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When categorized by main cell type, there are three kinds of mesothelioma, epithelial, sarcomatoid, and biphasic. Biphasic mesothelioma contains both epithelial and sarcomatoid cells. Relatively rare, only between 10 and 20 percent of mesothelioma cases are sarcomatoid. Sarcomatoid mesothelioma is the least common, but most aggressive form of mesotheliomas. Because the kinds of cells that make up this type of mesothelioma are more prone to metastasizing, sarcomatoid mesothelioma is typically more aggressive and deadly.
Each type of mesothelioma is aggressive and difficult to treat. Therefore, sarcomatoid mesothelioma often has the lowest survival rates. Median survival rates for sarcomatoid malignant pleural mesothelioma range from 3.5 to 8 months. Sarcomatoid mesothelioma is also very difficult to treat and comes with a poor prognosis. Surgery is rarely an option with this type of mesothelioma.
A meta analysis study which investigated the clinicopathologic correlation of 13 cases showed that the median age at diagnosis was 66 years for sarcomatoid mesothelioma patients. They found that median survival of patients from date of diagnosis to date of death was 5 months. Abdominal pain was found to be the most common presenting symptom in patients. Patient tumor samples were positive for cytokeratins. 69% of patients were previously exposed to asbestos.
Any type of cancer that arises from cells of the mesenchyme tissue type is called a sarcoma. The cells of sarcomatoid cancer are part of tissues that make up the circulatory and lymphatic systems, as well as connective tissue, like cartilage and bone. Epithelial cancers are comprised of different type of cells. These cancers are called carcinomas. Sarcomas are relatively rare and sarcomatoid mesothelioma accounts for only about 10 to 20 percent of all diagnoses of pleural mesothelioma and four percent of diagnoses of peritoneal mesothelioma.
Sarcomatoid cells of mesothelioma tumors have certain characteristics that distinguish them from epithelial cells. These cells are shaped like tiny spindles with spindle shaped with elongated and enlarged nuclei. Some cells may even have more than one nucleus. These cells are less firmly attached to each other than epithelial cells and are more likely to spread to other areas of the body.
There are also several subtypes of sarcomatoid cells that may be seen in this type of mesothelioma. Desmoplastic cells are bland in appearance without many distinguishing characteristics. This subtype is very difficult to diagnose. Transitional cells are spindle shaped but plumper than ordinary sarcomatoid cells. Lymphohistiocytoid cells are large and very rare, accounting for just one percent of all mesothelioma diagnoses.
Diagnosing Sarcomatoid Mesothelioma
To diagnose any type of mesothelioma, the first step is a basic physical examination. The physical exam is typically followed by some type of imaging scan. If pleural mesothelioma is suspected, the first scan is typically a chest X-ray to rule out more common conditions like pneumonia. More detailed scans like CT scans or MRIs can then be used to get a better picture of tumors in the body.
If imaging scans show abnormal tissues that could be potential tumors, the next diagnostic step is a biopsy. During a biopsy, a thin needle is inserted into the tumor to obtain a fluid or tissue sample. If the tumor is difficult to reach, the sample may be obtained through surgery, although this is less common.
After the sample is obtained, a pathologist examines the cells to determine if the tissue is cancer and if it is mesothelioma. At this point, the pathologist will also determine if the cancer is epithelial, sarcomatoid, or biphasic. The appearance of the cells under a microscope may be enough to make this diagnosis, but mistakes are not uncommon. Mesothelioma is notoriously difficult to diagnose because cells often have an appearance similar to other types of cancer like lung cancer.
Sarcomatoid mesothelioma is a cancer of fibrous tissue. This makes it easy to misdiagnose as a number fibrous conditions, both cancerous and noncancerous. These conditions include localized fibrous tumors, pleural liposarcoma, renal cell carcinoma, fibrous histiocytoma, and fibrous pleurisy.
