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Staging Systems
The most widely used staging scheme for MPM has been the classification by Butchart et al (63) (Table 1). Stage I disease has not been uniformly associated with improved prognosis, a finding that has led some investigators to question whether it really corresponds to the early stage of MPM. Boutin and Rey (60) have suggested that evaluation with VATS allows more accurate staging, and they have proposed a modified Butchart classification (Table 2) (64). This scheme appears to correlate better with prognosis. Twenty-six patients with stage IA disease (according to Boutin and Rey's classification) had an expected survival of 28.5 months when they underwent surgery (64). Visceral pleural invasion indicated a worse prognosis because the survival of patients with stage IB disease was only 11 months. Survival of patients with stage II disease was 10 months, a finding which suggested that mediastinal involvement occurs very soon after visceral pleural invasion and may even be synchronous in some cases.
A new TNM-based staging system has been proposed by the International Mesothelioma Interest Group (65). This system recognizes the importance of positive nodes, and although N0 tumors (no regional lymph node metastases) are either stage I or stage II, depending on the extent of the primary tumor, N1 tumors (metastases in bronchopulmonary or hilar lymph nodes) are automatically classified as stage III.
Most patients (50%) have stage II disease at diagnosis. Presentation with stage I (18% of cases) and stage III (28%) disease is less common, and presentation with stage IV disease is very uncommon (4%) (21).
Resectability
Resectability of MPM is determined at surgery. A tumor of any size may be defined as resectable if it is confined to one hemithorax and shows only superficial invasion of the diaphragm or visceral pericardium, localized invasion of the chest wall limited to a previous biopsy site, and a smooth uninvolved inferior diaphragmatic surface. Surgically unresectable tumors are those that have transdiaphragmatic extension, diffuse chest wall invasion, focal chest wall tumor extension well beyond a biopsy site, invasion of essential mediastinal structures (great vessels, esophagus, trachea, aorta, heart), or distant metastases (56).
According to Patz et al (56), the CT and MR imaging criteria for resectability include (a) preserved extrapleural fat planes, (b) normal CT attenuation values and MR signal intensity characteristics of structures adjacent to the tumor, (c) absence of extrapleural soft-tissue masses, and (d) a smooth inferior diaphragmatic surface on sagittal and coronal images. Criteria for unresectability include tumor encasement of the diaphragm; invasion of the extrapleural soft tissues or fat; infiltration, displacement, or separation of ribs by tumor; and bone destruction (56).
In their study of 41 patients with MPM staged by using CT and MR imaging, Patz et al (56) concluded that the most reliable indicators of resectability were a clear fat plane between the inferior surface of the diaphragm and adjacent abdominal organs and a smooth inferior diaphragmatic contour. Unresectability was considered definite when the tumor encased the diaphragm. Poorly defined margins of abdominal organs adjacent to the diaphragm and poorly visualized inferior diaphragmatic surfaces were indeterminate findings seen in both resectable and unresectable MPM (56).
Patz and colleagues (56) compared the value of the two modalities in predicting resectability and concluded that CT and MR imaging provide similar information in most cases. The sensitivity of CT in evaluating the resectability of a tumor in three anatomic regions (diaphragm, chest wall, mediastinum) was 94%, 93%, and 100%, respectively. The sensitivity of MR imaging in the same three regions was 100%, 100%, and 92%, respectively. MR imaging has a slight advantage over CT for evaluation of the diaphragm and chest wall because sagittal, coronal, and other imaging planes can be used to optimize visualization of questionable areas. However, because of its widespread availability, CT is still recommended as the initial study for staging MPM. Use of MR imaging is reserved for indeterminate cases (56).
Staging and Resectability