Malignant mesotheliomas in the thorax arise from either the visceral or the parietal pleura. Although uncommon they have assumed great importance in the past few years because of their increased incidence among persons with heavy exposure to asbestos. In coastal areas with shipping industries in the UNITED States and Great Britain and in Canadian and South African mining areas, up to 90% of reported mesotheliomas are asbestos-related. There is a long period of 25 to 45 years for the development of asbestos related mesothelioma and there seems to be no increased risk of mesothelioma in asbestos workers who smoke. This is in contrast to the risk of asbestos related bronchogenic carcinoma already high and is markedly magnified by smoking. Thus for asbestos workers those who are also smokers), the risk of dying of lung carcinoma far exceeds that of developing mesothelioma.
Asbestos bodies are found in increased numbers in the lungs of patients with mesothelioma and mesotheliomas can be induced readily in experimental animals by intrapleural injections of asbestos.
Malignant mesothelioma is a diffuse lesion that spreads widely in the pleural space and is usually associated with extensive pleural effusion and direct invasion of thoracic structures. The affected lung is ensheathed by a thick layer of soft gelatinous, grayish pink tumor tissue.
Microscopically malignant mesotheliomas consists of a mixture of two types of cells either one of which might predominate in an individual case. Mesothelial cells have the potential to develop as either mesenchymal stromal cells or epithelium like lining cells. The latter is the usual form of the mesothelium, an epithelium that lines the serious cavities of the body. The mesenchymal types of mesothelioma appears as a spindle cell sarcoma resembling fibrosarcoma (sarcomatoid type), whereas the papillary type consists of cuboidal, columnar or flattened cells forming a tubular and papillary structure ( epithelial type) resembling adenocarcinoma. Epithelial mesothelioma may at times be difficult to differentiate grossly and histologically from pulmonary adenocarcinoma. Special features that favor mesothelioma include the following (1) positive staining for acid mucopolysaccharide, which is inhibited by previous digestion by hyaluronidase; (2) lack of staining for carcinoembryonic antigen (CFA) and othere epithelial glycoprotein antigens, markers generally expressed by adenocarcinoma (3) strong staining for keratin proteins with accentuation of perinuclear rather than peripheral staining (4) on electron microscopy the presence of long microvilli and abundant tonofilaments but absent microvillous rootlets and lamellar bodies. The mixed type of mesothelioma contains both epithelial and sarcomatoid patterns. Cytogenetic abnormalities occur in mesotheliomas but not reactive mesothelial proliferations a diagnostically useful feature.
Clinical course
The presenting complaints are chest pain, dyspnea and as noted recurrent pleural effusions. Concurrent pulmonary asbestosis (fibrosis) is present in only 20% of patients with pleural mesothelioma. Fifty percent of those with pleural disease die within 12 months of diagnosis and few survive longer than 2 years. Aggressive therapy (extrapleural pneumonectomy, chemotherapy , radiation therapy) appears to improve this poor prognosis in some patients. The lung is invaded directly, and there is often metastatic spread to the hilar lymph nodes and, eventually , to the liver and other distant organs.
Mesotheliomas also arise in the peritoneum, pericardium, tunica vaginalis, and genital tract. Peritoneal mesotheliomas are particularly related to heavy asbestos exposure, 50% of such patients also have pulmonary fibrosis. Although in about 50% of cases the disease remains confined to the abnorminal cavity, intestinal involvement frequently leads to death from intestinal obstruction or inanition.
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