Neurologic involvement in ALK-positive histiocytosis patients from Group 1B or 2. (A-E) Axial images of the T1-weighted contrast-enhanced MRI scans of the heads of 2 pediatric cases with multiple solid brain tumors before and after treatment with ALK inhibition, demonstrating robust responses in both. (F) Sagittal image of the T1-weighted contrast-enhanced MRI scan of the spine showing leptomeningeal contrast enhancement along the descending cauda equina nerve roots. (G-I) Axial images of successive fluorodeoxyglucose PET-CT scans showing partial and complete response of a neuroforaminal tumor at level L5 after 2 cycles of cladribine (H) and subsequent treatment with alectinib (I), respectively. Coronal images (not shown) demonstrated that the tumor followed the course of the exiting nerves, highly reminiscent of nerve sheath tumors such as neurofibromas. (J-N) Axial images of successive T1-weighted contrast-enhanced MRI scans of the head of a child with a left insula tumor before and after subtotal resection and successful treatment with alectinib. (O-Q) Axial images of the T1-weighted contrast-enhanced MRI scans of the head of a child with a left oculomotor nerve tumor, demonstrating slight regression but continued contrast enhancement of the tumor after treatment with vinblastine/prednisone-based chemotherapy. (R) Coronal image of the T1-weighted contrast-enhanced MRI scan of the head showing a 30 × 25 × 34 mm large tumor with contrast enhancement in the prepontine cistern that followed the course of the trigeminal nerve and caused pressure on the pons. (S) Sagittal image of the T1-weighted contrast-enhanced MRI scan of the cervical spine showing a large (18 × 24 × 45 mm) intradural extramedullary tumor at level C1-C2.