Oligodendrogliomas with neurocytic differentiation. A report of 4 cases with diagnostic and histogenetic implications.

Oligodendroglioma represents a distinct type of diffuse glioma with a relatively favorable prognosis. Although an O2A-like glial progenitor cell of origin has been suggested, a neuronal-oligodendroglial progenitor cell is also of interest, particularly because variable degrees of neuronal marker expression have been reported in typical oligodendrogliomas. We present 2 female and 2 male patients (ages 34-54) with frontal lobe oligodendrogliomas containing a) morphologically distinct collections of small round cells with hyperchromatic nuclei, b) well-formed Homer Wright-like and perivascular rosettes, and c) demonstrable neuronal differentiation by immunohistochemistry and/or electron microscopy in the rosette-associated regions. Unlike extraventricular neurocytomas, these cases featured an infiltrative growth pattern and a classic oligodendroglioma immunophenotype in non-rosette bearing portions of each tumor. FISH analysis demonstrated chromosome 1p and 19q codeletions in 3 (75%) cases, both in regions with and without rosettes. Recurrences were common, although all patients are currently alive 4 months to 13 yr from initial diagnosis. Based on clinicopathologic and genetic features, we diagnosed these tumors as oligodendrogliomas with neurocytic differentiation. However, it is unclear whether they represent a) gliomas with divergent neuronal differentiation, b) a distinctive form of glioneuronal neoplasm, or c) a reflection of glioneuronal histogenesis in oligodendrogliomas in general. In any case, their occurrence suggests a histogenetic overlap between oligodendroglioma and extraventricular neurocytoma not previously recognized.