Medulloblastoma (and PNET)

            The majority of medulloblastomas occur in children between four and eight years old, and males outnumber females three to one. Primitive neuro-ectodermal tumors (PNET) may present at birth or early infancy. Medulloblastomas and PNETS arise from remnants of primitive neuro-ectoderm in the roof of the fourth ventricle. These tumors are very malignant and exhibit an aggressive biologic behavior, commonly invading the adjacent brain stem and leptomeninges. Widespread dissemination through the ventricular system and distant seeding to other areas of the neuraxis occurs in as high as 30%. Endnote

Medulloblastomas are primarily midline vermian lesions, but hemispheric locations are also possible. Since they arise close to the fourth ventricle, growth predominantly into the ventricle may make them simulate an intraventricular mass. Necrosis, hemorrhage and cavitation are common features, giving these tumors a heterogeneous appearance on MR, but not to the same degree as seen with ependymomas. Calcification is rare in medulloblastomas. Endnote , Endnote They are hypervascular lesions and show moderate contrast enhancement.  

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