Malignant peripheral nerve sheath tumor of the orbit

Malignant peripheral nerve sheath (MPNS) tumor is a rare neoplasm of the orbit. The authors report a 78-year-old woman with an anaplastic MPNS tumor of the orbit that arose from a pre-existing encapsulated tumor composed of cells having a low mitotic index and displaying ultrastructural features of Schwann cells. The encapsulated tumor was unusual for a schwannoma because of its greater cellularity, absence of Antoni B tissue, and rarity of nuclear palisading. This combination of morphologic findings has two possible interpretations: (1) the anaplastic portion of the tumor arose from a variant of benign schwannoma known as a cellular schwannoma, or (2) the entire tumor was an MPNS tumor with variable differerentiation and extensive growth within markedly thickened perineurium.