The patient went on to have a resection.
The sections show a densely cellular malignant tumour with adjacent cerebellar cortex. It has a nodular appearance with scattered pale islands, as well as forming sheets. No rosettes are seen. The tumour cells have high N/C ratio with enlarged hyperchromatic nuclei, nuclear moulding, granular chromatin, inconspicuous nucleoli and scanty cytoplasm. There is no anaplasia. Many mitoses are identified. The tumour cells are focally GFAP as well as weakly chromogranin and synaptophysin positive. The Ki-67 index is about 30%. INI-1 shows normal staining pattern. No nuclear staining for B-catenin is seen. YAP1 and GAB1 are both apparently up-regulated, indicating Sonic Hedgehog subgroup.
Medulloblastoma (Desmoplastic/nodular subtype), WHO Grade IV. Sonic Hedgehog subgroup.
In this situation, one can be quite specific with a pre-operative diagnosis. Not only is this lesion almost certainly a medulloblastoma (young adults are a definite second peak of incidence, typically in the cerebellar hemisphere rather than vermis) but the SHH subtype can also be predicted with confidence.