Malignant mesothelioma masquerading as a multinodular bronchioloalveolar cell adenocarcinoma with widespread pulmonary nodules.

A 72-year-old man had a unilateral pleural effusion and multiple bilateral pulmonary nodules. Thoracoscopic biopsy revealed multiple discrete nodules in the pleura and lung. The latter consisted of tall columnar malignant cells arranged on alveolar surfaces in a lepidic growth pattern. Mucin filled the alveolar lumina, both in the nodules and surrounding lung. It stained with Alcian blue but not with periodic acid Schiff, suggesting that it was a glycosaminoglycan, which was confirmed as hyaluronic acid by complete digestion with hyaluronidase. Tumor cells were calretinin, Wilms tumor-1, and high-molecular-weight cytokeratin 5/6 positive, and were negative for thyroid transcription factor-1, cytokeratin 7, and cytokeratin 20. Ultrastructurally, they had very long and abundant, slender microvilli typical of a malignant mesothelioma. This is the first example of a mesothelioma masquerading as a bronchioloalveolar carcinoma.