Background: Mastectomy remains an effective treatment for ductal carcinoma in situ (DCIS) but whether further therapy is warranted for close or positive margins is controversial. We aim to characterize the treatment practices of DCIS throughout the United States in patients who undergo mastectomy with close or positive margins to better understand the use of postmastectomy radiation therapy (PMRT). Materials and methods: Using the 2004-2013 National Cancer Database, we identified all female patients with a diagnosis of DCIS who underwent mastectomy. Distributional characteristics were summarized for overall and margin-stratified samples. Characteristic differences were assessed by region and receipt of radiation. Chi-square and independent sample t-tests were used to assess differences for categorical and continuous variables, respectively. Results: In 21,591 patients who met inclusion criteria, 470 patients with close/positive margins were identified. Sixteen percent of patients with close/positive margins received PMRT compared to 1.5% with negative margins (P < 0.01). There was no difference in PMRT and patient race, insurance status, treatment facility, or endocrine therapy. Patients with close/positive margins who received PMRT were more likely to be in an urban setting from the Midwest (24.6%) and Northeast (21.8%) compared to the West (11.0%) and South (10.7%) (P < 0.01). Conclusions: Use of PMRT for DCIS following mastectomy with close/positive margins differs across the country. Regional variations in treatment patterns reinforce a need to determine whether PMRT improves survival to establish treatment guidelines.