NPM1 -Mutated Myeloid Sarcoma/Leukemia Cutis Mimicking Blastic Plasmacytoid Dendritic Cell Neoplasm and Heralding Bone Marrow Acute Myeloid Leukemia.

Myeloid sarcoma and blastic plasmacytoid dendritic cell neoplasm (BPDCN) are hematologic malignancies of immature cells that frequently involve the skin. In the absence of bone marrow involvement or an associated myeloid neoplasm, distinguishing myeloid sarcoma with disseminated cutaneous involvement (leukemia cutis) from BPDCN is particularly challenging because of overlapping clinical and immunophenotypic features. We report a case of NPM1-mutated myeloid sarcoma presenting with rapidly progressive cutaneous lesions and no initial medullary disease, resulting in an initial misdiagnosis of BPDCN and treatment accordingly. The clinicopathologic evolution over an 8-month period is detailed through analysis of 8 skin biopsies and 5 bone marrow examinations. The disease ultimately progressed to refractory acute myeloid (monoblastic) leukemia, and the patient died approximately 8 months after initial presentation.