Characteristics and prognosis of 17 special histologic subtypes of invasive breast cancers according to World Health Organization classification: comparative analysis to invasive carcinoma of no special type

Breast cancer is a heterogeneous disease with various histopathologic subtypes. Except for invasive carcinoma of no special type (NST), other subtypes are rare with limited data. The purpose of this study was to analyze the characteristics and prognosis of special histopathologic subtypes of breast cancer compared to NST. A total of 136,140 patients were analyzed using the Korean Breast Cancer Society Registry database between January 1996 and March 2019. The clinicopathologic features and survival outcomes of special type breast carcinoma were compared with those of NST. The prevalence of special subtypes other than NST was 13.7% (n = 18,633). Compared to NST, patients with lobular, medullary, metaplastic, and micropapillary carcinoma had larger tumors (p < 0.001). Patients with mucinous, tubular, medullary, metaplastic, and cribriform carcinoma presented with less node metastasis (p < 0.001), contrary to patients with micropapillary carcinoma. Patients with lobular, mucinous, tubular, papillary, and cribriform carcinoma presented as luminal A subtype much more often (p < 0.001). Micropapillary carcinoma included more luminal B subtype (p < 0.001). Typically, medullary and metaplastic carcinoma included more triple-negative subtypes (p < 0.001). In survival analysis, only medullary (Hazard Ratio (HzR) 0.542, 95% CI 0.345 to 0.852, p = 0.008) and metaplastic carcinoma (HzR 1.655, 95% CI 1.317 to 2.080, p < 0.001) showed significantly different overall survival from NST by multivariate analysis. Breast cancer had distinct clinicopathologic features according to histopathologic subtype. However, special types of breast cancer had similar survival outcomes compared to NST when adjusting for other prognostic factors, except for metaplastic carcinoma and medullary carcinoma.