Hybrid peripheral nerve sheath tumor, with schwannoma/perineurioma components.
Benign peripheral nerve sheath tumors are classically divided into schwannomas, neurofibromas, and perineuriomas (8). Their diagnosis is usually straightforward, given the unique morphological and immunohistochemical characteristics of each type. In recent years, peculiar tumors with hybrid features of these entities have been described. Histologically, most of these tumors revealed a biphasic pattern, showing an abrupt transition between the two components.
Since the first report of an hybrid peripheral nerve sheath tumor in 1998 (2), a few cases have been described, with the great majority arising in peripheral nerves of the trunk and extremities (2, 4, 5, 7-9). Uncommon locations, such as colon, nasal cavity, orbit and lymph node, have also been reported (1, 3, 6, 10). However, there is no report to date, of an intracranial hybrid tumor arising from a cranial nerve. We report an unusual case of a hybrid peripheral nerve sheath tumor arising from the VIIIth cranial nerve. The tumor shows a hybrid, biphasic appearance, with a predominantly schwannoma component, and a minor perineurioma component. This hybrid nature is confirmed and highlighted by immunohistochemical staining profiles, with S100 positivity of the schwannoma component and immunostaining of the perineurioma component for claudin-1, Glut-1 and CD34.
Hybrid peripheral nerve sheath tumors usually have a benign clinical course with only exceptional recurrences, similar to conventional benign schwannomas and perineuriomas. No association with neurofibromatosis has been identified.
The histological differential diagnosis includes spindle cell lesions such as schwannomas, perineuriomas, neurofibromas and low-grade malignant peripheral nerve sheath tumor. Conventional schwannomas and perineuriomas are composed of a single cell type (Schwann cells and perineurial cells, respectively). Schwannomas are characterized by loose and dense areas (Antoni A and Antoni B) and often have Verocay body formations. The conventional intraneural perineurioma consists of whorled tight proliferations of cells around one or more axons. Neurofibromas often have collagen bundles and myxoid background and are composed of axons and multiple cell types intimately mixed within the tumor. The diagnosis of a hybrid peripheral nerve sheath tumor is made in the presence of two distinct histological areas and is confirmed by different patterns of immunostaining of the two components. As in schwannomas, degenerative nuclear atypia should not be confused with malignant transformation (MPNST) and is seen in the absence of any other anaplastic features (mitosis, necrosis, increased cellularity). The pathogenetic basis of the dual differentiation seen in hybrid lesions is poorly understood.
Contributed by Facundo Las Heras, MD, PhD, Robert Martuza, MD, Paul Caruso, MD, Sandra Rincon, MD and Anat Stemmer-Rachamimov, MD.