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 <5%, PR 15%, HER2 3+, MIB-1 51%). Neoadjuvant chemotherapy with trastuzumab resulted in a clinical complete response, and total mastectomy yielded a pathological complete response. Conclusion: This case illustrates an exceptionally rare occurrence of an ipsilateral HER2-positive breast tumor appearing 20 years after treatment for OBC. The absence of MRI at the initial diagnosis, the long disease-free interval, and the discordant tumor biology highlight the diagnostic challenge of distinguishing a new primary cancer from a delayed manifestation of occult disease. Furthermore, considering emerging evidence that some OBC may arise from axillary ectopic breast tissue, the present case - lacking any pathological features of ectopic tissue - is more consistent with a metastatic origin than with an ectopic primary, although a definitive distinction cannot be established. Lifelong surveillance is essential for patients with OBC, even after prolonged remission.