An 8-year-old boy presents with early morning headaches and projectile vomiting. MRI shows a heterogeneously enhancing mass arising from the cerebellar vermis filling the fourth ventricle, with markedly restricted diffusion on DWI and dilated lateral ventricles. What is the most likely diagnosis?

• A Medulloblastoma ✓
• B Pilocytic astrocytoma
• C Brainstem diffuse astrocytoma
• D Ependymoma of the fourth ventricle

Explanation

B solid midline vermian mass in a child with marked restricted diffusion reflects high cellularity, characteristic of medulloblastoma, the commonest malignant posterior fossa tumor of childhood. It typically causes obstructive hydrocephalus and seeds via CSF. The best distractor is pilocytic astrocytoma, but that is a cystic cerebellar hemispheric lesion with a slowly enhancing mural nodule and facilitates rather than restricts diffusion.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.