Metastatic ductal carcinoma concealed within poorly differentiated thyroid carcinoma: Case report and literature review

Tumor-to-tumor metastasis (TTM) is a rare and diagnostically challenging phenomenon. We present a unique case of a 58-year-old woman with a right breast mass and progressive neck swelling. Histology revealed poorly differentiated thyroid carcinoma (PDTC) with infiltrating ductal structures in desmoplastic stroma, raising suspicion for metastatic disease. Immunohistochemistry (IHC) confirmed invasive ductal carcinoma (GATA3+, HER2+) within PDTC (TTF1+, PAX8+), establishing a rare diagnosis of breast-to-thyroid TTM. Accurate diagnosis required a multidisciplinary approach, integrating clinical history, imaging, and detailed morphological assessment. Recognizing TTM is essential to avoid diagnostic errors, particularly in cases with coexisting malignancies. Comprehensive immunoprofiling remains critical for precise diagnosis and appropriate therapeutic planning in complex oncologic scenarios.