Neuroradiology

Primitive neuroectodermal tumour

(PNET), highly cellular malignant tumours composed almost completely of undifferentiated neuroepithelial cells from which originate the differentiated neuroectodermal elements of the CNS. They can have foci of differentiation along neuronal or glial lines, as well as a mesenchymal component. Under this term are now grouped tumours such as primary cerebral neuroblastoma, ependymoblastoma, medulloblastoma, medulloepithelioma and undifferentiated small cell neoplasms of the brain. Some authors include in this category pineoblastomas. They are usually large solid tumours, most frequently in the deep cerebral white matter, with relatively sharp margins, with frequent necrotic areas and calcifications.

Clinical presentation is mainly with seizures and signs of raised intracranial pressure.

On noncontrast CT, the solid portions of the tumour tend to be hyperdense; cystic areas and foci of punctate calcifications are common, and haemorrhage is seen in approximately 10%. After contrast there is always some enhancement, homogeneous or inhomogeneous according to the macroscopic composition in terms of cysts and calcification versus solid tumor.

On MR, PNET are highly variable in appearance, ranging from homogeneous to markedly inhomogeneous in relation to the macroscopic structure. They usually appear as a large intraparenchymal or intraventricular mass, with isointense components containing T1 hypointense and T2 hyperintense cystic areas. Enhancement is marked and usually homogeneous in the solid component.

GS