When categorized by main cell type, there are three kinds of mesothelioma: epithelial, sarcomatoid, and biphasic. The latter is a type of mesothelioma that contains both epithelial and sarcomatoid cells. Only between 10 and 20 percent of mesothelioma cases are sarcomatoid. The kinds of cells that make up this type of mesothelioma are more prone to metastasizing than epithelial cells, which means that this is a more aggressive and deadly type of cancer.
All kinds of mesotheliomas are difficult to treat and aggressive, but sarcomatoid mesothelioma often is associated with the lowest survival rates. It is also very difficult to treat, is almost never curable, and it comes with a poor prognosis. Surgery is often not an option for treatment with this type of mesothelioma.
Any type of cancer that arises from cells of the mesenchyme tissue type is called a sarcoma. The cells of a sarcomatoid type of cancer are part of tissues that make up the circulatory system and the lymphatic system, as well as connective tissue, like cartilage and bone. Epithelial cancers are different type of cells and these cancers are called carcinomas. Sarcomas are relatively rare and sarcomatoid mesothelioma accounts for just about 10 to 20 percent of all diagnoses of pleural mesothelioma and just four percent of diagnoses of peritoneal mesothelioma.
Sarcomatoid cells of mesothelioma tumors have certain characteristics that distinguish them from epithelial cells. They are spindle shaped; they have elongated and enlarged nuclei and may even have more than one nucleus; they are less firmly attached to each other than epithelial cells and so they are more likely to spread to other areas of the body more quickly.
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. This is then usually followed by various types of imaging scans. For suspected pleural mesothelioma, the first scan is typically a chest X-ray, which can 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 what could be tumors in the body.
If imaging scans show abnormal tissues that could be tumors, the next diagnostic step is to take a biopsy. This usually involves a thin needle inserted into the tumor to withdraw a sample of fluid or tissue. In some cases, if the tumor is hard to reach, a more invasive strategy may be needed to get a biopsy. Surgical biopsies are not common, but sometimes must be used to get a sample.
Once there is a sample of cells a pathologist can examine those cells to determine if the tissue is cancer and if it is mesothelioma. This is also the stage at which the pathologist will try to 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 it is easy to make mistakes. Mesothelioma of all types is notoriously difficult to diagnose because the cells may appear like the cells of other types of cancer, like lung cancer.
Sarcomatoid mesothelioma is a cancer of fibrous tissue, which means it can also be misdiagnosed as a number of non-cancerous and cancerous conditions that are fibrous in nature: localized fibrous tumors, pleural liposarcoma, renal cell carcinoma, fibrous histiocytoma, and fibrous pleurisy.
Subtypes of Sarcomatoid Mesothelioma
An important part of the diagnosis is determining the types of cells involved in the cancer, but this often goes beyond just looking at epithelial, sarcomatoid, and biphasic types. There are also many subtypes, and it requires very careful inspection by a pathologist to determine the details of the cancer cells, their shapes, growth structures, and patterns. Sarcomatoid mesothelioma is already rare, and there are subtypes that can make an individual’s cancer unique and that help determine the course of treatment:
- Desmoplastic Mesothelioma. Desmoplasmia is growth of fibrous tissue, and it can often be benign. When making a diagnosis it is easy to mistake this kind of growth for a benign condition when it is really characteristic of mesothelioma tumor growth into fibrous tissues. When pathologists investigate tissue samples from this kind of mesothelioma they may find collagen fibers in the tumor or collagen death.
- Lymphohistiocytoid Mesothelioma. An even rarer subtype of sarcomatoid mesothelioma, lymphohistiocytoid mesothelioma occurs when there are certain types of immune cells within the tumor. These include lymphocytes and histiocytes. This subtype is often misdiagnosed because it is so rare.
- Heterologous Mesothelioma. Also very rare, heterologous mesothelioma occurs when there are other cell types in the tumors in addition to the original mesothelial cells. There may be bone cells in the tumors, for instance. It is possible, although even rarer for this type to occur in epithelial mesothelioma. In some cases the cancerous tissue may contain multiple types of cells, while in others there may be just one non-mesothelial cell type.
- Rhabdomyoblastic Differentiation. Extremely rare is sacrcomatoid mesothelioma in which there is a specific type of muscle cell, known as a rhabdomyoblast, among the cancerous cells. Rhabdomyoblasts are early muscle cells that develop into full skeletal muscle tissue. Cancer in these cells is much more common in infants and children for this reason, and seeing them in mesothelioma is very rare, but it has been reported.
A more precise strategy for identifying cell types and diagnosing mesothelioma and its subtypes is called immunohistochemistry. This is a technique that involves staining tissue using diagnostic markers that are specific to a type of cell. The markers used in this diagnostic technique are antigens and antibodies, proteins used by the immune system to identify and target pathogens.
Different cell types contain antigens as specific markers for that type. The stains that pathologists use 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 the right antibody that serves as a marker for these cells. It is a much more accurate way of making a diagnosis, although it is not foolproof.
Unlike epithelial and biphasic mesothelioma, most specialists do not classify sarcomatoid mesothelioma as resectable. In other words it cannot be treated surgically. The cancer spreads too quickly to be able to surgically remove tumors in their entirety. Sarcomatoid mesothelioma tumors tend to grow into the chest wall, so removing them is 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. Even so, it is not often successful. This type of mesothelioma is simply too aggressive to treat easily. Adding to that difficulty is the fact that, due to diagnostic errors, most people with this kind of cancer do not get a diagnosis until it is in the later stages and has spread to other parts of the body.
Much of the treatment used for patients with sarcomatoid mesothelioma is used to extend the lifespan and to make the patient more comfortable. Chemotherapy, and sometimes radiation, can be used to shrink tumors, which may help a patient live longer. Shrinking the tumors can also help reduce symptoms like pain, difficulty breathing, and swelling and pain in the abdomen for peritoneal mesothelioma. Other treatments like draining fluid that builds up in the abdomen or chest cavity, can also be used to help patients feel better.
The prognosis for sarcomatoid mesothelioma is generally not good. Most patients are given a life expectancy of six months or less and fewer than 10 percent of people live more than one year after a diagnosis of sarcomatoid mesothelioma. The prognosis will vary depending on individual factors such as the stage of the cancer at the time of diagnosis, the age of the patient, the patient’s lifestyle habits, and the overall health of the patient.
If you have symptoms of mesothelioma, or you know you have been exposed to asbestos, getting a diagnosis as early as possible is crucial for receiving the best possible prognosis. The earlier you get a diagnosis and start treatment, the better your chances will be of surviving longer and being more comfortable.
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