Histiocytoid Sweet syndrome-like presentation of mature plasmacytoid dendritic cell proliferation

Mature plasmacytoid dendritic cell proliferation is a condition associated with myeloid neoplasms, most commonly chronic myelomonocytic leukemia. Plasmacytoid dendritic cells can resemble lymphocytes and histiocytes morphologically and immunophenotypically. Mature plasmacytoid dendritic cell proliferation may therefore go unrecognized if the diagnosis is not suspected and appropriate stains for plasmacytoid dendritic cells are not performed. Herein, we present a case of mature plasmacytoid dendritic cell proliferation masquerading clinically and histologically as histiocytoid Sweet syndrome. The patient, who had previously been diagnosed with mature plasmacytoid dendritic cell proliferation that presented as pink, edematous, pruritic papules and plaques, had initially resolved following induction chemotherapy for acute myelomonocytic leukemia. However, he presented later with indurated purpuric plaques on the trunk within weeks of receiving filgrastim for neutropenia. Biopsies demonstrated marked dermal edema, interstitial, superficial, and deep infiltrate with histiocytoid appearing cells concerning for histiocytoid Sweet syndrome. Further work-up demonstrated that the infiltrate was predominantly composed of CD3-, CD4+, CD34-, CD123+, CD56-, CD68-, myeloperoxidase negative mononuclear cells consistent with mature plasmacytoid dendritic cell proliferation. This case demonstrates that MPDCP should be considered in the differential diagnosis of eruptions that clinically and histologically look like histiocytoid Sweet syndrome but stain negatively for myeloperoxidase.