Case Report: From standard therapy refusal to rare metastases: a case of untreated HER2-positive breast cancer progressing to brain and thyroid metastases after seven years of follow-up during which the patient received only Chinese herbal medicine.

OBJECTIVE: HER2-positive breast cancer is an aggressive subtype associated with high rates of early recurrence and brain metastasis. Standard adjuvant therapy, including anti-HER2 targeted agents, significantly improves outcomes. This case illustrates the natural history of untreated HER2-positive breast cancer and documents an exceptionally rare metastatic site. METHODS: We report a case of a 63-year-old woman with HER2-positive (3+, FISH amplified), ER-positive (80-90%), PR-negative, Ki-67 15% invasive ductal carcinoma of the left breast (pT1cN2aM0, 7/12 axillary lymph nodes positive). RESULTS: The patient refused all standard adjuvant therapies (chemotherapy, radiotherapy, endocrine therapy, and anti-HER2 therapy) and opted for sole Chinese herbal medicine. After seven years of follow-up (with a disease-free interval of approximately four years, she developed a right cerebellar hemisphere metastasis (2.3×2.7 cm with extensive edema), widespread lymphadenopathy, and a pathologically confirmed thyroid metastasis-an extremely rare event in breast cancer. Fine-needle aspiration of the thyroid nodule demonstrated atypical cells, and subsequent immunohistochemistry confirmed breast cancer origin (GATA3+, GCDFP-15+, HER2 3+, ER+, TTF-1-, TG-). CONCLUSIONS: This case provides compelling in vivo evidence of the aggressive natural history of untreated HER2-positive breast cancer. The development of brain metastasis underscores the critical importance of anti-HER2 agents that penetrate the blood-brain barrier. The rare thyroid metastasis adds to the literature on unusual metastatic patterns. This report serves as a powerful warning of the consequences of abandoning evidence-based adjuvant therapy in high-risk breast cancer.