Medulloblastoma

Centered on the posterior wall of the fourth ventricle is a T2 FLAIR hyperintense, minimal enhancing lesion causing localized mass effect with nearly complete effacement of the fourth ventricle. The lateral and third ventricles are, however, normal in caliber, with no evidence of hydrocephalus. The lesion demonstrates low ADC values (~600 x 10^-6 mm^2/s) indicating high cellularity. No abnormal susceptibility.

Multiple punctate white matter lesions which are T2 FLAIR hyperintense and T1 hypointense are scattered throughout both cerebral hemispheres.

Conclusion:

Minimally enhancing lesion centered on the fourth ventricle with low ADC values is favored to represent a medulloblastoma. 

MICROSCOPIC DESCRIPTION: 

Sections show multiple fragments of tissue which is largely replaced by a cellular tumor. Tumor consists predominantly of small undifferentiated cells which have high nuclear to cytoplasmic ratios. The cells are primitive in nature with a stippled chromatin pattern with occasional mitoses. Tumor cells are embedded in a fibrillary background. No nodularity or areas of high grade anaplasia is seen in the biopsy. 

Immunohistochemistry shows the tumor cells stain positive with CD56 and synaptophysin. S-100 is positive. 

Staining for GFAP is present but this appears to be staining background astrocytic processes rather than the tumor. 

P53 stains approximately 10% of nuclei (considered wild type). 

CD3, CD20, CD45 are negative. 

GAB - YAP - Beta-catenin -Nuclear negative. 

FINAL DIAGNOSIS: medulloblastoma, classic histologic subtype, most likely non-SHH/non-WNT subtype. WHO Grade IV.