Pure Mucinous Breast Carcinoma: Integrating Molecular and Immunohistochemical Profiles for Accurate Diagnosis-A Case Report.

Background: Mucinous carcinoma of the breast (MCB) is a rare histological subtype accounting for approximately 2% of all breast cancers, and it is characterized by abundant extracellular mucin production, indolent clinical behavior, and a favorable prognosis compared with other invasive breast carcinomas. Early and accurate recognition is essential due to its distinct pathological and immunohistochemical features. Case Presentation: A 50-year-old woman with no significant medical or family history presented with a palpable mass in her right breast, so she underwent breast screening. The imaging revealed multiple irregular nodular lesions with clustered microcalcifications in the right breast (BI-RADS 4C), so she underwent a right modified radical mastectomy with axillary dissection. However, histopathological examination confirmed a well-differentiated pure mucinous carcinoma (> 90% mucinous component, Nottingham Grade I) without lymphovascular invasion or nodal metastasis (0/13). The tumor was ER/PR-positive, HER2-negative, with a low Ki-67 index (3%), and was completely excised with negative margins. Discussion: Pure MCB exhibits favorable biological behavior and excellent outcomes, particularly when accurately diagnosed through histopathological and immunohistochemical evaluation. Conclusion: Pure MCB is a rare, low-grade tumor with distinct pathological features. Accurate diagnosis and complete surgical excision are essential for optimal management, as patients typically achieve excellent long-term outcomes.