Mesothelioma is a rare malignant tumor of the mesothelium, typically a disease of the parietal or visceral pleura, but can be found (very rarely) in the peritoneum or pericardium as well. It is most often caused by exposure to asbestos, usually over a prolonged period of years; exposure to very large quantities of asbestos over several months can also lead to the development of mesothelioma. There is usually a long latency period of 30-40 years between exposure to asbestos and the onset of disease. Approximately 50-80% of mesothelioma cases are due to asbestos exposure, but other causes are also being investigated, including a relationship between the malignancy and simian virus (SV40).
Mesothelioma is most often seen in older men but can be found in both men and women of any age. Often the patient has a history of having worked with asbestos, or can be a relative of someone who had worked with asbestos and, presumably, exposure was due to residual asbestos fibers on work clothes.
A mesothelioma is a primary tumor of the layer of cells that line the peritoneal cavity, pleural cavity, or the pericardial cavity.
Mesotheliomas can arise from the surface lining of the peritoneum and are known as peritoneal mesotheliomas. However, in approximately 75 to 80% of the cases, mesotheliomas arise from the surface lining of the pleura. This type of mesothelioma is known as a pleural mesothelioma. Mesotheliomas of the pericardium, called pericardial mesotheliomas, are extremely rare. Of the pleural type mesotheliomas, 75% are diffuse tumors and the remaining 25% are localized. The diffuse tumors are generally malignant, while the localized tumors are generally benign.
Men are affected with mesotheliomas three times more often than women.
Due to the sensitivity of adjacent structures in the thorax such as the heart, esophagus, spinal cord and lung, radiation dosing is limited. However, it does appear to offer palliative treatment to about half of patients. Unfortunately there have been no studies that show that radiation increases long-term survival. Radiation has also been used along with surgery to help prevent seeding following biopsy and chest tube placement. A third way that radiation is being used in mesothelioma treatment is in combination with chemotherapy and surgery. The best outcomes for patients have come with chemo, post-surgery radiation and extra-pleural pneumonectomy. Unfortunately, recurrence is common even with the combined approaches. There has also been work by a Swiss research team using radiation prior to chemo and surgery with some promising results.
The Butchart system stages the mesothelioma based on the bulk of the primary tumor. Stage one involves either the right or left pleura and may also involve the diaphragm on the same side. Stage two involves the pleura on both sides, the heart, esophagus or chest wall. Thoracic nodes may be involved. If nodes other than those in the thorax are involved or if the tumor has gone into the peritoneal cavity, then it is considered stage three. Any involvement of other organs or evidence of metastasis is stage four. The Brigham system shows node involvement and resectability. Stage one tumors are resectable and there is no node involvement. Stage two tumors are still resectable but there are nodes involved. Stage three is unresectable and extends into the chest wall, heart, diaphragm or peritoneal cavity. There may be involvement of nodes outside of the thorax. Stage four involves any distant metastasis.
The most common cell type is epitheliod. This type is seen in fifty to seventy percent of patients. The epitheliod cell shape and appearance with staining under a microscope is similar to adenocarcinoma and they are often confused. Diagnosis is made by cell characteristics which include uniformity in size, cube shape, and a very visible nucleus within the cell. Life expectancy for patients with epitheliod mesothelioma is 8.5 months. Sarcomatoid mesothelioma is the least common type appearing in ten to fifteen percent of patients. Cell shape is oval and more irregular than epitheliod cells. The nucleus of the cell is also less visible than that of the epitheliod cell. The life expectancy with sarcomatoid cell type is seven months. Thirdly is biphasic mesothelioma which contains both epitheliod and sarcomatoid type cells. There are sharply defined areas of each cell type within a biphasic tumor. The life expectancy for this type of tumor is six months.
Those who have come in contact with asbestos have the greatest risk of developing mesothelioma. Over 50 percent of those with mesothelioma have had some type of contact with asbestos. This includes shipyard workers, construction workers, roofers, insulation installers and miners. Due to the majority of males in these professions, it is seen more often in males than females. The family members of those exposed to asbestos at work are also at risk because of the exposure to secondary asbestos in the air or on clothes worn home from work. Exposure to asbestos in tiles or ceilings at home is also a risk factor. The addition of smoking to asbestos exposure greatly increases the risk of developing mesothelioma. Among those exposed to asbestos, smokers were 55 percent more likely to get cancer as opposed to the normal population. In Turkey, exposure to erionite has also shown an increased risk of development of mesothelioma.
Pleurodesis is a palliative procedure that prevents the most common cause of dyspnea that is seen in pleural mesothelioma; pleural effusions. The most
commonly used material is an asbestos-free talc to produce the irritative effect that causes the pleurodesis. Partial pleurectomy is another method that prevents recurrence of pleural effusion. This is a surgical procedure that resects the pleura and pericardium from the apex of the lung to the base of the lung. Thoracostomy is a palliative surgery in which the fluid in the pleural effusion is drained and dyspnea and pleuritic chest pain are relieved. Extrapleural pneumonectomy is the most radical of the procedures. It involves removal of the involved lung, the hemidiaphragm, the visceral and parietal pleura and the pericardium.
Malignant Mesothelioma
Pleural thickening and nodularity on the right are common findings of malignant mesothelioma. Malignant mesothelioma is associated with asbestosis exposure in 77% of cases. Asbestosis exposure has been demonstrated in the following industries: ship building, construction, ceramics, paper mill, auto parts, railroad and insulation.
Pleural mesothelioma is often unilateral, with a nearly twofold predilection for the right pleura. Keep in mind basic anatomy; the right main stem bronchus is at less of an angle than the left allowing foreign material to reach lung tissue more easily.
Although the CT-Scan is not diagnostic, it is highly suggestive. A pleural biopsy of these areas is the diagnostic test of choice. Currently, treatment is a tri-modality approach starting with surgical resection of the tumor if possible, followed by radiation therapy to the region, and finally chemotherapy. The prognosis of the disease is poor with most patients living only 8-14 months even with treatment.
Localized:
a. Benign – with appropriate surgical resection (1 cm surgical margins), most patients have no further complications of the disease. Transformation from a benign to a malignant mesothelioma is a possibility without proper surgical resection.
b. Malignant – these rates very greatly across literature. Generally speaking, without treatment the disease will be fatal in 4-8 months. With multi-modality treatment (surgery, radiation therapy, and chemotherapy), most patients survive from 8-14 months with some surviving up to two years.
Diffuse:
Diffuse mesothelioma has the poorest prognosis, with an overall average life span after diagnosis of 1 year – with or without treatment. In general, the extent of tumor invasion (chest wall, mediastinum, and diaphragm being common) correlates with length of survival.