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The principal imaging findings of MPM-effusion, pleural thickening, and pleural masses-are also manifestations of many other disease processes. In patients older than 50 years of age, metastatic disease to the pleura represents the second most common cause of pleural effusion; the first is congestive heart failure (Figure 32). Pulmonary embolus, cirrhosis, collagen vascular diseases, tuberculosis, pneumonia, and empyema also must be considered in patients with large pleural effusions. It is interesting to note that 3%-5% of asbestos workers develop "benign asbestos pleurisy," a syndrome in which patients have recurrent, painful, sometimes hemorrhagic effusions without a malignancy (9). However, benign asbestos pleurisy is always a diagnosis of exclusion, and development of a unilateral effusion in an individual with a history of asbestos exposure should be considered suggestive of malignancy until proved otherwise (12).
Diffuse unilateral or bilateral pleural thickening can also occur in patients with asbestos-related pleural fibrosis and is typically accompanied by asbestosis (Figure 33). Infections (tuberculosis, fungal, actinomycosis), fibrothorax, empyema, other pleural malignancies, and metastases to the pleura may cause diffuse, nodular pleural thickening or multiple discrete pleural masses that cannot be reliably differentiated from MPM with imaging studies alone (9,50). Adenocarcinoma is the most common cell type to metastasize to the pleura (Figure 34). The most common primary malignant neoplasms that metastasize to the pleura and cause malignant pleural effusions are lung cancer (36% of cases), breast cancer (25%), ovarian cancer (5%), and gastric cancer (2%) (58). Lymphoma accounts for 10% of malignant pleural effusions (Figure 35) (58). Neoplasms that only occasionally show diffuse involvement of the pleura are invasive thymoma, leukemia, myeloma, and renal cell carcinoma (Figure 36).
None of these findings-pleural thickening, nodules or masses, effusions; lung encasement; frozen mediastinum; contraction of a hemithorax; invasion of chest wall, mediastinal, or abdominal structures; lung involvement; hematogenous or lymphangitic spread-is pathognomonic for MPM. Indeed, adenocarcinoma metastatic to the pleura cannot be reliably differentiated from MPM on the basis of imaging studies alone (54). However, specific CT findings are helpful in differentiating benign from malignant pleural disease. Pleural thickening with a circumferential distribution or lung encasement, nodular morphology, pleural thickening of more than 1 cm, and involvement of the mediastinal pleura are all suggestive of malignant disease, with specificities of 100%, 94%, 94%, and 88%, respectively; the sensitivities of these findings are 41%, 51%, 36%, and 56%, respectively (54).
It should be noted that the absence of pleural thickening does not exclude a diagnosis of malignancy. One report described three cases of MPM in which the only CT finding was pleural effusion, which proved to be malignant after thoracentesis. At surgery, small visceral pleural nodules were found, but the nodules were not visible on CT scans, even in retrospect (54).
Rib destruction and frank chest wall invasion are good indicators of malignancy. However, some infectious processes, such as actinomycosis, tuberculosis, and nocardiosis, can invade the chest wall, but usually at a single focus rather than at multiple sites, as seen with metastatic disease (50).
Some authors have suggested that the presence of definite hilar adenopathy indicates that the diagnosis is more likely to be adenocarcinoma metastatic to the pleura rather than MPM (31), but others disagree (54). Similarly, the tendency of MPM to have greater involvement of the inferior hemithorax is nonspecific and is present in the majority of patients with pleural metastases (54).
Differentiation between benign, asbestos-related pleural disease (eg, pleural plaques, benign asbestos pleurisy) and MPM is usually not difficult. Unilaterality or asymmetry is more suggestive of MPM. Calcification in the areas of pleural thickening or pleural masses is suggestive of benign disease (54).
Differential Diagnosis