Noticeable advances have been made in reducing the reoccurrence of breast cancer after a lumpectomy through invasive irradiation of the surrounding participating tissue. One effective postoperative procedure introduces a radionuclide applicator surrounded by an inflatable balloon into the evacuated breast cavity. Through, a series of polyethylene guide tubes, radioactive sources are introduced in a time-controlled program to irradiate tissue surrounding the lumpectomy site to a desired integrated dose of about 30 Gy at a depth of 1cm over a 5-day period in 10 treatment fractions. To be most effective and to minimize collateral radiation damage, reliable and accurate dose estimates must be performed in the patient treatment planning stage. The current treatment uses the TG-43 protocol  where radiation transport is estimated in a standard tissue phantom to provide best estimates of the delivered dose. Given today's computational power and comprehensive radiation transport algorithms, it is generally thought by those in the radiation oncology community that we should be able to more precisely predict doses, which is the subject of this presentation.