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Mesothelioma is classified into four main types based on the part of the body: i)Pleural mesothelioma is a cancer that develops in tissue surrounding the lungs, ii) pericardium mesothelioma originates in the pericardium membrane lining the heart, iii) peritoneum mesothelioma starts growing around the abdominal organs, and iv) tunica vaginalis mesothelioma is a cancer that forms on the membrane lining the testicles. The most common type is pleural mesothelioma which makes up around 70% of all cases. 10% to 20% of mesotheliomas are peritoneum.
The overall 5-year survival rate for patients with mesothelioma is only 10% because it is usually diagnosed at a advanced stage. However, even if the cancer is found at an early stage, the 5-year survival rate for people with mesothelioma is still between 13% and 16%. When the cancer metastasized to distant organs, the 5-year survival rate is less than 1%.
Mesothelioma can be further classified by cell type. These mesothelioma tumors may be categorized as being benign or malignant, epithelial, sarcomatoid, or biphasic, or as one of several other less common cell variants. Examination of the tumor cells, known as histology, can help doctors determine the type of cancer and what treatments will best help the patient.
Histology is the study of cells, and it is commonly used to aid diagnosis. For cancers like mesothelioma, examining the cells of a suspected tumor can tell a doctor if the patient has cancer, if it is malignant. In some cases, these cells can also help determine where the cancer originated. Mesothelioma cells are notoriously difficult to diagnose. They don’t always look different from other cancer cells. For example, pleural mesothelioma cells are very similar to lung cancer cells in morphology.
A biopsy is necessary to determine cell type for mesothelioma. A sample of fluid or tissue is taken from an area of the body thought to have a tumor. If there is a known tumor, the sample will be taken directly from that. The tissue sample is prepared on a microscope slide and stained to reveal the individual cells. A pathologist then examines the cells and attempts to identify them for proper diagnosis.
Immunohistochemistry, a branch of histology, uses antibodies to stain tissue and fluid samples. These antibodies attach to specific antigens or proteins on certain types of cancer cells. Using immunohistochemistry, a pathologist can target one particular cancer cell type, either confirming the cancer or ruling it out. In addition, pathologist can also tell you the degree of your tumor’s growth rate or angiogenesis –new blood vessel formation- based on the use of immunohistochemistry markers such as ki67 or CD31.
Benign and Malignant Cells
Unfortunately, mesothelioma cells are almost always malignant. Malignant means they are capable of growing and spreading to nearby tissues and other organs. If left untreated, malignant cells are always fatal. On the other hand, benign tumor cells do not spread through the body. Instead, they may grow into a tumor that becomes uncomfortable or causes symptoms. Benign mesotheliomas are rare. Some cell types considered benign mesotheliomas include papillary and adenomatoid.
When a pathologist studies the cells from a person with mesothelioma, he or she looks for the type of cell in the tumor. These cells may be epithelial, sarcomatoid, or both. Epithelial cells are the most common type and are found in 70 percent of malignant mesothelioma tumors. These cells have certain physical characteristics, like large nuclei and distinctive branching patterns, that are discernible under a microscope.
Epithelial cells typically respond well to treatment. Therefore, an epithelial mesothelioma diagnosis is generally better news than the alternative. Because epithelial cells adhere to one another, they are less likely to spread to other tissues.
In comparison to epithelial cells, sarcomatoid cells have a distinctive, elongated spindle shape that is easy to identify under a microscope. About 10 to 20 percent of mesotheliomas are diagnosed as sarcomatoid. Sarcomatoid cells are more likely to spread aggressively. Sarcomatoid mesothelioma resists treatment and often forms multiple nodules rather than one or more large tumors. The prognosis of the sarcomatoid type of cancer has the worst prognosis.
If a mesothelioma tumor includes both sarcomatoid and epithelial cells, it is called biphasic. A tumor will be diagnosed as biphasic if at least 10 percent its cells are one type, with the remaining cells falling into the other category. In other words, a tumor comprised of 90 percent sarcomatoid cells and 10 percent epithelial would be considered biphasic cancer. About 25 to 30 percent of mesotheliomas are biphasic.
Rare Cell Variations
Diagnoses may also include cell variants. Variants are rare and present unique treatment challenges. Deciduoid mesothelioma is a variant of epithelial cells. Originally, this unusual variant cell was mostly found in cases of peritoneal mesothelioma in young women. Approximately 20 to 30 cases of deciduoid mesothelioma are reported each year.
Another variation is lymphohistiocytoid mesothelioma. This rare variant is comprised of malignant inflammatory immune cells, including plasma cells, histiocytes, and lymphocytes. In addition, epithelial and sarcomatoid cells are present in the tumor. Less than 1 percent of pleural mesothelioma diagnoses are classified as lymphohistiocytoid.
Desmoplastic mesothelioma most often occurs in cases of peritoneal mesothelioma. This type of mesothelioma accounts for about 5 percent of all cases related to asbestos exposure. Desmoplastic mesothelioma occurs when the malignant tumor triggers the fibrous growth of connective tissue, like collagen. Since this is a sarcomatoid type of cancer, prognosis is almost always poor.
When at least half of the cells in desmoplastic mesothelioma are of the small cell type, it is considered to be small cell mesothelioma. Small cells are exactly what they sound like: smaller cells as compared to other types of cancer cells. Under the microscope, small cells look round, bland, and uniform.
Implications for Treatment
Understanding mesothelioma cell types is important for proper treatment. Different cells and tumors require different treatment approaches. Understanding the basic cell type and any rare variations can help a medical team determine the best treatment for an individual patient. If you have been diagnosed with mesothelioma, be sure that you receive all possible tests. This will result in the most complete diagnosis possible as well as the most comprehensive treatment strategy.
Page Medically Reviewed and Edited by Pinar Kanlikilicer, PhD
Dr. Pinar Kanlikilicer has a PhD in Biomedical Engineering. She completed her 5-years of postdoctoral training in the Department of Experimental Therapeutics at MD Anderson Cancer Center. She is currently working in the field of cancer as a research scientist.