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Breast carcinoma metastatic to follicular adenoma of the thyroid gland

Sir: We report a case of carcinoma of the breast metastatic to a follicular adenoma of the thyroid gland, initially diagnosed by fine needle aspiration biopsy (FNAB) and later confirmed in the hemithyroidectomy specimen. The patient, a 42-year-old white woman, was referred to the surgery clinic with a nonpalpable breast lump detected on mammography. On examination, a left-sided thyroid nodule was palpated. FNAB of the breast lesion was performed with stereotactic localization and revealed malignant cells in clusters, with a high nucleocytoplasmic ratio, pleomorphic and hyperchromatic nuclei and prominent nucleoli (Figure 1A). Examination of the thyroid FNAB showed two populations of cells: many uniform small epithelial cells forming microfollicles, and a discohesive element composed of polygonal larger cells with pleomorphic, hyperchromatic nuclei and a high nucleocytoplasmic ratio. A diagnosis of metastatic carcinoma was made despite equivocal immunostaining for oestrogen and progesterone receptors in neoplastic cells. The resected left thyroid lobe measuring 35 · 15 · 13 mm, contained a well-encapsulated heterogeneous firm white nodule, 15 mm in greatest dimension. Histologically, pleomorphic tumour cells of metastatic breast carcinoma were intermingled with follicles of a microfollicular adenoma of the thyroid