The evolution of an oligodendroglioma into a primitive neuroectodermal tumour.

Oligodendrogliomas may comprise a biologically heterogenous group of tumours. There is disagreement about the relationship between their histological features and subsequent behaviour, and evolution of histologically typical oligodendrogliomas into more malignant neoplasms has been described. We describe a supratentorial oligodendroglioma in a 4-year-old boy which recurred 9 months after initial resection when it showed foci of histologically primitive cells, not present in the initial specimen. It recurred again at the same site 20 months later following a course of radiotherapy, but showed on this occasion the appearances of a primitive neuroectodermal tumour (PNET) with astrocytic and ependymal, but not oligodendroglial, differentiation. The evidence suggests evolution of an oligodendroglioma into a PNET, and raises the possibility that some oligodendrogliomas may arise by differentiation from a pool of proliferating primitive cells which may occasionally become the dominant population.