Accounting for 7 to 20 percent of all mesothelioma cases, sarcomatoid mesothelioma is the least common of the disease’s three main histological cell types: epithelioid, sarcomatoid and biphasic. This type of tumor has also been called sarcomatous, diffuse malignant fibrous and spindled mesothelioma.
Most sarcomatoid tumors develop on the protective lining of the lungs, or pleura. 10 to 20 percent of all pleural mesotheliomas are sarcomatoid, and the cell type makes up fewer than 4 percent of all peritoneal mesothelioma.
Under a microscope, sarcomatoid mesothelioma cells appear elongated, spindle-shaped and often form a fibrous pattern that resembles a tumor called histiocytoma. Some epithelioid cells may be present within sarcomatoid tumors, but by definition they must make up less than 10 percent of the tumor’s mass. Variants of this form of cancer include transitional, lymphohistiocytoid and desmoplastic mesothelioma. Like most other types of mesothelioma, the sarcomatoid cell type is linked to asbestos exposure.
Diagnosing Sarcomatoid Mesothelioma
Diagnosing sarcomatoid mesothelioma is difficult because these tumors tend to mimic other benign and malignant conditions in appearance. When pathologists examine sarcomatoid tissue samples under a microscope, the cells resemble those of sarcoma tumors and localized fibrous tumors of the pleura. Because of the aggressive nature of this tumor type, prognosis is not as favorable as other cell types, and treatment options are not as plentiful. The poor survival rates associated with this cell type makes accurate diagnosis even more imperative.
Common Misdiagnoses for Sarcomatoid Mesothelioma
- Pleural liposarcoma
- Fibrous pleurisy
- Malignant fibrous histiocytoma
- Localized fibrous tumors
- Metastasized renal cell carcinoma
If a patient’s symptoms and work history suggest mesothelioma, doctors will first order an imaging test such as an X-ray, CT scan or MRI. However, to determine the cell type for a diagnosis, doctors need to retrieve an adequate amount of tumor tissue directly through a biopsy.
If the tissue sample is too small, a pathologist might not be able to see the full extent of the tumor’s features. This could lead to a misdiagnosis. Most sarcomatoid mesotheliomas, for example, contain cells that resemble those of soft tissue tumors.
Sarcomatoid Features and Diagnostic Techniques
Pathological examination reveals that sarcomatoid mesothelioma tumors form nodules that invade surrounding tissues, including the fat found in the parietal pleura. When viewed under a microscope, sarcomatoid tissue contains spindle-shaped cells arranged in a haphazard pattern with plump, elongated nuclei. The same pattern is often seen in fibrosarcoma tumors and thus contributes to misdiagnosis. Sometimes sarcomatoid mesothelioma cells can have more than one nucleus, which makes the tumor easily confusable with fibrous histiocytoma.
To better differentiate sarcomatoid mesothelioma from other tumors, pathologists may use a tissue staining technique called immunohistochemistry. This technique enhances cell samples with antibodies that react to specific proteins in the tumor tissue. This causes visible reactions that can be observed under a microscope.
For most types of mesothelioma, immunohistochemical staining with the antibody calretinin helps pathologists make an accurate diagnosis. However, sarcomatoid mesothelioma tissue usually fails to react to calretinin. Instead, pathologists may use a panel of antibodies that will show up positive for the sarcomatoid cell type, including CAM5.2, WT1, podoplanin and D2-40. Other types of tumors that resemble sarcomatoid mesothelioma will stain negative for these antibodies.
Variants of Sarcomatoid Mesothelioma
Another reason sarcomatoid mesothelioma is difficult to diagnose is that several distinct variants exist. These variants display different cell structures and characteristics, and can include:
- Transitional mesothelioma - This variant behaves like other sarcomatoid mesotheliomas but has plump, spindle-shaped cells.
- Lymphohistiocytoid mesothelioma - Composed of large spindle-shaped cells, lymphohistiocytoid mesothelioma tumors contain a large amount of inflammatory and immune cells mixed throughout. This variant makes up less than 1 percent of all mesotheliomas.
- Desmoplastic mesothelioma - This variant is one of the most difficult cell types to diagnose. At least half of this tumor is "patternless," and it contains a few elongated cells and collagen bundles. This cell type accounts for approximately 5 percent of all mesotheliomas.
Sarcomatoid Mesothelioma Symptoms, Treatments and Prognosis
Symptoms related to this type of cancer are similar to the symptoms of other mesothelioma cell types. For sarcomatoid mesothelioma of the pleura, symptoms can include shortness of breath, weight loss and weakness. In sarcomatoid mesothelioma of the peritoneum, abdominal "fullness," pain, anorexia and nausea have been reported.
As with other cell types, treatments for sarcomatoid mesothelioma usually depend on the stage and location of the cancer. The primary treatment options for this cell type are surgery, chemotherapy and radiation therapy, the same used for other cell types of mesothelioma.
However, sarcomatoid cells have proven to be more resistant to treatment than other types of mesothelioma. Surgery in particular can be difficult because sarcomatoid tumors are very rigid and often metastasize to the chest wall, making them especially difficult to remove. In cases of peritoneal sarcomatoid mesothelioma, the tumor usually encases the soft internal organs of the abdomen, which can make the identification and removal of the primary tumor challenging.
Chemotherapy has also displayed limited success rates for this mesothelioma cell type. However, a promising drug called selenite has proven effective for treating all types of mesothelioma, including the sarcomatoid cell type. Researchers found that selenite used in combination with doxorubicin induced apoptosis (cell death) in sarcomatoid mesothelioma cells. Another combination of chemotherapy drugs that produced encouraging results was CYVADIC, which incorporates cyclophosphamide, vincristine, adriamycin and dacarbazine.
The median survival for patients with sarcomatoid mesothelioma is typically less than six months, but some patients have been known to survive longer depending on factors like age and overall health. Patients with the lymphohistiocytoid variant of this cell type have survived for as long as six years, and there have even been reports of spontaneous tumor remission.
Finding clinical trials designed for sarcomatoid mesothelioma patients may be difficult. Certain specialty cancer centers like UCLA's Jonsson Comprehensive Cancer Center in Los Angeles, California offer mesothelioma programs dedicated to providing more treatment options and expertise than standard cancer care facilities. Alternative therapies and procedures may be considered if traditional treatment options are exhausted.