* Malignant Mesothelioma

    Malignant mesothelioma is an uncommon form of cancer that affects the mesothelium, or the lining surrounding several of the body’s internal organs. Asbestos exposure is almost exclusively responsible for the development of mesothelioma.

    The prevalence of lung cancer in those exposed to asbestos had been noted since the 1920s, and the first case report dealing with malignant mesothelioma, derived from records at Massachusetts General Hospital, was published in 1947. It was not until 1998, however, that mortality recording agencies began tracking malignant mesothelioma as a separate cause of death.

    Malignant mesothelioma can affect four parts of the body. The lungs are most frequently affected, which is classified as pleural mesothelioma. The second most common location for the tumors to originate is the abdomen, which is known as peritoneal mesothelioma. Another variation of the disease, pericardial mesothelioma, develops in the sac that surrounds the heart. The rarest location for mesothelioma to be found is in the lining of the testicles, known as the tunica vaginalis, in a form of the cancer that is medically classified as testicular mesothelioma.

Benign vs. Malignant Mesothelioma

    As with most forms of cancer, mesothelioma may be classified as either malignant or benign. Benign tumors are completely surgically resectable because they are non-cancerous and unable to spread.  On the other hand, malignant mesothelioma is cancerous, highly aggressive and hard to treat. While asbestos has been definitively linked to malignant mesothelioma, benign mesothelioma often results from other disorders such as infectious pleuritis, hepatic cirrhosis or systemic lupus erythematosus.

    Because a patient may present both benign and malignant mesothelioma tumors simultaneously, a doctor must be able to differentiate between the two before beginning treatment. One of the most important criteria for determining if a tumor is malignant or benign is the presence of stromal invasion.  Since benign tumors do not metastasize, if the tumor has spread to local glands, cells and connective tissue, the tumor is considered to be malignant.

    A doctor can consider the probability of malignancy based on how deep in the pleura the cellular proliferation appears. Benign cells typically conglomerate at a relatively shallow depth, while malignant cells can appear as far as the fat or muscle areas of the chest wall or inside of the lung parenchyma.

    Total cell enlargement – as opposed to just the enlargement of the cell’s nucleus – is another indicator of malignancy. Large aggregations of cells that feature nuclear irregularity and hyperchromasia (an elevated level of the DNA protein chromatin) almost always imply a malignant diagnosis. Reactive (benign) lesions involve linear cells that typically lack papillae, a dome-shaped structure on the cell surface.

Malignant Mesothelioma Cell Types

    A malignant mesothelioma tumor may be made up of three different cell types:

• Epithelial Cells – Epithelial cells appear the most frequently in malignant mesothelioma tumors, and this subtype accounts for about 50 percent of cases. These cells, which are uniform and sharply defined, feature prominent nuclei and often form complex branching patterns.
• Sarcomatoid Cells – These spindle-shaped cells typically lack strong characteristics, occurring in approximately 30 percent of reported malignant mesothelioma cases.
• Biphasic Cells – A malignant mesothelioma tumor is considered biphasic when both epithelial and sarcomatoid cells are present and each type accounts for at least 10 percent of the mass. Biphasic (mixed) cells are present in around 20 percent of all malignant mesothelioma reports.

Diagram of Malignant Mesothelioma Types.

•     These histological classifications are commonly used in developing a prognosis and treatment plan for malignant mesothelioma patients. Although the life span associated with malignant mesothelioma can be short, the subtype can account for differences in life expectancy ranging up to 200 days.

     Of the subtypes, mesotheliomas involving epithelial cells are typically most responsive to treatment, resulting in a better prognosis. Sarcomatoid cells are the most stubborn when treated, and some surgeons do not consider sarcomatoid tumors as eligible for resection. Biphasic cells are also considered less responsive to treatment than epithelial.

    Diagnosing what type of malignant mesothelioma a patient has can be challenging. One contributing factor is that it can be difficult to differentiate mesothelioma from adenocarcinoma, a cancer originating in glandular tissue, on small tissue specimens.

Source : http://www.asbestos.com