Answer: Paraganglioma

Histology: Essential features are encapsulation, compact non-infiltrating lesion (whose acinar architecture can be confimed by reticulin staining), somewhat epithelioid cytological features, perivascular orientation, bland nuclei, and rare mitoses.

Discussion: The differential diagnosis of a well-circumscribed neoplasm in the cauda equina includes 1) myxopapillary ependymoma (recognizing that many ependymomas in this region are neither “myxo-“ nor papillary, yet clearly fifferent from the classic “cellular ependymoma” as it occurs in the spinal cord proper),
2) schwannoma (neurofibromas are rare),
3) paraganglioma, and
4) meningioma (generally only of the rare clear cell type).

Paragangliomas of the filum terminale are readily identified if the possibility is entertained. This initial consideration usually falls to the pathologist since paragangliomas are not well known by radiologists or neurosurgeons. Paragangliomas can resemble ependymoma, but are more acinar ,(with “Zellballen”), and less fibrillar. Mucopolysacarrhide is absent. The paraganglioma’s epithelioid quality and “salt-and-pepper” nuclei are other helpful distinguishing features. The diagnosis is elementary, more or less, once the possibility is entertained. Immunoreactivity for synaptophysin and chromogranin are definitive. Scattered S-100-positive sustentacular cells are often present, albeit often only spottily.