18F-FDG PET/CT Findings in an Infant With Multisystem ALK-Positive Histiocytosis

ALK-positive histiocytosis is a rare histiocytic neoplasm seen predominantly in infants and children and may present as single-system or multisystem disease, often with neurologic involvement. We report a 5-month-old girl with pancytopenia and hepatosplenomegaly whose whole-body 18F-FDG PET/CT demonstrated intensely FDG-avid disseminated disease involving the brain, left parapharyngeal/skull-base region, lungs, liver, pancreas, kidneys, cervix/lower uterine body with vaginal extension, vulva, soft tissues, muscle, spinal canal region, and skeleton. Published FDG PET/CT descriptions of ALK-positive histiocytosis remain limited and have mainly focused on isolated CNS or localized extranodal disease; this case expands the imaging spectrum by demonstrating striking multiorgan and genital-tract involvement in a single examination.