Once cancer is suspected, a tissue biopsy is needed to confirm the diagnosis. If the pleura or tissue lining the chest and lungs is involved, a thoracoscopy is performed, usually under local anesthesia. A small opening is made in the chest wall and an instrument called a thorascope, through which the surgeon can see into the chest, is inserted between the ribs. Tissue and fluid can be collected through this opening. Similarly, if the peritoneum or tissue lining the abdominal cavity and abdominal organs is involved, a peritoneoscope is inserted into the abdominal cavity to collect biopsy tissue. The biopsy tissue is then examined under a microscope to find out if the cells in it are normal or cancerous. Depending on the cell type, a diagnosis of malignant mesothelioma can be made. Malignant mesotheliomas are of three cell types: epithelioid, mixed and sarcomatoid. In general, epithelioid malignant mesotheliomas are the most common and carry the best prognosis; sarcomatoid ones are the most rare with the shortest survival time.
The next step is to stage the cancer as localized, advanced or recurrent. This may be done through a combination of imaging and surgical exploration. There are three staging systems in use, each with four stages: the Butchart System, the TNM System and the Brigham System