Medulloblastoma

Pathology

Microscopic Appearance of Medulloblastoma

• Dense sheet of small, basophilic cells
• Little cytoplasm
• Round to oval hyperchromatic nuclei
• High mitotic index
• Evidence of neuronal or glial differentiation in up to 50% of cases
• Homer Wright rosettes are commonly present

Homer Wright Rosettes

• Homer Wright rosettes are a characteristic microscopic feature of medulloblastoma
• They are thought to represent abortive attempts at neuronal differentiation
• Homer Wright rosettes are not pathognomonic of medulloblastoma and can be seen in other tumors, such as ependymoma

Subtypes

Classic or undifferentiated medulloblastoma (A)

• Highly cellular tumor
• Sheets of small oval or round blue cells

Classic medulloblastoma with area of glial fibrillary acidic protein immunopositivity (B)

• Glial fibrillary acidic protein (GFAP) is a marker of glial differentiation
• The presence of GFAP-positive cells in medulloblastoma suggests that the tumor may have a glial component

Desmoplastic medulloblastoma (C)

• Reticulin-positive internodular zones and reticulin-free pale islands
• Desmoplastic medulloblastoma is a rare subtype of medulloblastoma that is characterized by the presence of reticulin-positive internodular zones and reticulin-free pale islands

Large cell medulloblastoma (D)

• Increased nuclear size with abundant mitoses
• Large cell medulloblastoma is a rare subtype of medulloblastoma that is characterized by the presence of large cells with increased nuclear size and abundant mitoses