Pleural mesothelioma is a malignant mesothelioma that forms in the pleura, the portion of the mesothelium that surrounds the lungs in the thoracic cavity. Pleural mesothelioma is almost always contracted when the patient inhales asbestos fibers in large numbers, often over an extended period of time. The fibers migrate into the lungs, where they become ensconced in the lung tissue. Some of the fibers move further, penetrating the lung tissue and entering the pleura (the name for the portion of the mesothelium that surrounds the lungs).

The majority of pleural mesothelioma cases involve the right lung. Medical researchers believe that this is because the right lung is usually larger than the left lung, and has more surface area for the fibers to penetrate. For similar mechanical reasons, most pleural mesotheliomas begin in the bottom lobe of the lung rather than the top lobe, because gravity pulls the fibers down once they have been inhaled.

Once in the pleura, the asbestos fibers begin traumatizing and irritating the pleural tissue. At this stage, the patient has no symptoms and cannot feel what is going on;cancer has not yet formed.

In response to the damage being done by the asbestos fibers, the body creates macrophages, warrior cells that are used to rid the body of foreign objects such as fibers. The macrophages surround and attack the asbestos fibers, releasing a fluid that breaks down the fiber and tries to destroy it. Unfortunately this fluid also irritates the pleura, creating scar tissue which is highly susceptible to mutations – mutations which lead to the development of mesothelioma. While the scar tissue is forming over time, the asbestos fibers are also directly damaging the DNA of the local cells, turning off the chemical controls that regulate and control cell growth. Over time these changes result in the first cancer cell – a cell which does not know how to die in a timely fashion, as the body’s other cells all do, and which does not respond to the body’s chemical commands for it to stop reproducing and replicating itself. This cell then goes on to spread itself throughout the mesothelium – the patient now has cancer.

At first the tumors are small – little more than bumps on the pleural tissue. However, the tumors grow together quickly and form a thin lining that adds to the thickness of the pleural sac. As time passes, the lining gets thicker and thicker as more cancerous cells develop, and the newly thickened lining begins pressing on the lungs, compressing them and compromising their ability to function. This is the point at which the patient generally begins to notice symptoms. At the same time as the pleural lining is thickening, the pleural tissue begins to secrete large quantities of fluid into the pleural area, adding still further to the pressure on the lungs. Breathing becomes painful and difficult.

The tumor now begins to form what appears to be a hard shell over the pleura, and starts expanding outward into the body. The tumor mass begins to shed cancer cells, which serve as tiny ambassadors of the disease to the rest of the body. The full-blown cancer begins to metastasize – and at this point it has become almost impossible to treat.