Mesothelioma Cell Types
In addition to the four main types of mesothelioma, which are classified by the part of the body the primary tumor effects, there are also classifications by cell type. Mesothelioma is cancer of the mesothelium, the tissue lining organs in the body. This includes cancer of the pleura around the lungs, of the pericardium around the heart, of the peritoneum around the abdominal organs, and of the tunica vaginalis around the testicles.
Additionally these mesotheliomas may be categorized as being benign or malignant, epithelial, sarcomatoid, or biphasic, or as one of several other less common types of cell variants that make up the tumors. Examination of the tumor cells, known as histology, is an important part of diagnosis that can help doctors determine the type of cancer and what treatments will best help the patient.
Histology is the study of cells and is used in medicine to aid in diagnoses. For cancers, like mesothelioma, examining the cells of a tumor or a suspected tumor can tell a doctor if the patient has cancer, if it is malignant, and in some cases, where the cancer originated. Mesothelioma cells are notoriously difficult to diagnose. They don’t always clearly look different from other cancer cells. For example, pleural mesothelioma cells look a lot like lung cancer cells.
To study the cells and determine cell type for mesothelioma, a patient must first be biopsied. This involves extracting a sample of fluid or tissue from an area of the body thought to have a tumor or from a known tumor. The sample is then prepared on a microscopic slide and stained so that the individual cells can be seen. A pathologist then examines the cells and attempts to identify them to aid in diagnosis and treatment plans.
A subtype of histology, called immunohistochemistry, uses antibodies to stain tissue and fluid samples. These antibodies attach to specific antigens, or proteins on certain types of cancer cells. With an immunohistochemistry approach, a pathologist can target one particular cancer cell type and either confirm or rule out its presence in a biopsy sample.
Benign and Malignant Cells
Mesothelioma cells are almost always malignant. This means they are capable of growing out of control and spreading to nearby tissues and other organs. Malignant cells, left untreated, are always fatal. Benign tumor cells, on the other hand, do not spread. They may grow into a tumor that becomes uncomfortable or causes symptoms, but it will not spread to other tissues. Benign mesotheliomas are rare, but possible. Some of the cell types that are considered benign mesotheliomas include papillary and adenomatoid.
When a pathologist studies the cells from a person with mesothelioma, one thing he or she looks for is the type of cell in the tumor: epithelial, sarcomatoid, or both. Epithelial cells are most common and are found in 50 to 70 percent of malignant mesothelioma tumors. They have certain physical characteristics that can be seen under a microscope, including large nuclei and branching patterns in how they grow.
Epithelial cells respond best to treatment, so getting a diagnosis of epithelial mesothelioma is better than the alternative. They are easier to treat than other cell types because they adhere to each other. This makes them less likely to spread to other tissues.
Sarcomatoid cells look different from epithelial cells. They have a distinctive spindle shape, are elongated, and are fairly easy to identify under a microscope. About ten to 20 percent of mesotheliomas are diagnosed as sarcomatoid. This type of cell is more likely to spread and more likely to do so aggressively. Sarcomatoid mesothelioma is more difficult to treat than epithelial. They resist treatment and tend to form multiple nodules rather than one or more large tumors.
If the cell types in a mesothelioma tumor include both sarcomatoid and epithelial cells, it is considered to be biphasic mesothelioma. It is only diagnosed as biphasic if there if ten percent or more of the cells are one type and the rest are of the other type. In other words a tumor made up of 92 percent sarcomatoid cells and eight percent epithelial would be considered sarcomatoid cancer. About 25 to 30 percent of mesotheliomas are diagnosed as biphasic.
Rare Cell Variations
In addition to classifying mesothelioma as one or both of the two major cell types, diagnoses may also include cell variants. Most of these are rare, but possible and may present unique challenge for treatment. One of these is deciduoid mesothelioma, a variant of epithelial cells that is reported in about 20 to 30 cases every year. Originally this unusual variant cell was mostly found in the cases of peritoneal mesothelioma in young women.
Another variation is lymphohistiocytoid mesothelioma. This is mesothelioma with malignant inflammatory immune cells, including plasma cells, histiocytes, and lymphocytes, along with epithelial and sarcomatoid cells. Less than one percent of pleural mesothelioma diagnoses are identified as this type of variant.
Desmoplastic mesothelioma most often occurs in cases of peritoneal mesothelioma and accounts for about five percent of all cases related to asbestos exposure. This type of mesothelioma occurs when the malignant tumor triggers the fibrous growth of connective tissue, like collagen. This is a sarcomatoid type of cancer, which means that prognosis is almost always poor for this diagnosis.
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 the cell type after a diagnosis of mesothelioma is important for treatment. Different treatment approaches are best used for different types of cells and tumors. Understanding the basic cell type and any rare variations can help a medical team come up with the best possible treatment for a patient. If you have been diagnosed with mesothelioma be sure that you are given all the tests possible for the most complete diagnosis and the most comprehensive treatment strategy.
Page edited by Dave Foster
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