[A misleading breast metastasis of medullary thyroid carcinoma].

Recognition of mammary metastases by pathologists is fundamental because their prognosis and treatment are different from those of primary mammary carcinomas. We report the case of a 54-year-old woman presenting on her mammography a left breast nodule known for 5 years, having discreetly increased in size. Breast ultrasound showed a regular 1.2cm hypoechogenic nodular formation. A microbiopsy was performed. On microscopic examination, we observed a tumor proliferation realizing nests within a small, richly vascularized stroma. The tumor cells had a moderately abundant, eosinophilic, granular cytoplasm and a rounded, slightly atypical nucleus. One mitosis was found for 10 fields at×400 magnification. Tumor cells did not express hormone receptors but chromogranin A, synaptophysin, TTF1 and thyrocalcitonin. The proliferation index established by the anti-Ki67 antibody was 5 %. The diagnosis was a secondary localization of a well-differentiated neuroendocrine tumor which immunohistochemical profile firstly suggests a thyroid origin. We later learned that the patient had a history of total thyroidectomy 13 years ago. It was a sporadic medullary carcinoma of the thyroid. Bone scintigraphy revealed a lacunar lesion of the posterior part of the right iliac wing suspicious of secondary location. This right iliac lesion was biopsied. It was also a localization of the medullary thyroid carcinoma. The final diagnosis is a metastatic medullary thyroid carcinoma, slowly progressive, the mammary metastasis having probably existed for 5 years.