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Small cell mesothelioma is an extremely rare subtype of mesothelioma. The name describes the appearance of the cells under a microscope. Cases of this type of mesothelioma are often categorized as epithelial. However, some small cell tumors include sarcomatoid and other types of cells. Mesothelioma cases diagnosed as a small cell have tumors that are comprised of about half small cells.
Small cell mesothelioma is commonly misdiagnosed as small cell lung cancer because there are some significant similarities. However, studies have found significant differences between the two. Since the treatment strategy for SCM is different than SCL, early recognition of the cytologic features is very important for the proper clinical management of the disease. As with most types of mesothelioma, the prognosis for small cell mesothelioma is not usually good. Treatments like surgery, chemotherapy, and radiation can be used to extend or improve a patient’s life, but a cure is not usually a possibility.
Characteristics of Small Cell Mesothelioma
Small cell mesothelioma is typically comprised of a mix of cell types, with approximately half described as small cells. These cells are small, uniform, round, and contain a single large nucleus. Cases of small cell mesothelioma tumors have been described in the pleura or peritoneum. Most of these patients had a history of asbestos exposure. Studies of small cell mesothelioma have found the additional cells in the tumors are usually mostly epithelial but may also be a biphasic mix of epithelial and sarcomatoid.
Small Cell Mesothelioma vs. Small Cell Lung Cancer
Mesothelioma and lung cancer are often difficult to distinguish when making a diagnosis. Lung cancer and pleural mesothelioma occur in the same region of the body, and their cell structures often look very similar. Both types of cancer can form in a small cell variety, both less common than other types.
One study investigated 13 cases of small cell mesothelioma to determine what features distinguished them from small cell lung cancer. These features can be used to make a more accurate diagnosis. The study found that small cell mesothelioma tumors lacked certain growth patterns seen in small cell lung cancer. These characteristics include rosettes, ribbons, and streams. The study also found unique characteristics in cell nuclei. Using immunohistochemistry, the study found three biomarkers (Chromogranin A, carcino-embryonic antigen (CEA) or leucocyte common antigen (LCA).) not present in any of the 13 tumors. This fact could also be used to diagnose small cell mesothelioma.
Diagnosis and Immunohistochemistry
Diagnosing mesothelioma is already difficult, but when tumor cells include characteristics of small cell cancer, it can be even more challenging. Diagnosis typically begins with a physical examination and imaging scans. For respiratory and chest symptoms, an X-ray can rule out things like pneumonia. More detailed scans of the abdomen or chest can show tumors or areas of abnormal tissue that may be cancerous.
After imaging scans, a doctor may order a biopsy. A biopsy uses tissue or fluid samples to aid diagnosis. Once samples are acquired, a pathologist examines them under a microscope to determine if they are malignant, originated in the mesothelium, are sarcomatoid or epithelial cells—or a mixture of both—and finally, if there are signs of cellular subtypes, like small cells.
To identify a small cell tumor, pathologists look for round, uniform, small cells. In a study of eight cases of small cell mesothelioma, between 80 and 100 percent of the cells in biopsy samples appeared to be small cells. The proportion of small cells to other types can vary, but when the majority have small cell characteristics, diagnosis is likely to be small cell mesothelioma.
Examining the physical appearance and growth patterns of the tumor cells is not always enough to provide an accurate diagnosis. The few studies that have examined small cell tumors discovered that a more accurate diagnosis includes both pathological study of cells and immunohistochemical staining. Immunohistamine staining uses immune system proteins as markers. There are certain proteins present in most cases of small cell mesothelioma. Other proteins are never present. These details help make a more accurate diagnosis.
An accurate diagnosis is crucial to determine the proper course of treatment for any type of cancer. For small cell mesothelioma, the treatment plan varies by individual and depends on the stage of the cancer. The later the stage, the more difficult it is to treat and the less likely it is that surgery will be a viable option.
Because small cell mesothelioma has a long latency period and is difficult to diagnose, many cases are only diagnosed at a later stage, limiting treatment options to chemotherapy and radiation, although these are unlikely to cure the cancer.
These treatment strategies can be used to reduce tumor size with the hope of extending a patient’s life. These treatments can also reduce symptoms and make the patient feel more comfortable.
In one study of eight patients with small cell mesothelioma, the average survival time was just over eight months. With so few cases to study, it is impossible to give an accurate prognosis. What the experts know, however, is that mesothelioma does not come with a good prognosis in most cases.
From what is known about small cell mesothelioma, it does seem that it is very aggressive and that life expectancy is relatively short. Treatments are mostly used as palliative care. If you have been diagnosed with any type of mesothelioma, make sure you know your rights with respect to asbestos exposure and the law.
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.