Breast metastasis from medullary thyroid carcinoma mimicking ductal carcinoma with neuroendocrine differentiation

BACKGROUND: Medullary thyroid carcinoma very rarely metastasizes to the breast. Hematogenous spread to the liver, lungs, or mediastinum is more common. CASE: We describe the morphologic and immunohistochemical features of a 63‐year‐old woman who presented with a BIRADS‐5 category nodule in the right breast and enlarged axillary lymph nodes. Core biopsy showed suggested breast cancer with neuroendocrine or apocrine differentiation. The immunohistochemical profile showed (RE−/RP−/HER‐2−) and Ki67 10%. Chromogranin and synaptophysin were positive; AR and GCDFP‐15 were negative. On reviewing the patient's clinical history, it was discovered that she had been treated for medullary thyroid carcinoma 15 years earlier. Additional stains showed positivity for TTF‐1, CEA, and calcitonin. These findings were consistent with a diagnosis of breast metastasis from medullary thyroid carcinoma. We discuss briefly the morphologic features and the possible key features in order to make an accurate diagnosis. CONCLUSION: This case highlights the importance of investigating a history of cancer in patients with discordant or unusual histologic or immunohistochemical findings, as this can help avoid misdiagnosis and inappropriate treatment.