MRI brain of a 6-year-old boy with morning vomiting and ataxia shows a solid midline vermian mass that is hypointense on T2, shows marked restricted diffusion on DWI, and enhances heterogeneously. It extends through the fourth ventricle into the cisterna magna. The most likely diagnosis is:

• A Pilocytic astrocytoma
• B Medulloblastoma ✓
• C Ependymoma
• D Brainstem glioma

Explanation

Medulloblastoma is a highly cellular small round blue cell tumour arising from the vermis, giving T2 hypointensity and marked restricted diffusion, features that separate it from pilocytic astrocytoma. Pilocytic astrocytoma, the key distractor, is a slowly growing WHO grade 1 tumour that is T2 hyperintense, often cystic with an enhancing mural nodule, and does not restrict diffusion.

Reference: Osborn's Brain, 2nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.