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Abstract
A 57-year-old man with malaise, ascites, and abdominal pain was found to have a peritoneum studded with numerous, small nodular tumor masses. Light microscopy revealed an anaplastic malignant tumor of uncertain differentiation. Mucin stains were negative. Electron microscopy revealed pleomorphic tumor cells with diffusely distributed cytoplasmic tonofilaments and well-developed true desmosomes. No long, thin, branching microvilli were present, yet tumor cells were strongly positive for both callus keratin (polyclonal) and monoclonal cytokeratin (AE1/3) in a diffuse cytoplasmic distribution (a pattern corresponding to the diffuse cytoplasmic tonofilaments). Tumor cells were negative for Leu-M1 and carcinoembryonic antigen. The findings were most consistent with malignant mesothelioma, and additional questioning, after tissue diagnosis, revealed a work history of asbestos exposure.