MICROSCOPIC DESCRIPTION: All the sections show a densely cellular malignant tumour, adjacent to cerebellar cortex. It has a nodular architecture with scattered pale islands. The tumour cells have high N/C ratio with elongated and hyperchromatic nuclei, nuclear moulding, focal prominent nucleoli and scanty cytoplasm. Within the nodules, the tumour cells are more widely spaced and with more fibrillary matrix. Scattered mitoses are seen, along with apoptotic debris. The features are those of desmoplastic/nodular medulloblastoma. The reticulin stain shows fibres in-between the nodules but absent in the pale islands. The tumour cells are focally synaptophysin and chromogranin positive. The topoisomerase index is about 20%. INI1 and NeuN are negative.
B-catenin is negative. GAB1 and YAP1 are focally positive. p53 is negative in most of the tumour cells.
DIAGNOSIS: Desmoplastic/nodular medulloblastoma, SSH-activated and TP53-wildtype (WHO Grade IV).