapy with fewer side effects of radiation exposure to nearby tissues. However, the invasive nature and high cost of this treatment mean that MammoSite should not be seen as the sole future direction of radiotherapy treatment delivery methods.