Mesothelioma Cell Types
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There are three major mesothelioma cell types, epithelioid, sarcomatoid, and biphasic, as well as several rare subtypes.[1] Examination of the tumor cells, known as histology, can help doctors determine the type of cancer and what treatments will best help the mesothelioma patient.
About Mesothelioma Cell Types
The four types of mesothelioma are pleural, peritoneal, pericardial, and testicular. These describe the location of the original tumors, but also important is the type of cell in the tumors.
When doctors diagnose mesothelioma, they use histology, the study of cells, to describe the main cell types and any rare subtypes. Mesothelioma is usually diagnosed as one of three main types:
- Epithelioid
- Sarcomatoid
- Biphasic
Biphasic mesothelioma is cancer with a mixture of epitheloid and sarcomatoid cells. Rare subtypes of these main types also exist.
Doctors describe mesothelioma cell types because it helps them determine a prognosis and the best course of treatment. Each cell type responds differently to treatments and progress and change in different ways.
The Study of Cells – Mesothelioma Histology
Histology is the study of cells. Mesothelioma pathology includes the use of histology to aid the diagnosis of cancers. For cancers like mesothelioma, examining the cells of a suspected tumor can tell a doctor if the patient has cancer and if it is malignant.
In some cases, these cells can also help determine where 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 doctor takes a sample of fluid or tissue from an area of the body thought to have a tumor. 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.[2]
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.
A pathologist can target one particular cancer cell type using immunohistochemistry, either confirming cancer or ruling it out.[1] In addition, a 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
The first way that pathologists describe biopsied cells is as malignant or benign
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 that are sometimes benign include:
- Papillary. A rare, benign type of mesothelioma is called well-differentiated papillary mesothelioma. It occurs most often in women with peritoneal tumors and can become malignant over time. The cells are described as papillary because they grow in finger-like shapes known as papillae.[3]
- Cystic. Cystic describes tumors that are made up of smooth cysts and flat or cube-shaped cells. Benign multicystic peritoneal mesothelioma most often occurs in women.[4]
- Adenomatoid. Very rarely, mesothelioma can develop as benign adenomatoid cells. It may occur in the mesothelium around reproductive organs or in the abdomen. Peritoneal adenomatoid mesothelioma most often occurs in men.[5]
- Fibrous. Local fibrous tumors in the mesothelium can occur anywhere but are more common in the pleura. They may or may not cause symptoms and rarely become malignant.[5]
Epithelial Cells
When a pathologist studies cells from a person with mesothelioma, they look 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% of malignant mesothelioma tumors.[1] These cells have specific physical characteristics, like large nuclei and distinctive branching patterns, which 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 spread and metastasize less readily.
Sarcomatoid Cells
Compared to epithelial cells, sarcomatoid cells have a distinctive, elongated spindle shape that is easy to identify under a microscope. About 10 to 20% of mesotheliomas are diagnosed as sarcomatoid.[1]
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 is worse than for epithelioid.
Biphasic Mesothelioma
If a mesothelioma tumor includes both sarcomatoid and epithelial cells, it is called biphasic.[1] Doctors will diagnose cancer as biphasic if at least 10% of its cells are one type, with the remaining cells falling into the other category.
In other words, a tumor comprised of 90% sarcomatoid cells and 10% epithelial would be considered biphasic cancer. About 25 to 30% of mesotheliomas are biphasic.
Cell Subtypes
Diagnoses may also include cell subtypes, or variants. Most variants are rare and present unique treatment challenges.
- Deciduoid mesothelioma is a variant with epithelial cells. Originally, this unusual variant cell was mostly found in cases of peritoneal mesothelioma in young women. Approximately twenty to thirty cases of deciduoid mesothelioma are reported each year.[6]
- Another variation is lymphohistiocytoid mesothelioma. This rare variant contains malignant inflammatory immune cells, including plasma cells, histiocytes, and lymphocytes. In addition, epithelial and sarcomatoid cells are present in the tumor. Less than 1% of pleural mesothelioma diagnoses are classified as lymphohistiocytoid.[7]
- Desmoplastic mesothelioma most often occurs in cases of peritoneal mesothelioma. This type of mesothelioma accounts for about 5% 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, the prognosis is almost always poor.[8]
- When at least half of the cells in desmoplastic mesothelioma are of the small cell type, oncologists call it small cell desmoplastic mesothelioma. Small cells are what they sound like. They are smaller cells as compared to other types of cancer cells. Under the microscope, small cells look round, bland, and uniform.
- Heterologous mesothelioma occurs when the tumors include cells from other parts of the body, like bone, muscle, and cartilage. In the few reported cases of this type, sarcomatoid cells predominated and the prognosis was poor.[9]
How Does Cell Type Affect 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. For instance, sarcomatoid cells spread much more rapidly, requiring the most aggressive treatments.
If you have been diagnosed with mesothelioma, be sure that you receive all possible tests. This will result in a complete diagnosis as well as the most comprehensive treatment strategy.
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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.