“ This is the most common malignant posterior fossa tumour in children. Most present at less than 5 years of age. They tend to seed along...
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This is the most common malignant posterior fossa tumour in children. Most present at less than 5 years of age. They tend to seed along the neuraxis (up to 30% at presentation). In total, 5% have metastases outside the CNS, mostly to the bone. Treatment is with surgery and radiotherapy to the whole neuraxis. Without metastases, 5-year survival is 80%.
CT
• Hyperdense midline posterior fossa mass on pre-contrast scan, arising from the cerebellar vermis or floor of the fourth ventricle
• Homogeneous enhancement post-contrast, variable surrounding oedema
• Hydrocephalus in 90%
• Foci of haemorrhage, cystic change, necrosis and calcification may be seen.
MRI
• Low to intermediate T1 signal with intermediate to high signal on T2.
• Enhancement more heterogeneous than on CT (more sensitive).
• Whole neuraxis must be imaged—check for foci of high T1 signal post-contrast, may be intra-dural, extra-dural or sub-pial.
Post-contrast axial computed tomography scan of the brain of a child showing a large lesion in the posterior fossa that enhances uniformly. There is perilesional oedema and hydrocephalus due to obstruction of the fourth ventricle. The appearance is in keeping with medulloblastoma.
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