Malignant perineurioma (Malignant peripheral nerve sheath tumor with perineural differentiation).
The term malignant peripheral nerve sheath tumor (MPNST) is inclusive of any sarcoma arising from the peripheral nerve, in most cases from the Schwann cells but not only from them. It is a rare form of sarcoma (3-10%) (2) and those of perineurial origin are exceedingly uncommon (4,3). Histopathological diagnosis of MPNST may be difficult because of the extreme morphological variability of this tumor. Conventional MPNST usually focally expresses S100 protein. In the present case this was completely absent in both the sarcoma and the most differentiated areas. Although considered by some authors to be highly specific of perineurial cells, GLUT1 and EMA may be co-expressed also in other tumors (1). Therefore ultrastructural examination was necessary to show highly diagnostic features supporting the perineurial cell nature of the lesion.
50-60% of MPNST cases are observed in patients with neurofibromatosis 1 and the risk of malignant transformation is higher in patients with deep neurofibromas. Another risk factor is represented by previous (4-40 years before) radiation exposure. In the present case the patient did not show any of the two risk factors but the malignant transformation of a benign perineurial tumor that had lasted about ten years.
Perineuriomas are benign peripheral nerve sheath tumors composed exclusively of perineurial cells, which surround individual nerve fascicles. They may be sclerosing (composed of cords of small epithelioid to spindle cells arranged in a dense collagenous stroma) or reticular, with anastomosing cords of elongated spindle cells, with lacy or reticular architecture. The malignant form has the same cytoarchitectural features but also hypercellularity, nuclear atypia, hyperchromasia and high mitotic rate.
Benign perineuriomas rarely recur. The malignant form may metastasize but its behavior is less aggressive than conventional malignant peripheral nerve sheath tumor (3).
Contributed by Elia Guadagno, Domenico Solari, Gianpiero Iannuzzo, Eduardo Clery, Mariarosaria Cervasio, Annarosaria De Chiara, Marialaura Del Basso De Caro