A 6-year-old boy presents with morning vomiting, gait ataxia, and headaches for 2 weeks. MRI brain reveals a midline cerebellar mass, T1 isointense, T2 hyperintense, with homogeneous enhancement and no significant edema. There is obstructive hydrocephalus. The lesion appears well-circumscribed and arises from the cerebellar vermis. What is the most appropriate next step in management?