Neuroradiology

Medulloblastoma

a highly malignant infratentorial primitive neuroectodermal tumour (PNET), sometimes termed PNET medulloblastoma.

Medulloblastomas are thought to arise from bipotential embryological cells located in the roof of the fourth ventricle.

About 75% of medulloblastomas arise in the vermis; the remaining are found in the cerebellar hemispheres; about 75% manifest clinically before 15 years of age.

Clinical presentation is characterized by vomiting, signs of raised intracranial pressure due to hydrocephalus, increasing head size in younger children and cerebellar ataxia in older children.

Macroscopically medulloblastomas are solid tumours (although cysts may also be found), well circumscribed and encapsulated, bulging anteriorly within the fourth ventricle. Histologically they are composed of very primitive, undifferentiated small, round cells. Meningeal seeding and dissemination along CSF pathways is extremely high. Dissemination may occur both in the spinal cord CSF space and in the intracranial subarachnoid spaces.

At CT medulloblastoma appears as a solid vermian mass, possibly extending to the cerebral hemispheres, isodense or slightly hyperdense on baseline study due to the high nuclear/cytoplasmic ratio of the cells; a homogeneous marked enhancement is observed following contrast injection. Rarely calcifications, haemorrhages and cysts are observed.

In the supratentorial compartment triventricular hydrocephalus is almost the rule.

On MR (Fig.1) the most common appearance is that of a slightly hypointense or isointense posterior fossa midline tumour on T1-weighted images and an inhomogeneous iso-hyperintense mass on T2-weighted images. Marked enhancement is usually observed.

Meningeal seeding is revealed almost exclusively by contrast injection although reduced CSF T2 hyperintensity may be observed in cases of massive seeding in the region of the cauda equina.

GS

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Fig.1

a, b. T1-weighted and T2-weighted MR images. An inhomogeneous mass, mainly hypointense on T1 and partly iso-hyperintense on T2 fills in the fourth ventricle on the midline arising from the vermis. c, d, e. T1-weighted MR images in the axial, sagittal and coronal plane, following i.v. injection of gadolinium. Inhomogeneous enhacement of some of the components of the tumour.
Medulloblastoma, Fig.1 (a)
Medulloblastoma, Fig.1 (b)
Medulloblastoma, Fig.1 (c)
Medulloblastoma, Fig.1 (d)
Medulloblastoma, Fig.1 (e)