The small cell group of thyroid tumors that includes lymphoma, poorly differentiated carcinoma, medullary carcinoma, secondary neoplasms, as well as tumors with uncertain histogenesis, remains as a valid diagnostic cul-de-sac due to its heterogeneous constitution. The existence of small cell thyroid tumors with EWSR1-FLI1 rearrangement together with neuroendocrine and/or carcinomatous differentiation raises not only differential diagnostic problems but also a very interesting therapeutic dilemma. This review explores the classification of small cell tumors of the thyroid taking into account the immunophenotype and molecular profile of such tumors.
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