Medulloblastoma (posterior fossa PNET)


Image 1

Clinical History: 
A 7 year-old boy who presents with nausea and vomiting.

Medulloblastoma (posterior fossa PNET)

MR Technique: 
5 mm axial FSE (17 echoes) pre-contrast T2- and axial and sagittal post-contrast SE T1-weighted images are presented, acquired at 1.5 T on a Siemens Avanto MR system. Sequence parameters were TR/ TE = 5240/111 and 400/10 (for both T1-weighted scans) respectively. Scan times were 2:32, 2:41 and 3:25 min:sec.

Imaging Findings:
There is a heterogeneously enhancing fourth ventricular mass, which completely fills and slightly expands the inferior half of the fourth ventricle. The mass was isointense on pre-contrast T1-weighted images (not shown) and slightly hyperintense on T2-weighted images. A few small intratumoral cysts were noted on the T2-weighted scan, which are seen in about half of cases (of medulloblastoma). There is mild to moderate bilateral lateral and third ventricular prominence with edema surrounding the frontal and occipital horns of the lateral ventricles (these findings are illustrated only in part on the post-contrast sagittal T1-weighted image). The ventricular prominence and findings on T2-weighted scans (transependymal CSF) are consistent with acute obstructive hydrocephalus. The cerebellar tonsils extend approximately 7 mm below the level of the foramen magnum, secondary to cerebellar herniation as a result of the fourth ventricular mass causing obstruction of the lateral and third ventricles. Enhancement is also noted diffusely throughout the cerebellar sulci, consistent with leptomeningeal metastases. The top two differential diagnostic considerations include medulloblastoma and ependymoma, with the leptomeningeal metastases more common at presentation with a medulloblastoma.