Abstract
A 69-year-old, male presented with chronic persistent urticaria beginning in early 2022, which responded to prednisone and triamcinolone. His rash returned on his chest with centripetal spread and left axillary lymphadenopathy in December and was less responsive to steroids in January. His IgE antibodies were negative. By early 2023, he had generalized violaceous plaques and tumors scattered on the proximal extremities and trunk concerning for CTCL, leukemia cutis, Sweet syndrome, and urticarial vasculitis. Biopsy revealed a diffuse monomorphous infiltrate of medium-sized blasts with agranular blue-gray cytoplasm, irregular nuclei, and one to multiple nucleoli in the dermis and invading the subcutaneous fat. There was no angioinvasion, coagulative necrosis or epidermal involvement. Initial sections raised a differential diagnosis of CTCL, diffuse large B-cell lymphoma, histiocytoid Sweet syndrome, leukemia cutis, and blastic plasmacytoid dendritic cell neoplasm (BPDCN). By immunohistochemistry, the cells were positive for CD4, CD56, CD123, and TCL1, but negative for CD3, lysozyme, and myeloperoxidase. Taken together, these findings were diagnostic of BPDCN. Subsequently, the patient was found to have 10% bone marrow involvement. BPDCN is an aggressive and rare hematologic malignancy that can affect the CNS, lymphoreticular system, and skin. It affects males three times more than females with an average onset in the mid-60s. There is a poor prognosis with a median overall survival of one year. The patient was started on an investigational medication targeting CD123, tagraxfusp. We present this case as it represents an unusual urticarial clinical presentation of a rare, high-grade hematologic malignancy.