An interesting and clever structure in the body is the pleura. The pleura is a double-layered membrane that surrounds each lung, acting as a protective covering. The two layers form a closed structure, and these two layers of the pleura are in contact with each other and can move back and forth against each other as the lungs inflate and deflate during normal respiration. Under normal physiologic conditions, there is a small amount of fluid in the space between the two layers of the pleura to keep the two pleural membranes moist and able to slide.
Many pathologic conditions may lead to the accumulation of fluid, pus, blood or air in the pleural space. Since the lungs and pleura are located in the thorax or chest cavity, a space confined by the rib cage and diaphragm, the total volume of these entities is restricted and fixed. Thus, if there is extra pleural fluid, it will perforce place pressure on the lung and either prevent it from expanding fully or, if the volume is large enough, cause it to collapse.
Many conditions may lead to such circumstances. These include congestive heart failure, pneumonia or penetrating chest injuries, such as by gunshot or knife wounds. We, of course, are primarily interested in the impact of malignancy on the pleura.
Cancer in the pleural space usually manifests itself as a pleural effusion—this is an abnormal collection of fluid in the pleural space. As I indicated above, if it is large enough, it can cause significant symptoms. To either diagnose the etiology of an effusion or to treat it, a needle can be put through the ribs into the space and fluid extracted for laboratory analysis. A pleural effusion from a malignant cause will show malignant cells on investigation.
As with other organs (liver, lungs, bone, etc.), there are two ways malignant cells can arise in the pleural space—they can be metastatic spread from another primary site, or it can be a primary tumor arising in the pleura itself.
Metastasis to the pleura can occur from a number of sites. Most common are the lung itself, the breast or lymphoma. Less commonly, metastatic spread may also arise from stomach cancer, ovarian cancer, kidney cancer or even colorectal cancer. Treatment of a malignant pleural effusion requires the treatment of the primary site, but in addition drainage of the fluid may be undertaken for symptomatic relief.
A primary tumor of the pleura is a mesothelioma. This is not a common tumor—there are only about 3,000 cases annually in the U.S. In the vast majority of instances, these appear to be associated with exposure to asbestos from some source. The association between asbestos and mesothelioma is so strong that the diagnosis of a pleural mesothelioma almost implies that asbestos exposure occurred at some point. There are also multiple cases reported of wives developing mesothelioma from washing their husband’s clothes that have been contaminated with asbestos fibers. The association was first described in 1960 in South Africa where asbestos is mined; asbestos has been restricted since 2002 in the U.S.
Apparently, the asbestos fibers are inhaled and are able to penetrate through the lung itself to reach the pleural cavity. Once there, it can take a decade or decades for a mesothelioma to develop.
In some circumstances, it appears that radiation exposure may cause mesothelioma as well. Karen Antman is a well known specialist in the treatment of sarcomas and mesotheliomas —mesothelioma is a form of sarcoma—who became head of oncology at Columbia in the early 1990s; she has been the dean of the Boston University School of Medicine since 2005. When she came to Columbia, she shared with me five cases of mesothelioma that had occurred in people who had received radiation therapy to the chest for treatment of Hodgkin disease and had no prior exposure to asbestos, which we published in the Journal of Clinical Oncology.
Mesothelioma is generally considered to be incurable. However, aggressive treatment with surgery and some new chemotherapeutic agents, especially a drug called pemetrexed, for the disease have been successful in prolonging survival for some of its victims. Average life expectancy is about a year, but some do live considerably longer.
The abdomen has a similar double layer of mesothelium surrounding it as well, referred to as the peritoneum, which also acts as a protective barrier for the abdominal contents. A primary malignancy of the peritoneal mesothelium may arise there as well, known as a peritoneal mesothelioma, and also attributable in most cases to asbestos exposure. It is estimated that 20-30% of mesotheliomas are peritoneal. Apparently, the inhaled asbestos fibers are brought up in saliva and then swallowed, make their way through the alimentary tract to the intestines where they somehow penetrate through the bowel walls into the peritoneum and induce the malignancy, similar to what takes place with pleural mesothelioma.
Alfred I. Neugut, MD, PhD, is a medical oncologist and cancer epidemiologist at Columbia University Irving Medical Center/New York Presbyterian and Mailman School of Public Health in New York.
This article is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment, and does not constitute medical or other professional advice. Always seek the advice of your qualified health provider with any questions you may have regarding a medical condition or treatment.
