Unusual morphologic presentations of blastic plasmacytoid dendritic cell neoplasm: report of two cases misdiagnosed as melanoma and leprosy.

We present 2 cases of blastic plasmacytoid dendritic cell neoplasm (BPDCN) showing unusual histological features. One patient, a 73-year-old male, presented with a nonpruritic macular erythema of the skin on the anterior and posterior chest wall, the biopsy of which was originally diagnosed as malignant melanoma. The neoplastic cells were negative for S100 and HMB45 and strongly positive for CD45, CD4, CD56, and CD123. The final diagnosis was a BPDCN associated with abundant melanin pigment and numerous melanophages. The second patient was a 73-year-old male with a 5-month history of small, slowly enlarging, bruise-like plaques on his limbs and chest. Histologic examination of the skin biopsy revealed an atypical cellular/myxoid infiltrate with numerous macrophages, which was originally diagnosed as consistent with lepromatous leprosy. The atypical cells were immersed in an alcian blue-positive myxoid matrix at pH 2.5. The Fite-Faraco stain was negative. Positive immunoreactivity was demonstrated for CD4, CD56, and CD123. Based on the histopathology and immunohistochemistry findings, a diagnosis of BPDCN with prominent myxoid matrix was rendered.