['We present a 4-year update on the efficacy, cosmetic results, and complications of MammoSite breast brachytherapy in patients enrolled in the American Society of Breast Surgeons registry trial.', 'A total of 1,449 breasts in 1,440 patients with early stage breast cancer undergoing breast-conserving therapy were treated with adjuvant, accelerated partial breast irradiation (APBI) (34 Gy in 3.4-Gy fractions) delivered with the MammoSite device. The median follow-up period for the entire group was 36.1 months.', 'The 3-year actuarial rate of ipsilateral breast tumor recurrence was 2.15%. The 3-year actuarial rate of axillary recurrence was .36%. Complication rates were as follows: infection, 9.5%; seroma, 26.8% (symptomatic seroma, 12.7%); and fat necrosis, 2.0%. The percentages of breasts with good or excellent cosmetic results were as follows: 12 months, 95%; 24 months, 94%; 36 months, 94%; and 48 months, 91%.'] Do four-year clinical update from the American Society of Breast Surgeons MammoSite brachytherapy trial?