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Mesothelioma arises most commonly from pleural or peritoneal surfaces. Patients with pleural mesothelioma first complain of chest pain and shortness of breath, while individuals with peritoneal mesothelioma usually present with abdominal pain, ascites, or an abdominal mass.

Pleural mesothelioma is the more common of the two, although very heavily exposed asbestos workers generally develop peritoneal primary lesions.

Because of the proximity of the pleura to the pericardium, pericardial involvement–manifested as atrial arrhythmia or as fibrinous pericarditis–often accompanies pleural mesothelioma. Reccurent unilateral pleural effusions also are common in pleural mesothelioma.

A benign asbestos-related pleural effusion must be differentiated from one resulting from malignancy. In the first 10 years after asbestos exposure, benign pleural effusions may develop in some patients and then resolve. A few of these patients will develop a malignancy years later.

For the mesothelioma patient there may be clotting abnormalities, frequently disseminated intravascular coagulation DIC or microangiopathic hemolytic anemia. Reports of tumor-related DIC, however, are most often associated with mucin-producing tumors, which mesotheliomas are not. The mechanism of induction of the clotting abnormality in these patients is not known.

Histologic confirmation of malignant mesothelioma is frequently delayed after the onset of symptoms by as much as six months, because only pleural fluid is evident early in the course of the disease, and because pleural cytologic evaluation often is equivocal. X-ray evidence is usually just supportive: Only about 20 percent of patients with pleural primary lesions have asbestosis evident on X-ray. Calcified plaques may look lide buttons on the diaphragm. “Eggshell” calcifications of the pericardium also are characteristic.

Due to the frequent equivocacy of the pleural cytologic evaluation, an open biopsy is generally required. A biopsy of normal lung tissue also may be performed since asbestos fibers can usually be identified in the lung tissue but not generally in the tumor.

There are three histologic types of mesotheliomas: sarcomatous, epithelial, and mixed. The pathologist has no trouble diagnosing either the sarcomatous or mixed variants. The epithelial type, which is the most common, can easily be mistaken for adenocarcinoma, particularly if the asbestos exposure is long forgotten. If the condition is misdiagnosed as adenocarcinoma, a search for a primary lesion may take a significant proportion of the patient’s remaining life.

To confirm a diagnosis of mesothelioma, th pathologist generally uses the periodic acid-Schiff PAS stain with diastase digestion. A PAS stain is positive in many mesotheliomas and adenocarcinomas, but after diastase digestion, the PAS disappears in mesotheliomas but remains present in adenocarcinomas.

In the most extreme cases, conservative pathologists refuse to confirm a diagnosis of mesothrlioma until after a carefully performed autopsy eliminates all other possible primary malignancies.

When the diagnosis is “adenocarcinoma” in a patient with a history of asbestos exposure, it’s wise to obtain an adequate biopsy for special studies. Close-needle pleural biopsy is seldom adequate; a samll thoracotomy is frequently necessary to provide the pathologist with a sufficiently large piece of tissue.

Many patients are involved in litigation or require substantial documentation of asbestos exposure for workmen’s compensation. A fiber count of more than 500 per gram of wet tissue weight has been established as documenting a “substantial asbestos exposure.” A count of 200 per gram of tissue may be seen in urban dwellers in the general population and apparently is not associated with added risk of asbestos-related malignancies.