Cancer of the pleura is most often metastatic, usually adenocarcinoma from the breast or lung.Primary malignant neoplasms of the pleura are relatively rare.Pleural neoplasms frequently produce a malignant pleural effusion, making cytology a useful tool in their detection.Thoracoscopy is also very useful tool for obtaining tissue samples for diagnosis because it allows the direct visualization of the pleural surfaces.

    Mesotheliomas are the most common primary malignant tumors of the pleura.Other rare primary tumors of the pleura include fibrosarcoma, angiosarcoma, and other sarcomas.Mesotheliomas arise from pleura in about 80% of cases and from the peritoneum in the remaining 20%.In the chest, the tumors can arise from either the visceral or parietal pleura. Approximately 75% of these tumors are diffuse and malignant. The rest are localized and usually benign. Mesotheliomas are more common in men, who represent about 75% of all cases.

    The benign mesothelioma, also known as a fibroma, is usually a small (1-2 cm) localized growth which may enlarge.These tumors always remain confined to the surface of the lung and do not normally produce a pleural effusion.Generally they become symptomatic when their growth impinges on adjacent structures within the chest.They are sometimes associated with hypoglycemia, and they are not associated with asbestos exposure.Histologically they are composed of whorls of reticulin and collagen fibers amongst cytologically benign spindled cells resembling fibroblasts.

    The more common malignant pleural mesotheliomas, however, are strongly associated with asbestos exposure.In the normal population, mesotheliomas are rare tumors with an incidence of 2 to 17 cases per 1,000,000 people; however, in asbestos workers the relative risk is over 1000 times greater. The risk of bronchogenic carcinoma is increased only 5 fold in asbestos workers, but the risk is 55 times greater in asbestos workers who smoked.It should be stressed that there is no increased risk of mesothelioma in smokers who have asbestos exposure, which is in direct contrast to the association between asbestos exposure and bronchogenic carcinoma, where the risk is markedly increased.

    Common occupational sources for asbestos exposure include the mining, milling, manufacturing processes. A history of shipyard work or replacement of automobile brake lining in the past places a patient at risk for mesothelioma. Epidemiologic studies have determined that 70% to 90% of cases have identifiable asbestos exposure.