Medulloblastoma

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Diagnosis Definition

Medulloblastoma is the most common malignant brain tumor in children; it is a genetically defined embryonal cell tumor (2016 revised World Health Organization classification)
80% occur between ages 1 and 10
Most arise from the cerebellar vermis and extend into the fourth ventricle; less commonly they can occur in young adults and arise from the cerebellar hemispheres

Imaging Findings

Composed of densely packed, small round cells, medulloblastomas have relatively high density on CT studies and show reduced diffusion on MRI compared with other posterior fossa tumors
On MRI, the tumor can present with variable T1 and T2 intensities and enhancement patterns, cyst formation, areas of hemorrhage or calcification, and leptomeningeal tumor seeding
The tumors can be well-defined or ill-defined
Medulloblastomas have a very high tendency to disseminate via the CSF to other parts of the brain, ventricles, and spinal canal; up to one-third of patients have dissemination at presentation

Pearls

Medulloblastoma is one of the rare brain tumors that can also have metastases outside of the CNS, although this is rare with current therapies
Because of the tumor’s tendency for CSF dissemination, contrast-enhanced MRI of the entire spine is integral to the imaging work-up and follow-up
The key to these tumors is the restricted diffusion with low ADC values, distinguishing them from ependymomas and pilocytic astrocytomas in children

References

Koeller KK, Rushing EJ. From the archives of the AFIP: medulloblastoma: a comprehensive review with radiologic-pathologic correlation. RadioGraphics 2003; 23(6):1613-1637
Yeom KW, Mobley BC, Lober RM, et al. Distinctive MRI features of pediatric medulloblastoma subtypes. American Journal of Radiology 2013; 200:895-903
Shih RY, Koeller KK. Embryonal Tumors of the Central Nervous System. RadioGraphics 2018; 38:525–541

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