Pure Mucinous Breast Carcinoma with Micropapillary Pattern (MUMPC): A Case Report

Pure mucinous breast carcinoma with micropapillary pattern (MUMPC) was proposed as a new histopathological variant of pure mucinous carcinoma (PMC) with tumor cells forming a micropapillary architecture. The Classification of Tumours of the Breast by the World Health Organization, however, does not differentiate MUMPC as a distinct subtype. There is currently no consensus whether tumors that exhibit these features are classified as PMC or invasive micropapillary carcinoma (IMPC) with associated mucin production. A 45-year-old woman was examined for a tumor in her left breast. Upon physical examination, an elastic hard mass of around 5 cm along with accompanying skin flare and ulceration was palpated in the upper outer quadrant of the left breast. Mammary ultrasonography revealed a clearly marginated hypoechoic tumor of 55.0 × 46.9 × 37.0 mm in size in the upper outer quadrant of the left breast. A vacuum-assisted biopsy (VAB) was performed in the same site and histopathological diagnosis of PMC was made. Contrast-enhanced magnetic resonance imaging (MRI) showed a T1W1 low-intensity signal and a T2W1 high-intensity signal at the primary focus, ring enhancement of the tumor margin, and stranding enhancement inside the tumor. A preoperative diagnosis of left breast cancer (PMC), cT4bN1M0, stage IIIB, luminal B-like was made. We performed a simple mastectomy with axillary lymph node dissection. A 55.0 × 48.1 × 37.1 mm tumor with the gelatinous cut surface was excised. Histopathological examination of the excised specimen revealed mucin lake formation in the tumor containing clusters of atypical cells. The atypical cells showed swollen, irregular nuclei and a papillary growth pattern that lead to the diagnosis of MUMPC.