Case Report: ALK-positive histiocytosis with a novel

Background: ALK-positive histiocytosis is a rare form of cutaneous histiocytosis and has been explicitly designated as a novel histiocytic neoplasm in recent classification systems. This disease may involve multiple organs throughout the body. Its diagnosis primarily relies on pathological confirmation, and molecular evidence of Case presentation: An 8-month-old Chinese male infant was admitted with 1-day history of fever and newly diagnosed anemia of 0.5-day duration. Physical examination revealed chronic rashes on the bilateral lower extremities, including the plantar surfaces. Laboratory investigations demonstrated anemia (hemoglobin level: 63 g/L), elevated inflammatory markers, and an increased serum S-100 protein level (9.11 ug/L). Abdominal ultrasonography and contrast-enhanced computed tomography (CT) revealed hepatosplenomegaly and diffuse hypoechoic or cystic low-density lesions in the liver (largest approximately 7 × 8 mm), which were initially suspected to represent a liver abscess. Histopathological examination of a biopsy specimen from the lower extremity rash revealed dermal proliferation of histiocytes, foamy cells, and short spindle cells, accompanied by scattered multinucleated giant cells and lymphocytes. Immunohistochemical analysis demonstrated positivity for CD68, S100, and ALK, and negativity for CD1a, CD3, CD20, and other markers. Fluorescence Conclusion: This study reports a rare case of infantile ALK-positive histiocytosis that was initially misdiagnosed as a liver abscess and identifies a novel