The rare asbestos cancer known as mesothelioma has many similarities to lung cancer; it affects the same general area of the body and exhibits many of the same symptoms. Yet there are some substantial differences—from the causes and development of the cancers, to the structure of the tumors, to their treatment.
One of the reasons that these two diseases are often conflated is that exposure to the toxic material asbestos can cause both of them. Yet the two occur in different tissues of the body. Lung cancer affects the lung tissue itself, whereas mesothelioma attacks the pleura, which is a thin membrane that both covers the lungs (and other organs) and lines the inside of the chest cavity. Mesothelioma may later metastasize to the lung tissue, but it always begins in the mesothelium. Although pleural mesothelioma is the most common form of the cancer, it can also strike the peritoneal or pericardial mesothelia, which surround and protect the stomach and the heart, respectively.
Asbestos exposure is the primary cause of mesothelioma, although there are some other possible causes of the disease—exposure to Thorotrast, a chemical once used in diagnostic X-rays, and to Simian virus 40, which contaminated several million polio vaccines. The relatively recent technology of carbon nanotubes has also fallen under scrutiny as being a possible source of mesothelioma cancer, but none of these purported causes have been conclusively linked to the cancer to the same extent as asbestos.
On the other hand, lung cancer may be caused by a number of factors, or a combination. Smoking, secondhand smoking, a family history of the disease, and exposure to radon gas are all definitively linked to a higher incidence of lung cancer diagnoses.
Although patients diagnosed with lung cancer may have large or multiple tumors, the tumors generally are individualized masses with clearly defined boundaries. This makes them easier for a surgeon to remove, thereby increasing the chances of actually curing the cancer. Mesothelioma, on the other hand, is characterized by diffuse malignancies; the “tumor” is not a contained mass but instead spreads across the surface of the mesothelium, rendering it extremely difficult to surgically resect. In later stages, mesothelioma can actually encase the lung in a rind-like fashion. For the same reasons, lung cancer is more receptive to chemotherapy and radiation, whereas both of these therapies are usually unable to inhibit the growth of the diffuse malignant tissues in mesothelioma.
Another significant difference between these two types of cancer is their rates of occurrence. Lung cancer is diagnosed in approximately 200,000 new patients each year in the United States; mesothelioma cases number only about 3,000. The decline in cigarette smoking will undoubtedly cause lung cancer rates to fall, and mesothelioma rates are expected to peak in the coming decade, because the disease has such a long latency period. Yet mesothelioma will always be a much rarer cancer than lung cancer.
Neither cancer has a particularly high survival rate, but the prognosis for lung cancer is generally less grim than that for mesothelioma. Depending on the stage, lung cancer patients have a five-year survival rate between 15 and 75 percent, but fewer than 10 percent of mesothelioma patients live two years or longer past diagnosis. Typically, those who learn that they have malignant pleural mesothelioma only live a few months.