Mesothelioma

Malignant mesothelioma affects the visceral and parietal pleura commencing as pleural nodules which coalesce to form a lobular sheet of tumour often several cm thick encasing the lungs, extending into the fissures and eventually invading the chest wall, mediastinal structures and diaphragm. Distant metastases occur late and are usually clinically silent though present in about half of patients at autopsy. There may be an associated exudate pleural effusion which is commonly haemorrhagic and low in pH and glucose. Cytology may reveal malignant mesothelial cells but some form of pleural biopsy is usually required for diagnosis.

The chest radiographic findings are of either a pleural effusion or of diffuse nodular pleural thickening which may extend into the fissures and occasionally encase the whole lung. Encasement of the lung fixes the position of the mediastinum; there is therefore no mediastinal shift and there is often ipsilateral volume loss. Chest ultrasonography differentiates between pleural fluid and thickening and, when there is an effusion, may reveal evidence of underlying pleural nodules or diffuse pleural thickening. Occasionally, the pleural nodules are too small to be detected by either ultrasound or CT. CT and MRI accurately show the extent of pleural involvement by tumour and any associated pleural fluid. The usual appearance is of extensive nodular pleural thickening encircling much or all of the lung, extending onto the mediastinal aspect and into the fissures (Fig.1). The irregular and circumferential nature of the thickened pleura and the involvement of the mediastinal aspect aid in the differentiation from benign pleural thickening. Calcification of the tumour is rare. In the late stages of the disease there may be direct extension into the chest wall and mediastinum and through the diaphragm into the abdominal cavity all of which is well demonstrated on CT or MRI; features of previous asbestos exposure may also be present and differentiation of early malignant mesothelioma from large, noncalcified pleural plaques may be difficult as benign plaques may enlarge on serial examinations. However, malignant mesothelioma often occurs without any evidence of either pleural plaques or pulmonary fibrosis.

CF - HM