Solitary cutaneous plexiform neurilemmoma (schwannoma): a clinicopathologic, immunohistochemical, and ultrastructural study of 11 cases.

Plexiform neurilemmoma (PN) is a rare benign peripheral nerve sheath tumor. The tumor is an uncommon nodular variant of schwannoma. Eleven cases of cutaneous plexiform neurilemmoma (CPN) were studied by clinicopathologic correlation, immunohistochemistry, and electron microscopy. The patients' ages ranged from 6 to 80 yr; the median age was 37 yr. The tumors were presented as single, soft to rubbery, movable, nontender, and sometimes painful nodules ranging from 0.5 cm to 2.5 cm. in diameter. The lesions were most commonly located on the extremities. The overlying skin surface was intact. These tumors were not associated with von Recklinghausen's neurofibromatosis or neurilemmomatosis. On gross examination the cut surface of the tumors showed grayish-white to yellow or tan coloration and had a well-defined border, but there was no evidence of a plexiform growth pattern. The microscopic features were characterized by single or multiple well-circumscribed nodules of spindle-shaped tumor cells. The nuclei were irregular and elongated, and the cytoplasm was eosinophilic and fibrillary without distinct cytoplasmic borders. Nuclear palisading was prominent, and Verocay bodies were present. Mitotic figures were rare (fewer than 2 per 20 high-power fields). Bodian stain showed presence of nerve fibers at the periphery of the tumor. The adjacent tissue showed wavy, spindle-shaped cells and collagen fibers in a myxoid stroma rich in hyaluronic acid, a pattern reminiscent of neurofibroma. The tumor cells showed positive reactivity with anti-S-100 protein in both the nuclei and cytoplasm. Glial fibrillary acid protein was focally positive, and neuron-specific enolase was negative. Electron microscopy displayed features of Schwann cells.(ABSTRACT TRUNCATED AT 250 WORDS)