Presence of malignant mesothelial cells in the sputum.

Malignant pleural mesothelioma and peripheral adenocarcinoma, of the lung, also known as pseudomesotheliomatous adenocarcinoma, have similar clinical and radiological characteristics and even similar microscopic findings, and this makes it difficult to differentiate them. Malignant pleural mesothelioma rarely invades the bronchial lamina or bronchioloalveolar spaces, and tumor cells are not usually found in the sputum. Therefore, the appearance of tumor cells in sputum more likely supports the diagnosis of peripheral lung cancer. We report a rare case in which malignant pleural mesothelioma cells were found in the sputum. For the differential diagnosis of a mass involving both the pleura and lung, physicians should consider that malignant mesothelial cells can be found in the sputum, although this is very rare.