Histology is a branch of biology and medicine that involves the study of the cells and tissues of animals and plants. Samples of tissue are stained on a slide and observed under a microscope to study the structure and composition of each cell. Mesothelioma histology is the study of the various types of mesothelioma cells. The study of diseased cells, such as those found in tumors, is a branch of histology called histopathology. Trained medical doctors, usually board-certified pathologists, examine the tumor tissue under a microscope and classify mesothelioma cells by type.
There are several types of mesothelioma cancer cells. Each cellular type of mesothelioma responds to different treatments and affects the individual patient's prognosis, so an accurate diagnosis of cancer cell type is essential to develop an effective treatment plan. After treatment is administered, studying tissue samples can also provide insight on the effectiveness of chemotherapy drugs and other treatments.
Histology also helps prevent misdiagnosis. For example, in women, peritoneal mesothelioma and ovarian cancer can be difficult to differentiate without analyzing the cell type through histology. If patients are pursuing legal action against an asbestos-producing company that may have caused their mesothelioma, a misdiagnosis can also negatively affect their chances of obtaining compensation.
Mesothelioma Cell Types
Epithelial mesothelioma occurs in 50 to 70 percent of most cases and is the most common cell type. Sarcomatoid mesothelioma occurs in 7 to 20 percent of cases, while biphasic occurs in approximately 20 to 35 percent.
Fast Fact: Frozen section fixation is a histology process where a surgically removed tumor is quickly frozen. A slice of the frozen tissue is stained and placed on a slide to determine if a tumor is malignant, usually while the patient is still in surgery.
Each specific cell type has different visible characteristics. For example, in the case of sarcomatoid mesothelioma, cells have elongated nuclei, while epithelial cells are most commonly distinguished by the identification of microvilli (microscopic protrusions of a cell) tissue or cell organelle.
The characteristics noted in the different cell types are very subtle. Unfortunately, this can make the diagnosis process of mesothelioma quite challenging. Specifically distinguishing mesothelioma cells from cells characteristic of adenocarcinoma, a cancer that affects glandular tissue, can be a serious challenge because the cells can appear very similar.
In rare cases, histological variances of mesothelioma cells occur, resulting in the following cell types:
- A rare variant of epithelial mesothelioma, deciduoid mesothelioma has been diagnosed in 45 cases. Roughly half of all deciduoid mesotheliomas originate in either the pleura or the peritoneum.
- A form of sarcomatoid mesothelioma, the framework of desmoplastic mesothelioma cells are more than 50 percent collagenized, or made up of fibrous tissue that produces collagen.
- This cell type is a rare and often misdiagnosed variant of sarcomatoid mesothelioma that contains histocyte-like dense lymphocytic infiltrates.
- A rare variant of mesothelioma, this cell type occurs when at least 50 percent of desmoplastic mesothelioma cells are formed by small cells.
Additional mesothelioma histological variances include adenoid cystic, tubulopapillary, glandular, histiocytoid, microcystic, macrocystic, signet ring, single file, diffuse – NOS, glomeruloid, poorly differentiated (large cell) or pleomorphi ,mucin positive, gaucher cell-like, in situ and well-differentiated papillary.
The Mesothelioma Histology Process
Before a diagnosis of cell type can be made, tissue samples must be obtained and prepared for study. Teams made up of surgeons, histotechnicians, histopathologists and other professionals work together to provide a patient's doctor with information.
When testing for mesothelioma, a medical test involving the removal and examination of cells, tissue or fluids from the body, known as a biopsy, is often performed. After the biopsy, the tissue is prepared by specialists called histotechnicians. These scientists process the tissue by preserving it and staining it with a number of special chemicals that will reveal the minute structures of mesothelioma cells.
- Fixing makes the tissue more rigid and allows it to be sliced with more ease. The tissue is placed in a chemical like formaldehyde.
- Embedding the tissue in paraffin or plastic resin in a block form to make the material easy to cut.
- Sectioning takes place in a machine called a microtome. The microtome can slice extremely thin pieces of the sample.
- Mounting occurs after the slices of embedded tissue are smoothed out in a water bath. The samples are placed on slides and baked to set the paraffin.
After the cancerous tissue is mounted and stained on a slide, the specimen is now ready for study. Then, histopathologists take the slide and view it under a microscope. They can determine what type of cancer may be present by viewing the structure of cells in the tissue.
In addition to staining, histotechnicians may choose to use different techniques for highlighting the details in cells that may be of interest, such as in situ hybridization and immunohistochemistry. In situ hybridization uses the application of fluorescent or radioactive probes to bind DNA and RNA and analyze the genes of a cell. Using this method, scientists can detect genetic abnormalities present in mesothelioma cells.
Immunohistochemistry is based on the principle that antibodies bind to specific antigens and protiens called oncoproteins in human tissue. Different antibodies are applied to tissues on a microscope slide. The tissue samples are then analyzed for specific visual patterns created by the interaction of antibodies and proteins.
Immunohistochemical staining is widely used in the diagnosis of mesothelioma cells and other abnormal cells. However, many medical experts say that immunohistochemistry should be used in context with other diagnostic data. In other words, the results of a mesothelioma immunohistochemistry analysis should not be considered the primary indicator for a mesothelioma diagnosis.
However, other studies have found that immunohistochemical stains improve the accuracy of diagnosis. In one Japanese study, it was estimated that 10 to 15 percent of patients receive an inadequate diagnosis. Since many patients require this diagnosis for legal action, the study recommended improving current diagnosis techniques by improving the quality of immunohistochemistry stains.
Two antibodies often used to aid in diagnosing mesothelioma are BerEP4 and vimentin. BerEP4 has shown to be helpful in the diagnosis of epithelial mesotheliomas. However, immunohistochemists are still researching the efficacy of BerEP4 as a diagnostic indicator for mesothelioma, as this antibody has appeared both in mesotheliomas and other similar tumors.
Like the antibody BerEP4, vimentin has shown to help diagnose mesothelioma, but the protein also appears in other tumors aside from mesotheliomas. Because vimentin appears in other malignant tumors, it is not considered a discriminatory marker for mesothelioma, but rather a piece of the diagnostic puzzle.