Subtypes of Sarcomatoid Mesothelioma
An important part of diagnosis is determining the types of cells involved in the cancer. However, this often goes beyond looking at epithelial, sarcomatoid, and biphasic types. There are also many subtypes. Careful examination by a qualified pathologist can reveal details of the cancer cells, including their shape, growth structure, and patterns. While sarcomatoid mesothelioma is already rare, there are subtypes that can make an individual’s cancer unique. These cancer subtypes will determine the proper course of treatment. Subtypes are:
- Desmoplastic Mesothelioma. Desmoplasmia is an often benign growth of fibrous tissue. When making a diagnosis, this growth can be mistaken for a benign condition when in reality, it is characteristic of mesothelioma tumor growth into fibrous tissues. When pathologists examine tissue samples from this kind of mesothelioma, they may find collagen fibers in the tumor or collagen death.
- Lymphohistiocytoid Mesothelioma. Lymphohistiocytoid mesothelioma occurs when there are certain types of immune cells within the tumor. These include lymphocytes and histiocytes. This subtype is frequently misdiagnosed because it is so rare.
- Heterologous Mesothelioma. Also very rare, heterologous mesothelioma occurs when there are other cell types present in addition to the original mesothelial cells. For example, tumors may contain bone cells in addition to mesothelial cells. In some cases, cancerous tissue may contain multiple types of cells, while in others there may be just one non-mesothelial cell type.
- Rhabdomyoblastic Differentiation. An extremely rare subtype of sacrcomatoid mesothelioma, rhabdomyoblastic differentiation contains a specific type of muscle cell, known as a rhabdomyoblast. Rhabdomyoblasts are early muscle cells that develop into full skeletal muscle tissue. Cancer in these cells is much more common in infants and children. Seeing them in mesothelioma is very rare.
Immunohistochemistry is a more precise strategy for identifying cell types and diagnosing mesothelioma subtypes. This technique uses diagnostic markers that are specific to cell type to stain tissue. The markers are antigens and antibodies, proteins used by the immune system to identify and target pathogens.
Different cells contain antigens as specific markers for that type of cell. The stains contain different types of antibodies that target and attach to a specific antigen. To determine if the cells in a biopsy sample are sarcomatoid mesothelioma cells, a pathologist will use a specific stain with an antibody that serves as a marker for these cells. Although this method is not completely foolproof, it is a much more accurate form of diagnosis.
Unlike epithelial and biphasic mesothelioma, most specialists do not classify sarcomatoid mesothelioma as resectable, meaning it cannot be treated surgically. The cancer spreads too quickly for surgery to be able to remove tumors in their entirety. Sarcomatoid mesothelioma tumors tend to grow into the chest wall, making removal difficult and risky. With peritoneal sarcomatoid mesothelioma, the tumors may grow into other organs, which also presents risks.
Without surgery as a treatment option, specialists must rely on other strategies. Chemotherapy is the most commonly used treatment for sarcomatoid mesothelioma, although it is not often successful. This type of mesothelioma is simply too aggressive for easy treatment. Unfortunately. a systematic review of 30 sarcomatoid malignant pleural mesothelioma trials showed that there are fewer responses in patients with sarcomatoid histology than in the other mesothelioma subtypes.
Common diagnostic errors further complicate the situation, delaying proper diagnosis until this kind of cancer is in the later stages and has spread to other parts of the body.
Much of the treatment used for sarcomatoid mesothelioma is used to extend the lifespan and make the patient comfortable. Chemotherapy and radiation can shrink tumors, which may help a patient live longer. Shrinking tumors can also help reduce symptoms like pain, difficulty breathing, swelling, and abdominal pain. Other treatments can also be used to help patients feel better.
The prognosis for sarcomatoid mesothelioma is generally poor. Most patients are given a life expectancy of six months or less. Fewer than 10 percent of patients live beyond a year after diagnosis. Prognosis varies depending on individual factors such as the stage of the cancer, the age of the patient, lifestyle habits, and overall health.
If you have symptoms of mesothelioma, or have been exposed to asbestos, early and accurate diagnosis is crucial for receiving the best possible prognosis. The earlier you receive a diagnosis and start treatment, the better your life expectancy and comfort level will be.
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.