Five-year outcome of patients classified in the "unsuitable" category using the American Society of Therapeutic Radiology and Oncology (ASTRO) consensus panel guidelines for the application of accelerated partial breast irradiation: An analysis of patients treated on the American Society of Breast Surgeons MammoSite^® registry trial

Purpose: We applied the ASTRO Consensus Panel (CP) guidelines for the application of accelerated partial breast irradiation (APBI) to patients treated with this technique on the ASBS MammoSite^® registry trial to determine potential differences in outcome of patients classified in the "unsuitable" category. Methods: Of 1,449 cases treated with APBI on the registry trial, 176 fit the criteria for the unsuitable category: 130 cases were <50 years of age, 13 had positive margins, 38 had positive nodes, 6 had tumors >3 cm, and 9 had an EIC >3 cm. Rates of ipsilateral breast tumor recurrence (IBTR) and regional nodal failure (RNF) were assessed. Median follow-up was 53.6 months. Results: The 5-year actuarial rate of IBTR for unsuitable cases was 5.25% (RNF rate was 0.63%). By comparison, the 5-year actuarial IBTR rates for various subsets of patients were: all 1,449 cases, 3.89% (p = 0.2365); all 1,449 cases excluding unsuitable cases [n = 1,273] (3.6%, p = 0.1683); invasive only cases [n = 1,255] (3.86%, p = 0.2464); and invasive only cases excluding unsuitable invasive cases [n = 1,105] (3.89%, p = 0.2396). On univariate analysis for variables potentially associated with IBTR in all 1,255 cases with invasive cancer (including age, tumor size, nodal status, overall stage, margin status, ER status, presence of an EIC, and ASTRO unsuitable category), only negative ER (-) status was associated with the 5-year rate of IBTR (p = 0002). No other variable (including unsuitable CP designation) was associated with IBTR. Conclusions: The ASTRO CP guideline designation of unsuitable did not differentiate a subset of patients with a significantly worse rate of IBTR when treated with the MammoSite^® breast brachytherapy catheter to deliver APBI.