Nonneurologic disease manifestations in ALK-positive histiocytosis patients with single-system disease (Group 2). (A-D) Successive fluorodeoxyglucose PET-CT images of an adult female with a large right clavicular tumor at diagnosis (A; time, 0) and after treatment with radiotherapy (B; time, 5.5 months), 6 weeks of vinblastine/prednisone-based chemotherapy (C; time, 9.5 months), and 2 months of alectinib (D; time, 14 months). (E-I) T1-weighted contrast-enhanced (E), T2-weighted (F), diffusion-weighted (G), apparent diffusion coefficient (H), and plain T1-weighted (I) MRI images of the left lower leg of a child at diagnosis showing a single soft tissue tumor that infiltrates the musculature and shows contrast enhancement and restricted diffusion. (J-K) Photographs of the retroauricular scalp lesion of an infant, with a clear change in clinical appearance after 3 months (K).