A Rare Hybrid Presentation: Coexistence of Necrotizing and Histiocytoid Variants of Sweet Syndrome (SS) in a Patient With Acute Myeloid Leukemia (AML).

Sweet syndrome (SS) is a rare, acute, febrile neutrophilic dermatosis marked by neutrophilic infiltration into the skin, blood, and various organ systems. It manifests as painful skin eruptions, which can range from violaceous papules and plaques to vesicles and ulcers, accompanied by peripheral blood neutrophilia and fever. Sweet syndrome has three subtypes: classical, drug-induced, and malignancy-associated. There are different clinical and pathological variants of SS. However, rare variants, such as necrotizing SS and histiocytoid SS, can complicate the clinical and pathological picture, particularly in the context of hematologic malignancies. We report a case of a female patient in her late 50s with acute myeloid leukemia (AML) who developed rapidly progressing, painful, necrotic skin lesions. Clinically, the presentation raised immediate suspicion for necrotizing fasciitis. However, histopathological examination revealed a dense dermal infiltrate of mononuclear cells resembling histiocytes. Immunohistochemical staining confirmed these cells were immature myeloid precursors (MPO+/CD68+), consistent with histiocytoid SS. Despite the clinical appearance of full-thickness necrosis, the patient responded dramatically to high-dose systemic corticosteroids, avoiding the need for extensive surgical debridement. This case highlights the rare overlap of necrotizing and histiocytoid variants of SS as a paraneoplastic phenomenon in AML. It underscores the importance of recognizing these atypical presentations to prevent misdiagnosis of infection and to ensure the timely initiation of immunosuppressive therapy. Clinicians should remain vigilant, as these variants may serve as a sentinel marker for underlying or relapsed leukemia.