Background: The risk of axillary failure (AF) after accelerated partial breast irradiation (APBI) using MammoSite brachytherapy is unknown and has been source of concern as the axillary region is not treated with this technique. We aimed to determine the rate of AF in patients treated with APBI and identify factors associated with its occurrence. Methods: Data from the American Society of Breast Surgeons MammoSite Registry Trial were reviewed and patients with AF were identified. Clinical, pathologic and treatment-related variables were analyzed to determine which factors were associated with AF. Results: A total of 1440 patients underwent MammoSite APBI. A total of 1449 cases were treated (9 patients received bilateral treatment), 1255 cases (87%) of invasive breast cancer and 194 cases (13%) of ductal carcinoma in situ (DCIS). The median length of follow-up was 59 months. There were 10 patients who had an AF. Of these, 9 patients had an initial diagnosis of invasive cancer and 1 had an initial diagnosis of DCIS. The 5-year actuarial rate of AF was 0.79%. The only independent risk factor for AF identified by multivariate analysis was the presence of high-grade disease (P = .008). The 5-year overall survival rate in patients with an AF was 77.8% (there was 1 death related to breast cancer). Conclusions: The rate of AF after MammoSite APBI is low and appears to be similar to that achieved with wholebreast irradiation.
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