Pleural mesothelioma cancer represents about 75% of all mesothelioma cases. The disease is caused by the inhalation of asbestos fibers that settle in the lungs. These asbestos fibers slowly and gradually become inserted in the lung lining (the pleura) and after a time these fibers cause chronic inflammation that eventually leads to growth of cancerous tumors or in some cases asbestosis.
Pleural mesothelioma cancer normally appears as multiple tumor masses which affects the parietal surface (inside closer to the lung) and visceral surface (outside further from the lung) of the pleura. Typically the parietal surface has greater involvement than the visceral. The right lung typically shows a slightly higher incidence of mesothelioma, apparently due to the fact that the right lung is larger and has a greater amount of pleural surface area. Also, the lower lungs typically show more tumor masses than the upper lung. This is thought to be due to gravitational factors influencing how the asbestos fibers settle in the lungs after they have been inhaled. Large growths in the pleura are normally noted in patients upon diagnosis. These growths lead to a complete obliteration of the lung cavity later in the disease. The tumors can spread from the lung pleura to other organs, including the heart and abdomen. This aggressive disease can also invade the lymph nodes and the entire blood system.
The pleura has two layers: the inner (visceral) layer, which is next to the lung; and the outer (parietal) layer, which lines the chest wall. The two layers of the pleura are usually in contact and slide over each other as we breathe. The membranes produce fluid, which allows them to slide over each other easily.When a mesothelioma develops in the pleura (pleural mesothelioma), the delicate membranes thicken and may press inwards on the lung. Fluid may also collect between the two layers of the pleura: this is known as a pleural effusion.
Tumors also grow in the pleura, but these are tumors that start in other parts of the body and are not considered pleural mesothelioma. These tumors have metastasized to the pleura. The most common form of non-pleural mesothelioma cancer that occurs is lung cancer, representing about 36% of the cancer occurring in the pleura. The next most common forms are breast cancer (25%), ovarian cancer (5%) and gastric cancer (2%). Lymphoma also accounts for a portion of cancer that has metastasized to the pleura.
