Mucinous micropapillary carcinoma of the breast: an aggressive counterpart to conventional pure mucinous tumors.

Mucinous micropapillary carcinoma of the breast, also described as "pure mucinous carcinoma with micropapillary pattern," has recently come to attention as an unusual form of invasive breast cancer exhibiting dual mucinous and micropapillary differentiation. Despite increasing awareness of this morphologic variant, its clinical significance has not yet been elucidated. Here, we present 15 additional examples of these rare tumors to highlight some important differences between mucinous micropapillary carcinoma of the breast and ordinary pure mucinous carcinomas. The key features of mucinous micropapillary carcinoma of the breast included (a) largely or entirely mucinous appearance (>90% mucinous morphology), (b) distinctive micropapillary arrangement of the neoplastic cells, (c) intermediate to high nuclear grade, (d) "hobnail" cells, and (e) frequent psammomatous calcifications. In contrast to ordinary pure mucinous carcinomas, 20% of mucinous micropapillary carcinomas of the breast were characterized by human epidermal growth factor receptor 2 positivity, and 23% were p53 positive. More than half of mucinous micropapillary carcinomas of the breast (60%) demonstrated lymphovascular invasion, sometimes extensive. Synchronous axillary lymph node metastases were detected in 33% of patients and, on 2 occasions, involved more than 10 nodes. With a median follow-up of 4.5 years, we identified 1 patient (7%) with chest wall recurrence of mucinous micropapillary carcinoma of the breast after mastectomy. We conclude that mucinous micropapillary carcinomas of the breast constitute a clinically aggressive subset of mucin-producing breast carcinomas characterized by an increased capacity for lymphatic invasion and regional lymph node metastasis, reflective of their dual phenotype. Recognition of the morphologic and biologic heterogeneity within breast cancer subtypes should allow for a more accurate classification of the individual tumors and better patient stratification for treatment.