The patient went on to have a resection. 

Histology

Sections show pieces of cerebellum infiltrated by tumour. The tumour is composed of sheets of cells. The malignant cells are relatively monomorphous with irregularly-shaped polygonal nuclei, granular chromatin and minimal cytoplasm. The cell density is somewhat variable, with highly cellular areas and less cellular areas where the tumour cells are set in a fibrillary background. Apoptoses are scattered throughout the tumour but mitotic figures are relatively infrequent. There are scattered true Homer Wright rosettes and perivascular psuedorosettes, as well as columns of cells. There is one small area there are some pale nodules of cells set within the tumour; these pale nodules are negative with reticulin stain, while the intermodular tumour is positive.

Immunohistochemistry:

• Synaptophysin shows patchy but strong positivity.
• Chromogranin shows weak focal positivity.
• CD45 is negative.
• GFAP shows patchy positivity.
• INI1 staining is retained.
• Beta-catenin shows strong cytoplasmic staining but no nuclear staining.
• p53 shows wild-type staining.
• ATRX staining is retained (not mutated)
• IDH1 R132H is negative (not mutated).
• GAB1 is diffusely and strongly positive.

FINAL DIAGNOSIS: Medulloblastoma, desmoplastic/nodular (histologically defined), and SHH-activated and TP53-wildtype (genetically defined).