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Follicular Thyroid Carcinoma with Unusual Radioiodine-Refractory Breast Metastasis Mimicking Primary Breast Malignancy

The breast is an unusual site for metastasis from malignant tumors, although it may be the first manifestation of an extramammary primary malignancy typically arising in the lung, skin (melanoma), ovary, or lymph nodes (lymphoma). Breast metastasis from thyroid carcinoma is extremely rare and may take years to develop, as differentiated thyroid carcinoma is usually indolent and slow-growing and metastasizes to the lymph nodes, lungs, and bones. Here, we present the case of a middle-aged woman (after subtotal thyroidectomy for multinodular goiter 24 y previously) showing discordant 18F-FDG and iodine avidity in proven metastatic disease involving the lymph nodes, bones, spleen, and breast on whole-body 131I and 18F-FDG scans due to tumor heterogeneity.