A cross section of a nerve at low power (5x) showed diffuse expansion of the nerve by tumor. The tumour was highly cellular and the cells were disposed in two identifiable patterns. The majority of the tumour was arranged in nests and lobules. The individual cells had well defined, generous eosinophilic cytoplasm which was finely granular and contained occasional vacuoles. This epithelioid architecture (130x) was somewhat reminiscent of melanoma. The minor pattern noted was that of spindle-shaped cells (65x) arranged in interwoven fascicles and nuclei with undulating nuclear envelopes. The tumour had 2 mitoses per 10 high power fields. Necrosis was not present. In some areas there was a lymphoplasmacytic infiltrate (130x).
Immunohistochemistry showed strong positivity for S100, NSE and vimentin (all 130x). Cytokeratin, EMA, actin, Leu7, KP-1, CEA, LCA and HMB-45 were all negative.
Elongated cell processes and dense cytoplasmic bodies were evident on ultrastructural examination which showed basement membrane (arrow) and entrapped axons (A). Long spacing collagen (Luse bodies) was present as well as mesaxon formation. Basement membrane material was plentiful and redundant throughout the lesion.
Mib-1 immunohistochemistry was performed on our case giving a 6.6% labelling index compared with averages of 1.5% (+/- 0.83) and 0.6% (+/- 0.29) from 5 randomly selected acoustic neuromas (schwannomas) and 5 neurofibromas respectively.