Case 46. Bruise-like nodules of blastic plasmacytoid dendritic cell neoplasm on the background of diffuse petechiae

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare, aggressive hematologic malignancy primarily in elderly males. Almost all patients present with cutaneous tumors characterized by purpuric hue secondary to thrombocytopenia. Lymphadenopathy and pancytopenia are noted to have varying degrees, and CNS involvement is seen in up to 30% of cases. BPDCN arises from plasmacytoid dendritic cells. Those are large cells typically positive for CD4, CD56, BDCA-2, CD123, and TCL1 and negative for markers of myeloid, T-lymphoid, B-lymphoid, or monocyte lineage. Up to 10–20% of patients with BPDCN have a concurrent history of hematologic malignancy, including myelodysplastic syndrome, chronic myelomonocytic leukemia, and acute myeloid leukemia. BPDCN often undergoes a leukemic transformation with a median survival of less than 1 year. A new subset of skin-limited BPDCN with an excellent prognosis was recently defined; however, it is not clear which patient will remain to have a skin-limited disease since cases of metastatic spread after a presentation with an isolated cutaneous nodule were also described.