Pilocytic astrocytoma

There is a heterogeneous mixed cystic and solid large mass within the posterior fossa involving the inferior aspect of the fourth ventricle and adjacent cerebellar vermis. The solid components of the tumor demonstrate intermediate-to-high T2 signal with corresponding T1 hypointensity. The solid components heterogeneously enhance, more so along the margins of the cystic components. Inferiorly, the tumor fills and subsequently expands the fourth ventricle extending through the obex where there is mass effect upon the medulla and upper cervical spinal cord.

There is obstruction of the CSF outflow of the fourth ventricle at the obex and foramina of Luschka, which results in dilatation of the fourth ventricle and widening of the cerebral aqueduct. There is marked dilatation of the lateral ventricles and third ventricle, noting significant inferior displacement of the floor of the third ventricle which effaces the suprasellar cistern and prepontine cistern. There is severe flattening of the pituitary gland and widening of the sella, presumably a sequela of long-standing increased intracranial pressure. There is edema along the periphery of the lateral ventricles, consistent with transependymal flow of CSF related to the obstructive hydrocephalus.

There is susceptibility artifact consistent with calcification and likely superimposed blood products given its appearance on filtered phase imaging.

Mass effect within the posterior fossa contributes to inferior herniation of the cerebellar tonsils. Mass effect also displaces the medulla, pons, and midbrain anteriorly which results in effacement of the premedullary and prepontine cisterns. There is also some superior extension of the cerebellum toward the tentorial incisura, without frank superior transtentorial herniation. The quadrigeminal plate cistern and the cerebral sulci along the cerebral convexities are effaced. There is increased CSF space about the bilateral optic nerve sheaths with inversion of the optic discs, consistent with sequela of increased intracranial pressure and presumed papilledema.