An unusual case of cutaneous blastic plasmacytoid dendritic cell neoplasm with concomitant B-cell lymphoproliferative disorder.

Blastic plasmacytoid dendritic cell neoplasm is a rare hematologic malignancy characterized by aggressive clinical behavior and frequent cutaneous involvement. We describe a case of a 64-year-old man with a rapidly enlarging subcutaneous forearm mass. Histologic examination of the excisional biopsy specimen revealed a diffuse proliferation of atypical hematolymphoid cells in the dermis extending to the deep subcutaneous soft tissues. Occasional aggregates of small lymphocytes were noted to be distributed within the mass. The tumor cells expressed CD4, CD45, CD56, CD123, and terminal deoxynucleotidyl transferase (TdT) but not CD3, CD20, or CD34. A diagnosis of blastic plasmacytoid dendritic cell neoplasm was rendered. Chromosome analysis revealed a 45 X, -Y karyotype. In addition, flow cytometry identified a small population of monoclonal B cells. A staging bone marrow aspirate and biopsy was performed, which showed normal cytogenetics and no evidence of involvement by blastic plasmacytoid dendritic cell neoplasm. Flow cytometric evaluation of the bone marrow revealed a CD5-negative, CD10-negative monoclonal B-cell population consistent with a B-cell lymphoproliferative disorder. This is a very unusual example of cutaneous blastic plasmacytoid dendritic cell neoplasm with a novel cytogenetic finding and concomitant B-cell lymphoproliferative disorder. Although previously not reported, our case shows that blastic plasmacytoid dendritic cell neoplasm may be associated with lymphoid malignancy. The relationship between the 2 neoplasms, however, is unclear. A high degree of suspicion and bone marrow examination in patients with a new diagnosis of blastic plasmacytoid dendritic cell neoplasm is required to avoid this potential diagnostic pitfall.