Primitive neuroectodermal tumor with choroid plexus differentiation.

A case of a cerebral juvenile undifferentiated round cell tumor with unusual formation of glandular structures is presented. The round cell component showed focal expression of glial fibrillary acidic protein, S-100 protein, neurofilament protein and neuron-specific enolase. The glandular part consisted of papillary epithelial structures with a central PAS-positive lumen. Ultrastructural findings supported the interpretation of these parts as choroid plexus. This route of differentiation was recognizable at the first operation at the age of 5 years and was much more prominent in the recurrence found 7 1/2 years later. The unusually benign course of this primitive neuroectodermal tumor (PNET) may be due to its intrinsic progressive tendency towards choroid plexus differentiation.