Several studies have shown that the majority of in-breast recurrences following lumpectomy are at or near the original tumor site while ipsilateral breast recurrences further a field occur rarely. This suggests that the radiation dose could be delivered exclusively to the tumor bed, allowing larger fractions to be used without increasing toxicity and shortening the total treatment time. We investigated the use of the MammoSite irradiation system with a view to analyzing complications, cosmesis and patient comfort. Between 2004 and 2007 intracavity brachytherapy was given to 30 patients using the MammoSite device. The reference isodose was prescribed to the lumpectomy cavity with a 1 cm margin. Geometric parameters and anatomic position of the applicator after implantation were checked via CT, x-ray and ultrasound. Analysis was done for patient quality of life, cosmesis, early and late complications. Forty-nine patients received a proposal for MammoSite brachytherapy. Nine declined, 40 enrolled while 10 were excluded for various reasons (Table 5). A total of 30 patients were actually treated to 34 Gy (2 x 3.4 Gy) in 5 days. We observed 3 cases (10%) of infection within 3 months of implantation. Symptomatic seroma was seen in five patients (16.6%) at 6 months, in three (10%) at 12 months, and in just one patient (3.3%) at 18 months. Good to excellent cosmetic results were achieved in 75% by patient and physician ratings. Accelerated partial breast irradiation using the MammoSite catheter produces favorable short-term outcomes, limited toxic effects on skin, and optimal cosmetic results. Patient tolerance for the treatment is very high. Critical issues may regard the importance of good cavity conformance and adequate balloon-skin distance in avoiding possible dose excesses to the skin. For a selected patient group, this could be a valid alternative to conventional whole breast irradiation.