Mesothelioma is a rare cancer that can be divided by cell type: epithelial, sarcomatoid, and biphasic. Experts further divide these into subtypes. Some mesothelioma subtypes are so rare that little is known about how they progress or respond to treatment.
About Mesothelioma Cell Types
By using cell type, experts group mesothelioma into three broad categories:
- The most common category is epithelial mesothelioma, so named because it impacts epithelial cells.
- Mesothelioma of sarcomatoid cells is less common.
- The third group is called biphasic mesothelioma, which includes a mix of both epithelial and sarcomatoid cells.
Most of the rare subtypes of mesothelioma can be classified as occurring in either epithelial or sarcomatoid cells.
Epithelial Subtypes of Mesothelioma
There can be a great variety of characteristics like sizes, shapes, growth patterns in epithelial cells. Pathologists study tissue samples under a microscope and use cell patterns to identify the mesothelioma subtype when diagnosing mesothelioma.
There are several epithelial subtypes recorded in case studies, although they are uncommon:
- Adenomatoid. This type is also known as glandular mesothelioma, because it involves the cells that line glands, usually in reproductive organs. Adenomatoid mesothelioma is often benign.
- Papillary. The papillary cell type is finger-shaped, a structure also known as papillae. Some are recognized as tubulopapillary, with tubule structures. Others are micropapillary and lack a fibrovascular core seen in tubulopapillary cells. Micropapillary mesothelioma spreads more readily to lymph nodes.
- Deciduoid. This type of mesothelioma consists of sheets of cells that are large and round. Extremely rare, deciduoid mesothelioma has so far been seen mostly in patients with peritoneal mesothelioma. It has a poor prognosis.
- Clear cell. This rare type of mesothelioma has clear cell cytoplasm. This clear cell interior can be seen when examined under a microscope. Because it is so rare, this type of mesothelioma is difficult to diagnose. If clear cancer cells are found in the pleura or peritoneum, doctors must determine if they are mesothelioma cells or cells that metastasized from other locations.
- Mucin-positive. This type of mesothelioma is extremely rare. It occurs when epithelial mesothelioma cells test positive for mucin, the protein that makes up mucous. A positive test for mucin was once used to distinguish between mesothelioma and adenocarcinoma, but it has been recorded that mesothelioma cells may contain mucin; although, this is rare.
- Adenoid cystic. An adenoid cystic pattern of epithelial cells in mesothelioma is also rare. This structure includes a cribriform pattern, which means multiple small holes, tubular structures, and fibrous supporting tissue. This can easily be misdiagnosed as adenoid cystic carcinoma and adenocarcinoma.
- Signet ring. The rare signet ring form of epithelial mesothelioma cells are characterized by a large vacuole, which is a cavity within the cell. This makes it look like there is a ring inside the cell with a large stone on one side. Signet ring mesothelioma must be differentiated from adenocarcinomas and carcinomas with the same structure.
- Other types. There are other subtypes of epithelial mesothelioma that are so rare they have only ever been observed in a handful of patients. These include glomeruloid, histiocytoid, bakery roll, gaucher-like, single file, macrocystic, microcystic, placentoid, and pleomorphic.
Rare Sarcomatoid Mesothelioma Subtypes
Sarcomatoid mesothelioma is more aggressive than epithelial types. These cells spread more rapidly and lead to metastasis more quickly. There are a few rare subtypes of sarcomatoid mesothelioma:
- Desmoplastic. This type of mesothelioma tumor contains a lot of dense, fibrous tissue. It can be confused with fibrous pleurisy in the lungs, a benign condition. The cells of desmoplastic mesothelioma are bland and spindle-shaped.
- Heterologous. This is a rare variety of mesothelioma that includes cells from other tissues in the body. Types of heterologous mesothelioma include osteosarcomatous (bone cells), chondrosarcomatous (cartilage cells), and least common, rhabdomyosarcomatous (muscle cells).
- Lymphohistiocytoid. This rare subtype can be epithelial or biphasic but is more often sarcomatoid. It is often misdiagnosed as Hodgkin or non-Hodgkin lymphoma. The cells are histiocytoid cells and lymphocytes, cells of the immune system. They are large and polygonal in shape.
Diagnosing and Treating Rare Mesotheliomas
Diagnosis of the subtype of mesothelioma is crucially important. It is essential to make sure mesothelioma is not mistaken for another type of cancer, like adenocarcinoma, which would require a different treatment plan.
Pathologists must carefully examine biopsy samples to determine not just the shape and pattern of the cancer cells but also from what tissue they originated.
Treatment for any subtype of mesothelioma depends on the patient and how far the cancer has progressed. Surgery is often used if the cancer has not metastasized. Surgery removes as much cancerous tissue as possible and can be followed by radiation therapy or chemotherapy to destroy remaining cells.
With rare types of mesothelioma, delayed or incorrect diagnosis is common. This is because distinguishing between rare types of mesothelioma is difficult. Many people with a rare type will not be properly diagnosed until the cancer has already metastasized.
A sixty-one-year-old female patient with clear cell mesothelioma that developed in the peritoneum died due to a large amount of ascites and respiratory failure eleven days after her admission to the hospital in Japan. Diagnosis happened after she died. Late diagnosis of the disease makes treatment difficult and leads to poor survival rates.
For this reason, early diagnosis is crucial. If you have any symptoms of mesothelioma or have been exposed to asbestos, regular screening will improve treatment options and possible outcomes.Get Your FREE Mesothelioma Packet
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.