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Background: Micropapillary carcinoma (MPC) is known to have a worse prognosis than the other subtypes of breast cancer. Occasionally, MPC is observed in association with invasive ductal carcinoma not otherwise specified (IDC NOS), as well as mucinous carcinoma. Methods: We examined the immunohistochemical expression of an estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) in 127 cases of surgically resected MPC or IDC NOS with MPC. Further, we classified these cases based on their immunohistochemical profile. Results: Among the IDC NOS with MPC cases, 47 were luminal A (62.7%), 10 were luminal B (13.3%), and 9 were HER2 (12.0%). The MPC cases included 4 luminal A (50.0%), 2 luminal B (25.0%) and 1 HER2 (12.5%) subtypes. Of the mucinous carcinomas with MPC, 4 were grouped as luminal A (57.1%), 1 as luminal B (14.3%), and 2 as HER2 (28.6%) subtypes. However, among the mucinous carcinomas, 33 were categorized as luminal A (89.2%), 3 as luminal B (8.1%), and 1 as HER2 (2.7%) subtype, indicating a low incidence of HER2 subtype as compared to the other subtypes. Conclusions: The luminal B and HER2 subtypes were prevalent in carcinomas with MPC. This result explains the poor prognosis of breast carcinomas with an MPC pattern.
