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Mesothelioma, the cancer most often associated with asbestos exposure, can be categorized by where it originates. Categories include pleural, peritoneal, pericardial, and testicular. Mesothelioma can also be categorized by the types of cells that make up most of the tumors. Cell categories include epithelial, sarcomatoid, or biphasic, which is a mixture of the two types. Epithelial mesothelioma is the most common mesothelioma cell type, comprising as much as 70 percent of cases.
Epithelial mesothelioma is less aggressive than sarcomatoid mesothelioma. Although epithelial mesothelioma is malignant and will spread and metastasize, it does so more slowly than other types. This slower spread rate makes treatment more hopeful. In some cases treatment can actually be curative. However, this is still a relatively aggressive cancer that usually caries a reduced life expectancy.
Epithelial cells are found throughout the body. They make up the tissue lining various parts of the body, including the skin, throat, blood vessels, and organs. These cells act as the body’s safety barrier. Epithelial cells are usually packed tightly together. Depending on where they are located in the body, epithelial cells come in a variety of shapes. These cells may be flat, cube shaped, or columnar.
When epithelial cells mutate, they may become cancerous. WHen this happens, they tend to lose ordered shape. A mesothelioma tumor largely comprised of mutated epithelial cells is classified as epithelial or epithelioid mesothelioma. Pleural, peritoneal, pericardial, and testicular mesothelioma may all be epithelial. There are also several sub-types of epithelial mesothelioma cells. These include:
- Small cell- Small cell cancers are made up of small, uniform cells. These can be easily mistaken for the cells that make up small cell lung cancer.
- Adenomatoid- These cells are both epithelial and characteristic of cells that arise in the body’s glands.
- Tubulopapillary- This cell sub-type makes a distinct pattern of papillary structures and small tubules.
- Deciduoid- This is a rare epithelial sub-type shaped like large polygons. It is easy to misdiagnose because it is so rare.
- Solid- Solid cell types may be poorly or well-differentiated. Well-differentiated solid cells are round and orderly. In the poorly differentiated type, cells are not well organized.
- Cystic- Cystic mesotheliomas are most common in the peritoneum and are often benign.
- Glomeruloid- These cells are round or oval-shaped and are not common in mesothelioma. They are not thought to be related to asbestos exposure.
Diagnosis of Epithelial Mesothelioma
A diagnosis of epithelial mesothelioma may come after mesothelioma itself is diagnosed. A physical exam and imaging scans are used to identify an unusual growth in the pleura or lungs. An imaging scan, like a CT scan, cannot tell a doctor if the cancer is lung cancer or mesothelioma nor whether it is benign or malignant.
More specific diagnosis requires a biopsy. In a biopsy, a needle is used to harvesttissue or fluid samples. If the suspected tumor is difficult to access, a surgical procedure may be necessary to obtain an adequate sample. Next, a pathologist examines physical characteristics of the cells under a microscope as well as how the cells are ordered. This allows the growth to be identified as mesothelial or lung cancer cells. The biospy can also identify if the cells are are epithelial or sarcomatoid. If epithelial cells are found, the pathologist will then determine which sub-type they are. The distinction is important for treatment.
There is also an innovative new technique that makes diagnosis easier. The technique is called immunohistochemistry and involves antibodies. The antibodies are used to detect the antigens specific to each type of cancer cell. Several of these antibodies can be used to help diagnose epithelial mesothelioma.
Depending on the stage of the cancer, as well as the extent of spreading, surgery may be a good treatment option. Epithelial mesothelioma is generally considered a resectable type of mesothelioma. This means surgery can potentially remove the entire visible tumor. On the other hand, sarcomatoid mesothelioma is typically nonresectable.
For pleural epithelial mesothelioma, an extrapleural pneumonectomy, or EPP, is a possible treatment. EPPis a surgical procedure that removes the lung along with the pleura where the cancer originated. This is typically only done when the cancer has not yet spread to the lymph nodes and there is hope for a cure. Patients must be in generally good general health for this complicated and risky surgery.
A less aggressive surgical strategy is called debulking. Debulking is a procedure where a surgeon removes as much of the tumor as possible. Surgery may also be done to remove the pleura on one side of the chest or part of a lung. Such surgeries are usually combined with chemotherapy and radiation for best results. These treatments may be used before surgery to reduce tumor size or after surgery to eliminate remaining cancer cells. Chemotherapy is very common for the treatment of pleural mesothelioma. First-line chemotherapy regimens include platinum compounds together with another active agent such as gemcitabine, pemetrexed, or vinorelbine.
Other treatment options for the treatment of mesothelioma are targeted therapy and/or immunotherapy. Targeted therapy is a cancer treatment that uses drugs which targets specific oncogenic genes or proteins in the tumor. Dasatinib, for example, a multitargeted Src kinase/PDGFR inhibitor, was shown to have an antitumor activity against malignant epithelial mesothelioma patients in phase II clinical trials. Immunotherapy is used to treat cancer patients using body’s self defense mechanism; immune system. Cancer immunotherapies mostly consist of antibodies that bind and inhibit functions of proteins, called checkpoints. Most common targets for immunotherapy are PD-1 or PDL1. PDL1, for example, is a protein that helps keep immune cells from attacking normal cells in the body. However, cancer cells express this protein in high amounts to “trick” the immune system and to be safe. There are multiple phase II clinical trials investigating the efficiency of immunotherapy drug–Nivolumab alone or in combination with another drug called ipilimumab for malignant epithelial mesothelioma patients. Results are promising especially for relapsing patients.
One factor connected to improved mesothelioma survival rates is epithelial cell type. Because these cells tend to adhere to one another, they metastasize less readily than sarcomatoid cells. Epithelial cells will eventually metastasize if part of a malignant tumor. However, they will spread more slowly, giving a patient an overall better prognosis.
Other factors affecting prognosis include age, gender, general health, nutrition, body weight, and ability to conduct normal everyday activities. Also, red blood cell, platelet, and white blood cell counts contribute to prognosis. The stage of the cancer at the time of prognosis will also affect outcome. The earlier the stage, the better the prognosis will be. Therefore, getting an early diagnosis is essential. If you have been exposed to asbestos during your career, you should receive regular mesothelioma screenings.
You should also be aware of symptoms. Be prepared to see your doctor if you develop signs of this devastating cancer such as pain in chest or ribs, fatigue, loss of appetite, or night sweats. If you are diagnosed with mesothelioma, there are legal avenues to seek compensation. This includes lawsuits, settlements, and asbestos trust funds. A qualified lawyer can help you navigate your resources and help you make a case.
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.