Invasive Micropapillary Carcinoma of the Breast with an Encapsulated-Like Pattern: An Unreported Entity?

Invasive micropapillary carcinoma (IMPC) is a rare subtype of breast cancer, characterized by infiltrative growth and morula-like cell aggregates without fibrovascular cores. We recently encountered an unusual invasive breast carcinoma with a thick fibrous capsule resembling encapsulated papillary carcinoma but with micropapillary features. A 71-year-old woman presented with a palpable mass in the left breast. MRI revealed a 10 cm enhancing oval mass. Core needle biopsy was performed to diagnose invasive carcinoma with micropapillary features [estrogen receptor (ER) and progesterone receptor (PR)-positive, HER2-negative]. The patient received neoadjuvant chemotherapy, followed by a mastectomy with sentinel lymph node biopsy. The mastectomy revealed a well-circumscribed 11 cm mass surrounded by a fibrous capsule. The tumor consisted of tubulo-micropapillary structures without fibrovascular cores, lined by a single layer of cuboidal cells containing nests of foamy macrophages. Rare foci of lymphovascular invasion were observed. Two sentinel lymph nodes were negative. To our knowledge, invasive micropapillary carcinoma with an encapsulated pattern has not been previously reported.