Case Report: Breast metastasis from biliary tract cancer: a rare case and literature review.

Breast metastasis from biliary tract cancer (BTC) is an extremely rare clinical occurrence. We report the case of a 59-year-old woman who was incidentally found to have a hepatic mass, leading to a comprehensive workup prompted by progressive chest discomfort. Clinical examination revealed a firm left breast with nipple retraction and a palpable axillary mass. Imaging confirmed a non-mass enhancement lesion in the left breast along with extensive metastases to the liver, lungs, bone, and mediastinal lymph nodes. Core needle biopsies of the breast and abdominal wall lesion showed morphologically similar adenocarcinoma. Immunohistochemistry (IHC) was instrumental in the diagnosis, demonstrating positivity for CK19 and IMP3, focal positivity for CDX2, and a high Ki-67 index (80%), while key breast markers (GATA3, ER, PR, and HER2) were all negative. This immunoprofile strongly supported a biliary origin. The patient was diagnosed with advanced BTC with multiple metastases, including to the breast. She received supportive care and was referred for systemic therapy evaluation. This case highlights that BTC breast metastasis can mimic primary breast carcinoma, underscoring the critical role of pathology and IHC in achieving an accurate diagnosis, which is essential for guiding appropriate, individualized treatment and avoiding unnecessary surgery.