Abstract Purpose/Objective(s): Recent data from Intergroup Study E5194 prospectively defined a low risk subset of ductal carcinoma in situ (DCIS) patients where radiation therapy was omitted after local excision alone. At 5 years, they found a 6.8% ipsilateral breast event rate (IBTR) in selected low/intermediate grade (LIG) patients and a 13.7% IBTR in selected high grade (HG) patients. The purpose of our study was to determine the IBTR in DCIS patients treated on the American Society of Breast Surgeons (ASBrS) MammoSite® Breast Brachytherapy registry trial who met the criteria of E5194 treated with local excision and adjuvant APBI.Material/Methods: A total of 194 patients with DCIS were treated between 2002 and 2004 on the Mammosite registry trial; of these, 69 patients met the enrollment criteria for E5194: 1. LIG: low to intermediate grade with a pathological size > 0.3cm but < 2.5cm and margins >3mm (n=40) or 2. HG: high grade, pathological size <1cm and margins >3mm (n=29). All patients were treated with lumpectomy followed by adjuvant APBI (34 Gy in 3.4 Gy fractions). Median follow-up for surviving patients was 50.7 months (range, 0-73.4). Hormonal therapy was given to 51% of these patients (compared to 30% of patients on E5194). The clinical-pathologic data and long term outcomes for each patient were entered into a database for statistical analysis using SAS (v 8.2).Results: The median age of the entire cohort was 62.5 years. In the LIG cohort, the median size was 0.8cm. In the HG cohort, the median size was 0.6cm. In the LIG cohort, the 5-year IBTR was 0%, compared to 6.8% at 5 years in E5194. In the HG cohort, the 5-year IBTR was 5.3% compared to 13.7% at 5years in E5194. The overall the 5-year IBRT was 2% and there were no cases of elsewhere or regional failures in the entire cohort. The 5-year contralateral breast event rate was 0% and 5.6% in LIG and HG patients, respectively (compared to 3.5% and 4.2%, respectively, in E5194). The 5-year disease free survival and overall survival rate was 92.4% and 96.5%, respectively. In all patients with DCIS (n=194) treated with Mammosite on the registry trial, the 5-year actuarial IBTR, DFS, and OS was 3.1%, 93.8% and 98.2%, respectively.Conclusion: Adjuvant accelerated partial breast irradiation using Mammosite is a relatively convenient form of radiotherapy which shortens treatment time to 1 week. This study found that patients who met the criteria of E5194 treated with APBI had extremely low rates of recurrence (0% vs. 6.8% in the low to intermediate grade and 5.3% vs. 13.7% in the high grade). We conclude that all patients with DCIS who were eligible for E5194 have appreciable benefit from adjuvant APBI. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 951.