Mucinous Carcinoma Masquerading as Fibroadenoma in a Postmenopausal Woman: A Histopathological and Immunohistochemical Case Report

Background: Mucinous carcinoma of the breast is a rare special histological type of invasive breast carcinoma characterizedby abundant extracellular mucin surrounding malignant epithelial cells. It occurs predominantly in older and postmenopausalwomen and generally demonstrates a favorable clinicopathological profile, with frequent hormone receptor expression andlow rates of HER2 overexpression. Its circumscribed appearance may occasionally result in a benign clinical or radiologicalimpression. Case Report: A 58-year-old postmenopausal woman presented with a palpable lump in the left breast. Based onthe clinical assessment, the lesion was considered likely to represent a fibroadenoma. A true-cut biopsy was performed.Histopathological examination revealed an invasive malignant epithelial neoplasm comprising tumor cells arranged withinabundant extracellular mucin pools, consistent with mucinous carcinoma of the breast. Immunohistochemical evaluationdemonstrated estrogen receptor (ER) positivity with an Allred score of 6/8, while progesterone receptor (PR) and humanepidermal growth factor receptor 2 (HER2/neu) were negative. The findings established the diagnosis of mucinous carcinomadespite the clinically benign impression. Conclusion: Mucinous carcinoma of the breast can present as a well-defined orclinically benign-appearing breast mass and may mimic fibroadenoma, particularly in older women. Recognition of thecharacteristic mucinous morphology on core biopsy, supplemented by appropriate immunohistochemical evaluation, isessential for accurate diagnosis. This case highlights the importance of tissue diagnosis in breast masses that clinically resemblebenign lesions.