A Rare Case of Inflammatory Luminal B (HER2-Positive) Breast Cancer Misdiagnosed in a 30-Year-Old Woman: A Case Report

A 30-year-old woman presented with a six-month history of a rapidly enlarging, painful mass in the left breast, accompanied by erythema and intermittent fever. Initial imaging suggested a benign lesion, with differential diagnoses including granulomatous mastitis and phyllodes tumor. She was treated empirically with antibiotics and corticosteroids, but her symptoms worsened. Follow-up imaging revealed further mass enlargement and persistent inflammatory changes. A core needle biopsy confirmed human epidermal growth factor receptor 2 (HER2)-positive, lymphocyte-poor invasive ductal carcinoma, Nottingham Grade III, with a high Ki-67 index and extensive necrosis. Staging studies showed no distant metastases. The patient is recommended to undergo neoadjuvant chemotherapy with HER2 targeted therapy, with plans for radical mastectomy and axillary lymph node dissection. This case underscores the diagnostic challenge of distinguishing inflammatory breast conditions. Inflammatory breast symptoms unresponsive to conservative treatment should prompt early biopsy and multidisciplinary evaluation to avoid delays in diagnosis, especially in young women presenting with atypical features.