Ipsilateral Breast Tumor Detected 20 Years after Occult Breast Cancer: A Diagnostic Challenge in Distinguishing New Primary Cancer from Late Recurrence

Introduction: Occult breast cancer (OBC) is defined as axillary lymph node metastasis without an identifiable primary breast tumor. Although advances in imaging have reduced the incidence of "true" OBC, long-term outcomes extending beyond a decade remain rarely reported. Recent literature has also suggested that a subset of OBC may originate from ectopic breast tissue located within axillary lymph nodes, suggesting biological heterogeneity within this rare entity. Case Presentation: A 54-year-old woman presented with right axillary lymphadenopathy. Comprehensive imaging showed no intramammary lesion, and surgical biopsy confirmed metastatic breast cancer, consistent with OBC. Axillary lymph node dissection revealed seven metastatic nodes (ER 50%, PR 0%, HER2 3+). She received adjuvant chemotherapy and a non-steroidal aromatase inhibitor for 10 years without recurrence. Twenty years later, screening mammography identified a new spiculated mass in the ipsilateral breast. Core needle biopsy revealed HER2-positive invasive ductal carcinoma (ER