An unclassified cerebral small cell tumor in a patient with human T-cell lymphotropic virus type 1-induced primary extranodal lymphoma.

We describe an autopsy case of a thalamic tumor in a patient with human T-cell lymphotropic virus type 1 (HTLV-1)-induced extranodal lymphoma of the skull. Neither brain tumors associated with adult T-cell leukemia nor HTLV-1-induced extranodal lymphoma of bone have previously been reported. The tumor, which resembled an oligodendroglioma or dysembryoplastic neuroepithelial tumor, was composed of medium-sized cells with dark, round-to-polygonal nuclei and a mucinous matrix that formed pericellular lakes. Tumor cells were immunoreactive with S-100 protein and neuron-specific enolase but not with glial fibrillary acid protein, synaptophysin, or vimentin. Tumor cells had prominent rough endoplasmic reticulum and free ribosomes with short processes, compatible with an oligodendroglial or small neuronal nature. Many atypical lymphocytes had infiltrated the leptomeningeal space, subependymal zone along the ventricular walls, fornices, corpus callosum, and right frontal lobe. Multinucleated or bizarre astrocytes and macrophages were found exclusively in the right frontal white matter. The coexistence of this unclassified peculiar brain tumor with bizarre astrocytosis suggests a cytopathic effect of HTLV-1 on human glioneuronal cells.