This color-enhanced, sagital (from the side), T1-weighted MR image shows a large mass arising from the vermis/roof of the fourth ventricle and filling it, severely compressing the brainstem and causing obstructive hydrocephalus in addition to downward herniation of the cerebellar tonsils. This is a medulloblastoma, an aggressive type of primitive neuroectodermal tumor (PNET) which can seed the subarachnoid spaces with metastatic foci. Medulloblastomas are one of the most common posterior fossa tumors of childhood.