A 45-year-old presents with a well-circumscribed cerebellar mass. Histology shows a biphasic pattern with compact bipolar cells in Rosenthal fiber-rich areas alternating with loose microcystic areas containing eosinophilic granular bodies. BRAF V600E or BRAF-KIAA1549 fusion is characteristic. This is a:

• A Glioblastoma (IDH-wildtype)
• B Medulloblastoma
• C Ependymoma
• D Pilocytic astrocytoma (WHO grade 1) ✓

Explanation

Pilocytic astrocytoma is a WHO grade 1 glioma characterized histologically by a biphasic pattern: compact areas with elongated 'pilocytic' (hair-like) bipolar cells containing Rosenthal fibers (hyaline eosinophilic deposits of GFAP-containing intermediate filaments) alternating with loose microcystic areas with eosinophilic granular bodies. The molecular hallmark is BRAF-KIAA1549 fusion (most common) or BRAF V600E mutation. It has an excellent prognosis after surgical resection. Glioblastoma is IDH-wildtype with pseudopalisading necrosis; medulloblastoma has Homer-Wright rosettes; ependymoma has perivascular pseudorosettes.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.