Electron microscopy in the diagnosis of malignant schwannomas. A report of six cases.

The diagnosis of malignant schwannoma may be difficult or impossible by light microscopy alone, if the neoplasm is poorly differentiated and if there is no clinical evidence for its arising from a nerve-trunk. Six cases of malignant schwannoma in which electron microscopy confirmed or established the diagnosis are reported. In four cases, the anatomic origin of the neoplasm arising from a large nerve was demonstrated at the time of operation, and in one of these the patient had von Recklinghausen's disease. In three of the four cases, as well as in the fifth case, in which the tumor was associated with an overlying malignant melanoma, the distinctive schwannoma patterns of Antoni types A and B tissue were absent. In the sixth case, there were no clinical clues as to nerve sheath origin, but both the light and electron microscopic features of the neoplasm were very characteristic of Schwann cell derivation. The most consistently present, ultrastructural features of malignant schwannoma are the spindle shape of the cells and the terminal cytoplasmic processes. If the processes show intertwining and if lateral cell borders are interdigitated, the diagnosis is more readily confirmed. Likewise, basal lamina is an important diagnostic feature but may be scant and focal in the very cellular and less differentiated tumors. Microfilaments are usually present, albeit in varying numbers, and they do not form the dense bodies seen in smooth muscle cells. Other diagnostically useful, but less consistent characteristics include microtubules, primary and secondary lysosomes, and dense-core type granules. Long-spacing collagen was present in only one case and in none of those reviewed from the literature. This is also an important finding. It is probable that most malignant nerve sheath neoplasms arise from Schwann cells, rather than from fibroblasts or perinurial cells, but electron microscopy often will be necessary to confirm this impression.