Small Cell Mesothelioma
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Small cell mesothelioma is an extremely rare subtype of mesothelioma commonly misdiagnosed as small cell lung cancer. The treatment strategy is different for each, so early recognition of the cytologic features is very important for treatment choices and the patient’s outcome.[1]
About Small Cell Mesothelioma
Small cell mesothelioma includes 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.
Because it is so rare, there are few studies that describe what small cell mesothelioma cancer cells look like. A single case study of a woman with the diagnosis described several features of a small cell mesothelioma biopsy:[1]
- Large nuclei in the cells
- Minimal cytoplasm
- Powdery chromatin
- Infrequent mitosis
- No nuclear molding
Symptoms of Small Cell Mesothelioma
Pleural small cell mesothelioma causes chest pains, coughing, and shortness of breath. The most common symptoms of peritoneal small cell mesothelioma are abdominal pains, swelling and fluid in the abdomen, and gastrointestinal symptoms.
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, and both are 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. The study found that small cell mesothelioma tumors lacked certain growth patterns seen in small cell lung cancer: rosettes, ribbons, and streams.[2]
Diagnosing Small Cell Mesothelioma
Diagnosing mesothelioma is already difficult, but it can be even more challenging when tumor cells include characteristics of small cell cancer.
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.
They also determine if the cells 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% of the cells in biopsy samples appeared to be small cells.
The proportion of small cells to other types can vary, but diagnosis is likely to be small cell mesothelioma when the majority have small cell characteristics.
Immunohistochemistry of 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.
The study that looked for unique distinguishing factors in small cell mesothelioma patients found three biomarkers not present in any of the 13 tumors:[2]
- Chromogranin A
- Carcino-embryonic antigen (CEA), or
- Leukocyte common antigen (LCA)
These kinds of patterns in immunohistochemical stains can help pathologists differentiate between cancers that otherwise look very similar.
Treatment for Small Cell Mesothelioma
An accurate diagnosis is crucial to determining 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 cancer. The later the stage, the more difficult it is to treat and the less likely that surgery will be a viable option.
Because small cell mesothelioma has a long latency period and is challenging to diagnose, many cases are only diagnosed at a later stage, limiting treatment options to chemotherapy and radiation. However, these are unlikely to cure the cancer.
Treatment teams use these strategies 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.
Prognosis
What the experts know, however, is that mesothelioma does not come with a good prognosis in most cases. In one study of eight patients with small cell mesothelioma, the average survival time was just over eight months.[3] With so few cases to study, it is impossible to give an accurate prognosis.
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 primarily for 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.
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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.