Mucinous carcinoma of the breast is a rare histological subtype of invasive breast carcinoma characterized by abundant extracellular mucin production and generally favorable biological behavior. However, because it may present as a circumscribed or partially lobulated lesion on imaging, it can mimic benign breast masses and create a diagnostic challenge. We report the case of a 52-year-old asymptomatic postmenopausal woman in whom a left retroareolar breast lesion was detected incidentally during routine screening mammography. Initial mammography revealed a 15 mm partially irregular lobulated mass categorized as BI-RADS 4. Subsequent ultrasonography and breast magnetic resonance imaging confirmed the lesion, and interval enlargement resulted in a BI-RADS 5 assessment. Excisional biopsy demonstrated a well-circumscribed 2.3-cm gelatinous tumor. Microscopically, more than 90% of the lesion consisted of extracellular mucin pools containing neoplastic epithelial cells arranged in adenoid and cribriform-like structures, consistent with pure mucinous carcinoma. Immunohistochemically, the tumor showed strong, diffuse estrogen receptor and progesterone receptor positivity (95% each), HER2/c-erbB-2 negativity (score 0), MUC2 positivity, and a low Ki-67 proliferation index (<5%), supporting a luminal A-like immunophenotype. Based on the available pathological tumor size, the tumor was categorized as pT2. Regional nodal status could not be pathologically assessed because a sentinel lymph node biopsy was not performed (pNX). The patient underwent surgical excision followed by adjuvant chemotherapy and radiotherapy, and was scheduled for endocrine therapy. No evidence of recurrence or disease progression was observed during 12 months of follow-up. This case highlights the importance of screening mammography in the detection of clinically occult mucinous carcinoma and underscores the value of radiologic-pathologic correlation in lesions with deceptively benign imaging features.

Keywords: mucinous carcinoma, breast, screening mammography, histopathology, immunohistochemistry