Blastic plasmacytoid dendritic cell neoplasm presenting as a subcutaneous mass in an 8-year-old boy.

Blastic plasmacytoid dendritic cell neoplasm (also referred to as CD4+/CD56+ hematodermic neoplasm) is a rare hematological malignancy typically seen in older adults. The disease presents with nonspecific cutaneous lesions and advances toward a fatal leukemic phase despite an often favorable initial response to chemotherapy. Fewer than 200 cases have been documented, and it is exceedingly rare in children. We report a case of histopathologically proven blastic plasmacytoid dendritic cell neoplasm arising in an otherwise healthy and asymptomatic 8-year-old boy who noticed a painless mass within the subcutaneous tissues below the left calf. In addition to contrast-enhanced MRI, the imaging work-up included PET/CT, which showed metastatic involvement within left inguinal and retroperitoneal lymph nodes. The case is notable both for the young age of the patient and for the absence of characteristic cutaneous manifestations of this disease.