The pleural cavity is the space that lies between the pleura, the two thin membranes that line and surround the lungs. The pleural cavity contains a small amount of a thin fluid known as the pleural fluid, which provides lubrication as the lungs expand and contract during respiration.
If an excess amount of fluid is formed in the pleural cavity, a pleural effusion may develop. There are many causes of pleural effusions, ranging from heart failure to lung diseases to lung cancer. In order to diagnose a pleural effusion, your doctor may recommend a thoracentesis, a procedure in which a needle is placed into the cavity to remove pleural fluid. The pleural fluid is then analyzed under a microscope to look for the cause.
With lung cancer, a pleural effusion may have cancerous (malignant) cells present. If so, this may affect the stage of your cancer as well as possible treatments. It's important to note, however, that many pleural effusions in people with lung cancer are not cancerous -- don't have cancer cells present. For those who have cancer cells in their pleural fluid, the following article talks about what this means in regard to the stage of your disease, and possible treatments.