Pseudomesotheliomatous carcinoma of the lung with histochemical and immunohistochemical study.

An autopsy case of pseudomesotheliomatous carcinoma of the right lung in a 56-year-old man occupationally exposed to stone dust is presented. From open biopsy specimens in which polyhedral epithelium-like cells appeared arranged in nests, sheets, and trabecula without apparent tubular pattern, a malignant pleural mesothelioma was suspected. At autopsy, the right pleural cavity was obliterated by the tumor mass which covered the collapsed pulmonary parenchyma and was clearly demarcated from it. The gross appearance of the tumor was similar to that of malignant pleural mesothelioma. Histologically, marked interstitial fibrosis of the subpleural parenchyma of both lungs was observed, and the tumor tissue was interspersed in the parenchyma adjacent to the tumor mass. The tumor showed both intracytoplasmic and intercellular positive materials with colloidal iron, alcian blue (pH 2.5), and toluidine blue stains, which entirely disappeared after streptomyces hyaluronidase digestion. A small amount of intracytoplasmic PAS-positive material resistant to diastase digestion was also observed. Immunohistochemical staining for carcinoembryonic antigen, which is said to be negative in malignant pleural mesothelioma, was positive intracellularly. There were no histologic findings indicating asbestos exposure. From these findings, the authors made a diagnosis of poorly differentiated adenocarcinoma, which was characterized by the production of hyaluronic acid.