Malignant melanoma masquerading as primary breast carcinoma, a diagnostic dilemma in metastatic disease: Case report and a brief review of the literature

Metastatic melanoma to the breast is a rare but diagnostically challenging entity, frequently mimicking primary breast carcinoma due to overlapping clinical and radiological features. We present the case of a 59-year-old postmenopausal woman who presented with a progressively enlarging left breast mass and axillary lymphadenopathy, highly suggestive of invasive ductal carcinoma (IDC) on imaging. Initial histopathological examination provisionally favored poorly differentiated ductal carcinoma. However, a comprehensive immunohistochemical (IHC) analysis, revealing negative breast-specific markers (ER, PR, HER2, GATA3, mammaglobin) and strong positivity for melanocytic markers (S100, SOX10, MART-1, HMB-45, Melan-A), which established the diagnosis of metastatic melanoma. Subsequent molecular analysis identified a BRAF V600E mutation, and a small rectal mass that ultimately identified as the primary site. The patient achieved a sustained partial response with anti-PD-1 immunotherapy. This case features the critical importance of a high index of suspicion and thorough IHC evaluation in atypical breast presentations, even in the presence of compelling clinical and radiological evidence for primary breast cancer. It highlights a significant diagnostic dilemma and the transformative impact of precision oncology in managing such rare metastatic melanoma to the breast.