A 7-year-old boy presents with a 2-week history of morning headaches, vomiting, and unsteady gait. On examination, he has papilledema and truncal ataxia. MRI brain shows a midline mass in the 4th ventricle that is hypointense on T1-weighted imaging, hyperintense on T2-weighted imaging, and shows heterogeneous enhancement on contrast-enhanced T1. There is obstructive hydrocephalus. Diffusion-weighted imaging shows restricted diffusion within the mass. What is the most likely diagnosis?