March 2004: a 24-year-old woman with bifrontal headaches.

A 24-year-old woman with bifrontal headaches was found to have a well-circumscribed lesion in the frontal lobe subcortical white matter. Microscopic examination showed clusters of small round cells separated by hypocellular neuropil-like areas, and a distinct border between tumor and surrounding white matter. Synaptophysin was diffusely positive in neuropil-like areas, and many tumor cells expressed NeuN. Based on these findings, a diagnosis of "extraventricular neurocytoma" was made. A double-label immunofluorescence stain was performed with NeuN and Ki-67 antibodies to determine if NeuN+ cells remained in the mitotic cycle. No colocalization of these markers was found, thus supporting the hypothesis that neuronal differentiation (as marked by NeuN expression) is incompatible with continued proliferation of tumor cells, as well as normal neurons.