Medulloblastoma

Enhancing posterior fossa mass noted in the midline. 

A lobulated midline heterogeneously enhancing mass ( 29 x 38 mm ) extending inferiorly from the tentorium is again demonstrated. It is mildly heterogeneously T2 hypointense. There is prominent restricted diffusion within the mass. A cleft of CSF extends around the mass separating it from the cerebellum, suggesting it is extra-axial, although in the cerebellum this can be a difficult distinction to make. The mass effaces the fourth ventricle causing hydrocephalus. Ventricular size is stable and there is evidence of a small amount of transependymal edema. Mass effect displaces pons anteriorly effacing the prepontine cistern and the cerebellar tonsils inferiorly through the foramen magnum (4 mm ).

MR venography and post contrast imaging demonstrates a normally enhancing superior sagittal sinus and a dominant right transverse sinus. The left transverse sinus is partially hypoplastic.

MICROSCOPIC DESCRIPTION:

Paraffin sections show fragments of a densely hypercellular tumor. Tumor cells have primitive neuroectodermal features with irregularly shaped hyperchromatic and vesicular nuclei and minimal cytoplasm. There is prominent nuclear molding. Scattered Homer-Wright type rosettes are noted. Frequent apoptotic bodies are noted. There are moderate numbers of mitotic figures. No microvascular proliferation is seen and there is no necrosis.

Immunohistochemistry demonstrates divergent neuronal and glial differentiation with strong staining for neuronal markers, beta tubulin, synaptophysin, CD56 and neuronal nuclear antigen (NeuN) and patchy strong staining for GFAP. Nuclear staining for INI-1 is preserved and is strong. No staining for epithelial membrane antigen (EMA), pancytokeratin AE1/AE3, CD3, CD5, CD20 or CD30 is seen. The features are of primitive neuroectodermal tumor/medulloblastoma.

The topoisomerase labeling index is approximately 20%. Further immunohistochemical characterization shows strong perinuclear staining for GAP-1 and patchy mild nuclear staining for YAP-1. Beta-catenin staining is confined to perinucelar cytoplasm with no nuclear staining seen. This profile is most consistent with Group 2 (Sonic Hedgehog group) medulloblastoma of classical histological sub-type (WHO Grade IV)

DIAGNOSIS: Features most consistent with Group 2 (Sonic Hedgehog group) medulloblastoma of classical histological sub-type (WHO Grade IV).

See: Taylor et al. Acta Neuropathologica 2012;123:465-472