Malignant mesothelioma is an uncommon, but no longer rare, cancer that is difficult to diagnose and poorly responsive to therapy. Malignant mesothelioma is the most serious of all asbestos-related diseases. A layer of specialized cells called mesothelial cells lines the chest cavity, abdominal cavity, and the cavity around the heart. These cells also cover the outer surface of most internal organs. The tissue formed by these cells is called mesothelium. The mesothelium helps protect the organs by producing a special lubricating fluid that allows organs to move around. For example, this fluid makes it easier for the lungs to move inside the chest during breathing. The mesothelium of the chest is called the pleura and the mesothelium of the abdomen is known as the peritoneum. The mesothelium of the pericardial cavity (the "sac-like" space around the heart) is called the pericardium. Tumors of the mesothelium can be benign (noncancerous) or malignant (cancerous). A malignant tumor of the mesothelium is called a malignant mesothelioma. Because most mesothelial tumors are cancerous, malignant mesothelioma is often simply called mesothelioma. Mesothelioma was recognized as a tumor of the pleura, peritoneum and pericardium in the late 1700's. However it was not until much later, in 1960, that this particular type of tumor was described in more detail and even more importantly, its association with asbestos exposure was recognized. The first report linking mesothelioma to asbestos exposure was written by J.C.Wagner, and described 32 cases of workers in the "Asbestos Hills" in South Africa. Since then, the relationship between mesothelioma and asbestos exposure has been confirmed in studies around the world. The incidence of mesothelioma in the United States remains very low, with 14 cases occurring per million people per year. Despite these numbers, the noticed threefold increase in mesothelioma in males between 1970 and 1984, is directly associated with environmental and occupational exposure to asbestos, mostly in areas of asbestos product plants and shipbuilding facilities.
Mesothelioma is a formerly rare form of cancer in which malignant (cancerous) cells are found in the mesothelium, a protective sac that covers most of the body's internal organs. It principally affects the pleura (lining of the lungs) and peritoneum (surrounding the lower digestive tract). Most of the people who develop mesothelioma have worked on jobs where they inhaled asbestos particles. Working with asbestos is the major risk factor for mesothelioma. The typically long delay between first exposure to asbestos and death from mesothelioma (seldom less than 15 years, but possibly as long as 60 years) means that deaths occurring now and most of those expected to occur in the future reflect industrial conditions of the past rather than current work practices. This latency period means that the effectiveness of current controls cannot yet be assessed from the mesothelioma mortality figures. A history of asbestos exposure at work is associated with about 80 percent of all cases. However, mesothelioma has been reported in some individuals without any known exposure to asbestos.
Peritoneal mesothelioma is a rare disease. The total number of cases per year in the United States is estimated between 100 and 500. A number of patients have a history of asbestos exposure. Because of the frequent dissemination of pleural mesothelioma to the peritoneal cavity, one must rule out spread from a primary pleural malignancy as the cause of peritoneal disease. No genetic, dietary, employment or geographic associations have been reported.
Peritoneal mesothelioma is unusual in that it demonstrates a wide spectrum of biological
aggressiveness. The cystic variant of mesothelioma may cause recurrent episodes of severe lower abdominal pain but may not result in the death of the patient for many years. In contrast, the most aggressive mesothelioma variants may show metastases from the peritoneal surface to mesenteric lymph nodes at the time of initial surgery. Patients are diagnosed as having a malignant mesothelioma by histologic and immunocytochemical study. Frequent mitoses and increased size of the nucleus indicate an aggressive malignant process. Dissemination by cancer seeding and peritoneal fluid production would result in disease progression. As the peritoneal fluid produced by mesothelial nodules increased, dissemination to sites of peritoneal fluid resorption would be expected. Patients who are diagnosed with peritoneal mesothelioma often present to their physician with a large volume of ascites.
The widespread progression of malignant cells on peritoneal surfaces results in copious fluid production. The fluid production can be attributed to the retention of a functional property of normal mesothelial cells. In these patients the peritoneal space becomes a free conduit for mesothelioma cells to migrate from place to place. In the production of ascites fluid, the cancer cells provide themselves with a carrier solution to disseminate throughout the abdominal and pelvic spaces. Treatment of Peritoneal Mesothelioma Due to a lack of symptoms early in the natural history of peritoneal mesothelioma, a large majority of patients are first diagnosed with a large volume of disease diffusely spread throughout the abdomen and pelvis. The disease accumulates in largest volume at sites of peritoneal fluid reabsorption and at dependent sites by gravity. The small bowel surfaces and mesenteries are not spared of mesothelioma implants as in the mucinous appendiceal neoplasms. Promising results of treatment from a new strategy: cytoreductive surgery plus perioperative intraperitoneal chemotherapy: Four groups have now reported on approximately 300 malignant peritoneal mesothelioma patients. The National Cancer Institute in Bethesda, MD, The Washington Cancer Institute in Washington, DC, The Columbia Mesothelioma Center in New York and the National Cancer Institute in Milan, Italy. Each group has reported their experience with between 50 and 100 patients. With current treatment all the groups report a median survival of 5 years or better. The median survival in the past was approximately 1 year (see Tables 1 and 2). As a result of this apparent major improvement in survival with a new treatment strategy, it has become standard of care for these patients.
