A 25-year-old patient has a spinal cord tumor. Histology shows elongated bipolar cells with hair-like processes arranged in a biphasic pattern (loose microcystic areas alternating with compact Rosenthal fiber-rich areas). Ki-67 is 1%. The diagnosis and molecular marker expected is:

• A Pilocytic astrocytoma WHO grade 1 — KIAA1549-BRAF tandem duplication/fusion ✓
• B Diffuse astrocytoma WHO grade 2 — IDH1 mutation with ATRX loss
• C Ependymoma WHO grade 2 — perivascular pseudorosettes, NF2 or YAP1 fusion
• D Schwannoma — Antoni A/B areas with Verocay bodies, S100+

Explanation

Pilocytic astrocytoma (PA) is a WHO grade 1 tumor characterized histologically by the biphasic pattern of loosely textured microcystic areas and compact areas with Rosenthal fibers (hyaline eosinophilic inclusions in astrocyte processes), with very low Ki-67. The molecular hallmark is the KIAA1549-BRAF tandem duplication (seen in >80% of cerebellar/spinal PAs), creating constitutive MAPK pathway activation. It is circumscribed rather than diffuse and carries an excellent prognosis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.