S100 expression by atypical megakaryocytes: a previously unreported potential pitfall in dermatopathology.

Several types of large atypical epithelioid cells can mimic tumoral melanocytes and can therefore be a source of misdiagnosis of melanoma. Megakaryocytes are one of these types, and are not frequently mentioned in the literature. In the current report, we present the case of a 76-year-old man presenting with acute panmyelosis with myelofibrosis. The bone marrow biopsy contained atypical megakaryocytes that expressed S100 in the cytoplasm. We discuss how such aberrant expression could be a source of some diagnostic problems in dermatopathology, including cutaneous melanoma, metastasis of melanoma in bone marrow and metastasis of melanoma in sentinel lymph nodes.