MICROSCOPIC DESCRIPTION: 1&2: Paraffin sections show a densely hypercellular medulloblastoma involving cerebellum. Tumour cells have intermediate and large round and oval vesicular and hyperchromatic nuclei and a variable amount of pale cytoplasm. These are arranged predominantly in diffuse sheets. Several irregularly shaped pale islands of tumour cells are noted and there are areas of dense reticulin deposition. No rosettes are identified. Frequent mitotic figures and apoptotic cells are noted. No necrosis is identified. Tumour appears to merge gradually with adjacent cerebellum. Cerebellar cortex shows a mantle of atypical cells immediately deep to the pia arachnoid with tumour cell percolating throughout the molecular layer of cerebellar cortex. IMMUNOHISTOCHEMISTRY: There is patchy staining for GFAP and a moderate number of cells show nuclear staining for NeuN. No staining for Nestin or synaoptophysin is seen in tumour cells. The topoisomerase labelling index is approximately 40%.
DIAGNOSIS: 1&2: Cerebellar tumour: Medulloblastoma - Desmoplastic/nodular phenotype; Sonic Hedgehog molecular sub-type (see Comment).
COMMENT: Immunohistochemistry performed on the original tumour showed absence of nuclear staining for Beta catenin and upregulation of YAP1 and GAB1 indicating a Sonic Hedgehog (SHH) molecular subtype. Sections will be referred for immunostaining for YAP1, GAD1 and Beta-catenin. A supplementary report will be issued.
SUPPLEMENTARY REPORT: Beta catenin diffuse strong cytoplasmic; nuclear negative. GAB-1 diffuse moderate to strong cytoplasmic staining. YAP-1 cytoplasmic negative staining. CONCLUSION: Most consistent with SHH molecular subtype.