Loss of cytokeratin 20 and acquisition of thyroid transcription factor-1 expression in a merkel cell carcinoma metastasis to the brain.

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.