Metastatic medullary thyroid carcinoma masquerading as breast lesion: lessons from a rare case

Abstract

Medullary thyroid carcinoma (MTC), derived from parafollicular C-cells, accounts for 5–8% of thyroid malignancies. It has a propensity for hematogenous metastasis, commonly involving liver, lungs, and bones. Breast metastases from MTC are exceedingly rare, with about 40 documented cases reported worldwide. Herein this case describes a 25-year-old female who initially underwent total thyroidectomy and right cervical lymphadenectomy for MTC, staged as pT3bN1bM0 (stage IVA). Following surgery and adjuvant radiotherapy, she developed persistently elevated calcitonin levels. Subsequent Ga-68 DOTANOC PET-CT revealed local recurrence, but the patient declined re-exploration. Three years later, she presented with a breast lump, clinically and radiologically suspected to be fibroadenoma. Wide local excision with intraoperative frozen section revealed metastatic MTC. Immunohistochemistry confirmed the diagnosis (INSM1 positive, GATA3 negative). Breast metastasis from MTC is extremely uncommon and may mimic benign breast lesions, delaying diagnosis. This case highlights the importance of calcitonin surveillance, the role of intraoperative frozen section, and immunohistochemistry in differentiating metastatic MTC from primary breast tumors.