Three main mesothelioma cell types characterize how tumors form. Half of cases are the epithelioid cell type. One in 10 cases is the more aggressive sarcomatoid cell type. The rest are mostly biphasic cases, where tumor cells are at least 10 percent epithelioid and at least 10 percent sarcomatoid.
Histology is a branch of biology that involves the study of cells and tissues.
Histology technicians use microscopes to view tissues and cells up close. They prepare samples of tissue with chemical stains. The stains make the cells’ features stand out to help with identification.
Mesothelioma histology involves the study of cancerous mesothelial cells. It is considered a branch of histopathology, the study of diseased cells. Histopathology falls within the larger field of pathology.
It takes special training to identify cancer cells. Board-certified pathologists often become specialized in identifying different types of cancer. A small number of pathologists specialize in identifying mesothelioma cells.
There are several types of mesothelioma cancer cells. Each cell type responds to treatment differently and affects the individual patient’s prognosis.
An accurate diagnosis of the cancer cell type is essential to developing an effective treatment plan.
Histology also helps prevent mesothelioma misdiagnosis. For example, peritoneal mesothelioma and ovarian cancer can be difficult to differentiate. Analyzing the cell type helps doctors tell the difference.
Pathologists look for three different types of cells when mesothelioma is suspected. These cell types are called epithelial, sarcomatoid and biphasic.
Epithelial cells are uniform and sharply defined. They feature prominent nuclei and often form complex branching patterns. They divide quickly but tend to stick together, which means it takes longer for them to spread throughout the body. This cell type makes up 50 percent of all mesothelioma cases.
Spindle-shaped sarcomatoid cells typically lack strong characteristics. They spread more easily than epithelial cells because they don’t stick together the way epithelial cells do. This rare cell type characterizes 10 percent of cases.
Malignant mesothelioma is considered biphasic when both epithelial and sarcomatoid cells are present. Each type accounts for at least 10 percent of the tumor mass. The biphasic (mixed) cell type accounts for 30 to 40 percent of mesothelioma cases.
Each cell type has different visible characteristics. For example, sarcomatoid cells have elongated nuclei. Epithelial cells have microvilli (microscopic protrusions of a cell) or cell organelles.
Doctors use these histological classifications to confirm the diagnosis. Along with staging information, they can later develop a prognosis and treatment plan.
Each cell type responds differently to treatment.
Epithelial cells are typically the most responsive to treatment, which often leads to a better prognosis.
Sarcomatoid cells are the least responsive to treatment. Some mesothelioma specialists do not consider sarcomatoid tumors eligible for surgical removal.
The biphasic cell type is also considered less responsive to treatment than the epithelial cell type. But, the exact prognosis depends on the ratio of epithelial cells to sarcomatoid cells.
The appearance of the different cell types is subtle. This fact can make the diagnostic process quite challenging. For example, distinguishing mesothelioma cells from adenocarcinoma cells can be a serious challenge. Only the most experienced mesothelioma pathologists can easily tell the difference.
It takes a team to diagnose mesothelioma. The team includes surgeons, histotechnicians, histopathologists and other professionals. They work together to provide the patient’s oncologist with information.
A surgeon removes a sample of tumor tissue through a biopsy procedure. The sample is prepared by a histotechnician. This scientist preserves and stains the sample with a number of special chemicals. These chemicals reveal the microscopic appearance of the cells.
The histotechnician works closely with a pathologist to identify a cancer’s cell type.
Fixing: The tissue is preserved with a chemical such as formaldehyde.
Embedding: The tissue is embedded in a block of paraffin or plastic resin to make the material easy to cut.
Sectioning: A machine called a microtome slices extremely thin pieces of the sample.
Mounting: The slices of embedded tissue are smoothed out in a water bath and then placed on slides and baked.
“Frozen section fixation” is used to diagnose cancer during surgery. A surgically removed tumor is quickly frozen while the patient is still in surgery. A slice of the frozen tissue is then stained and placed on a slide for a rapid assessment by a pathologist to determine if a tumor is malignant.
After the cancerous tissue is mounted and stained on a slide, the sample is now ready to view under a microscope. They observe the size, shape and anatomical structure of the cells to identify the tumor’s cell type.
Other techniques are used by pathologists to learn more about cells. These techniques include in situ hybridization and immunohistochemistry.
In situ hybridization uses fluorescent or radioactive probes to bind DNA and RNA. Using this method, scientists can analyze the genes of a cell. They can also detect genetic abnormalities.
Immunohistochemistry is based on the principle that antibodies bind to specific antigens. Antibodies also bind to proteins called oncoproteins.
Different antibodies are applied to tissues on a microscope slide depending on what type of cancer is suspected. Visual patterns are created by the interaction of antibodies and oncoproteins. These patterns help pathologists diagnose mesothelioma.
Immunohistochemistry is regularly used in the diagnosis of mesothelioma. This technique improves the accuracy of diagnosis when used in context with other diagnostic data.
Karen Selby joined Asbestos.com in 2009. She is a registered nurse with a background in oncology and thoracic surgery and was the director of a tissue bank before becoming a Patient Advocate at The Mesothelioma Center. Karen has assisted surgeons with thoracic surgeries such as lung resections, lung transplants, pneumonectomies, pleurectomies and wedge resections. She is also a member of the Academy of Oncology Nurse & Patient Navigators. Read More