Histiocytoid Sweet syndrome with haloed myeloid cells masquerading as a cryptococcal infection.

: Histiocytoid Sweet syndrome is a histopathologic variant of Sweet syndrome characterized by an infiltrate of mononuclear cells that have a histiocytic appearance and represent immature granulocytes. The primary histopathologic differential diagnosis for histiocytoid Sweet syndrome includes leukemia cutis and true histiocytic dermatoses. However, it does not usually include a deep mycotic infection. Herein, we describe a case of histiocytoid Sweet syndrome in a 75-year-old man in which histopathologic examination showed a dense dermal infiltrate composed of mature neutrophils and numerous yeast-like mononuclear cells with a surrounding halo, suspicious for cryptococcal organisms. Immunohistochemical and histochemical studies demonstrated expression of myeloperoxidase by the yeast-like forms and an absence of periodic acid-Schiff and mucicarmine staining. Microbiologic culture studies were negative for fungal organisms. The case met all the criteria for Sweet syndrome and, given the cytomorphology of the mononuclear cells with vesicular irregularly rounded nuclei and myeloperoxidase expression, the case was most consistent with histiocytoid Sweet syndrome. The perinuclear haloes in this case most likely represent cytoplasmic vacuolization of the immature, histiocytoid appearing neutrophils secondary to a novel nonapoptotic caspase-independent death pathway. This case highlights the histopathologic similarities that may be present between cryptococcal infection and histiocytoid Sweet syndrome, and the utility of immunohistochemical markers and histochemical staining to differentiate between the two entities.