Loss of Cytokeratin 20 and Acquisition of Thyroid Transcription Factor-1 Expression in a Merkel Cell Carcinoma Metastasis to the Brain

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Merkel cell carcinoma (MCC) is recognized by its morphologic features and by its classic immunophenotypic properties. Although MCCs demonstrating nonclassic immunoreactivities have been described, a case documenting a change in immunophenotype during the course of disease progression has not been previously reported. We report a case of MCC that initially demonstrated cytokeratin 20 positivity but lost expression in subsequent metastases. Likewise, thyroid transcription factor-1 was initially negative in the tumor but expression was present in metastatic lesions.

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