A 28-year-old woman presents with a posterior fossa tumour causing ataxia. MRI shows a cystic cerebellar mass with a mural nodule. Pathology reveals spindle-shaped cells in biphasic pattern with Rosenthal fibres and eosinophilic granular bodies. Which is the most likely diagnosis?

• A Medulloblastoma — dense small blue cell tumour arising from vermis
• B Ependymoma — perivascular pseudorosettes, arising from the 4th ventricle
• C Pilocytic astrocytoma (WHO Grade 1) — most common cerebellar tumour in children/young adults ✓
• D Haemangioblastoma — abundant lipid-laden stromal cells with prominent vascularity

Explanation

Pilocytic astrocytoma (WHO Grade 1) is the most common brain tumour in children and young adults; in the posterior fossa it presents as a cystic mass with an enhancing mural nodule. Histologically it shows a biphasic pattern with compact bipolar pilocytic cells containing Rosenthal fibres and loose microcystic areas with eosinophilic granular bodies. KIAA1549-BRAF fusion is the characteristic molecular alteration.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.