Medulloblastoma pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Haytham Allaham, M.D. [2]

Overview

Medulloblastoma arises from the cerebellar stem cells, which are normally involved in the anatomical development of the cerebellum and other posterior fossa structures. Medulloblastoma is an invasive and rapidly growing brain tumor which frequently metastasize to different organs of the body. Genes involved in the pathogenesis of medulloblastoma include CTNNB1 gene, PTCH1 gene, MLL2 gene, SMARCA4 gene, DDX3X gene, CTDNEP1 gene, KDM6A gene, and TBR1 gene. Medullobastomas is associated with a number of syndromes that include Gorlin syndrome and Turcot syndrome. On gross pathology, a pink, solid, and well circumscribed mass is a characteristic finding of medulloblastoma. On microscopic histopathological analysis, round tumor cells, elevated mitotic rate, increased nucleus:cytoplasmic ratio, and presence of Homer-Wright rosettes are characteristic findings of medulloblastoma.

Pathogenesis

• Medulloblastoma arises from the cerebellar stem cells, which are normally involved in the anatomical development of the cerebellum and other posterior fossa structures.
• Medulloblastoma is usually located at the infratentorial region, where it forms a mass between the brain stem and the cerebellum in the vicinity of the fourth ventricle.
• Medulloblastoma is an invasive and rapidly growing tumor.
• Unlike most brain tumors, medulloblastoma may spread through the cerebrospinal fluid and metastasize to different organs of the body.

Genetics

• Development of medulloblastoma is the result of multiple genetic mutations.
• Genes involved in the pathogenesis of medulloblastoma include:

• CTNNB1 gene on chromosome 3
• PTCH1 gene located on chromosome 9
• MLL2 gene located on chromosome 12
• SMARCA4 gene located on chromosome 19
• DDX3X gene located on X chromosome
• CTDNEP1 gene located on chromosome 17
• KDM6A gene located on X chromosome
• TBR1 gene located on chromosome 2

• Medulloblastoma may be classified into at least four major subtypes based on a molecular classification criteria which include:

• WNT subgroup:

• Present among 10% of medulloblastoma patients
• Cytogenetic markers include classic monosomy 6
• Molecular markers include beta-catenin

• SHH subgroup:

• Present among 25% of medulloblastoma patients
• Cytogenetic markers include chromosome 9q loss
• Molecular markers include SFRP1 or GAB1

• Group 3 medulloblastoma:

• Present among 30% of medulloblastoma patients
• Cytogenetic markers include isochromosome 17q
• Molecular markers include MYC activation in 50% of this subtype

• Group 4 medulloblastoma:

• Present among 35% of medulloblastoma patients
• Cytogenetic markers include isochromosome 17q
• Molecular markers are still under investigation

Associated Conditions

• Medullobastomas is associated with a number of syndromes that include:

• Gorlin syndrome
• Turcot syndrome
• Coffin-Siris syndrome
• Cowden syndrome
• Gardner syndrome
• Li-Fraumeni syndrome
• Rubinstein-Taybi syndrome

Gross Pathology

• On gross pathology, a pink, solid, and well circumscribed mass is a characteristic finding of medulloblastoma.
• The following image demonstrates the gross pathology observed in medulloblastoma:

• 

  Image courtesy of Dr Frank Gaillard Radiopaedia(original file "here"). Creative Commons BY-SA-NC

Mircoscopic Pathology

• On microscopic histopathological analysis, round tumor cells, elevated mitotic rate, increased nucleus:cytoplasm ratio, and presence of Homer-Wright rosettes are characteristic findings of medulloblastoma.
• Medulloblastoma may be classified into two subtypes based on WHO histological classification criteria which include classic medulloblastoma and variant medulloblastoma
• Other variants of medulloblastoma include desmoplastic medulloblastoma, large cell medulloblastoma, anaplastic​ medulloblastoma and medulloblastoma with extensive nodularity.
• The table below differentiates between the five main groups of medulloblastoma according to WHO histological classification:

Grade	Histologic features
Classic medulloblastoma	Dense sheet like growth of cells, hyperchromatic round-to-oval nuclei, increased mitotic activity, conspicuous apoptosis, Homer-Wright rosettes, and necrosis is less common
Desmoplastic medulloblastoma variant	Brain invasion, 4 or more mitosis/10 HPF, necrosis, increased cell count, high nucleus:cytoplasm ratio, increased nucleoli size, presence of sheeting
Medulloblastoma with extensive nodularity variant	Cellular uniformity, fine fibrillary matrix, and occasional presence of mature ganglion cells
Large cell / anaplastic medulloblastoma variant	Both variants demonstrate high mitotic activity, pleomorphism, large round nuclei with variable eosinophilic cytoplasm, hemorrhage, and necrosis is common

• Shown below is a series of microscopic images observed in medulloblastoma:

• 

  Medulloblastoma smear

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  Medulloblastoma smear

• 

  Medulloblastoma smear demonstrating Homer-Wright rosettes

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  Medulloblastoma smear demonstrating cerebellar infiltrative growth

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  Medullloblastoma smear demonstrating demsomplastic nodular growth

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  Anaplastic large cell medulloblastoma smear

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  Medulloblastoma smear

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  Desmoplastic medulloblastoma smear on reticulin stain demonstrating "pale islands"

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  Desmoplastic medulloblastoma on MIB-1 immunostaining

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  Medulloblastoma smear demonstrating areas of geographic necrosis

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  Medulloblastoma smear demonstrating epitheloid ribboning and nuclear moulding of tumor cells

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  Medulloblastoma smear demonstrating partial MAP2 immunoreactivity

References

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