A 4-year-old child presents with a midline cerebellar mass. Histology shows a small round blue cell tumor with Homer Wright rosettes. Molecular testing reveals activation of the sonic hedgehog (SHH) signaling pathway. Which of the following is the most likely diagnosis?

• A Ependymoma
• B Medulloblastoma, SHH-activated ✓
• C Atypical teratoid/rhabdoid tumor (AT/RT)
• D Pilocytic astrocytoma

Explanation

Medulloblastoma is the most common malignant pediatric brain tumor, arising in the cerebelloma. The 2021 WHO classification divides medulloblastoma into four molecular groups: WNT-activated, SHH-activated, Group 3, and Group 4. SHH-activated medulloblastomas show Homer Wright rosettes on histology and activation of the sonic hedgehog pathway. Ependymomas show perivascular pseudorosettes. AT/RT shows loss of INI1. Pilocytic astrocytoma shows Rosenthal fibers and is not a small round blue cell tumor.

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.