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On a Friday morning, a surgical biopsy was performed on a rapidly enlarging mass on the posterior aspect of the distal end of the right upper arm of a 61-year-old woman. A previous fine-needle aspirate had revealed a malignant neoplasm, probably a sarcoma. A frozen section from the surgical biopsy confirmed this diagnosis, and, at the request of the surgeon and oncologist involved in the case, the pathologist in the surgical pathology suite immediately provided the electron microscopy unit with tissue to see whether a specific diagnosis could be available prior to the weekend. By mid-afternoon, using a microwave technique to enhance fixation and processing, sections were available for review and a report issued.
The fine-needle aspiration biopsy (Fig. 1) was reported as "positive for malignant cells, most consistent with a high-grade pleomorphic sarcoma". Review of the excisional biopsy (Figs. 2 and 3) revealed a pleomorphic sarcoma with marked nuclear pleomorphism, giant cells (some multinucleated) and increased mitotic tumor cells many of which were abnormal. There were no features to suggestive of rhabdomyosarcoma or any other specific line of differentiation. Immunocytochemistry, reported after the electron microscopy findings were available (see below), showed a strong positive reaction for vimentin (Fig. 4) and S-100 protein, and a weak reaction for neuron-specific enolase. Actin, myosin, desmin, cytokeratins and factor VIII were all negative.
To access light micrographs, use Figure 1.
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The variably sized and shaped tumor cells had only minimal numbers of cytoplasmic organelles. (Fig. 5). Between the closely associated tumor cells were collections of elongated, narrow cellular processes often aligned in an orderly, and at time rather parallel, fashion (Figs. 5 and 6). Only an occasional poorly formed intercellular junction joined adjacent cells or their processes. In some cases, the narrow cellular processes tended to encircle each other or small intercellular spaces (Fig. 7) and focal basal lamina was formed in relation to intercellular spaces (Fig. 7). This combination of features indicates a Schwann cell type of differentiation in this soft tissue neoplasm.
To access electron micrographs, use Figure 5.
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Malignant peripheral nerve sheath tumor - Soft tissue (upper arm).
The findings were specific and not only ruled out such possibilities as rhabdomyosarcoma, certain other sarcomas, malignant melanoma and metastatic carcinoma, but established a diagnosis of a poorly differentiated malignant peripheral nerve sheath tumor Immunocytochemistry (positive S100 protein, neuron-specific enolase and vimentin, and negative desmin, actin and cytokeratins) a few days later only served to confirmed the ultrastructural diagnosis.
The case illustrates the rapid (1-3) and precise diagnosis that
can be achieved in tumors that by either fine-needle aspiration
cytology or routine light microscopic preparations are simply
undifferentiated. Defining soft tissue sarcomas is a primary role
for diagnostic electron microscopy (3-6).
