Paraffin sections show fragments of a densely hypercellular tumour. Tumour cells have primitive neuroectodermal features with irregularly shaped hyperchromatic and vesicular nuclei and minimal cytoplasm. There is prominent nuclear moulding. 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 tumour/medulloblastoma.
The topoisomerase labelling 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