Serial 18F-FDG PET/CT Revealing Mixed Histiocytosis in a Pediatric Patient

A 6-month-old boy presented with a left parietal soft tissue swelling and CT findings of multiple calvarial lytic lesions. F-18-FDG PET/CT demonstrated hypermetabolic lesions in the left parietal, right occipital, and right femoral bones. The left parietal lesion was excised, and pathology was consistent with Langerhans cell histiocytosis. Interim PET assessment following induction chemotherapy demonstrated a "mixed metabolic response" with discordant rise in metabolic activity of the right femoral lesion. Subsequent core biopsy of the femoral lesion revealed a non-Langerhans cell histiocytosis, likely juvenile xanthogranuloma. Here we describe a rare pediatric case of mixed histiocytosis, unveiled by serial F-18-FDG PET/CT.