Written By: Devin Golden

Epithelioid Mesothelioma Diagnosis

An epithelioid mesothelioma diagnosis is the most common cell type of this cancer. This diagnosis usually provides patients with the most treatment options and the best prognosis for survival.

Karen Ritter, RN BSN

Medically Reviewed By

Karen Ritter, RN BSN

Registered Nurse

Karen Ritter, RN BSN

Medically Reviewed By

Karen Ritter, RN BSN

Registered Nurse


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Important Facts About Epithelioid Mesothelioma Diagnosis

  • Epithelioid mesothelioma is the most common cell type of mesothelioma cancer.
  • Diagnosing epithelioid mesothelioma involves a tissue biopsy and the evaluation of the tissue samples under a microscope.
  • After patients are given a diagnosis of epithelioid mesothelioma, doctors will determine the stage of their cancer.

How Is Epithelioid Mesothelioma Diagnosed?

Epithelioid mesothelioma is diagnosed by examining tissue cells under a microscope. Doctors can identify epithelioid cells based on their appearance and other unique characteristics they possess.

Mesothelioma can be one of three cell types: epithelioid; sarcomatoid; and biphasic, which is a combination of epithelioid cells and sarcomatoid cells. A key part of diagnosing mesothelioma is identifying any epithelioid cells or sarcomatoid cells that make up the patient’s tumors.

Epithelioid mesothelioma is the most common cell type diagnosed for this cancer. It accounts for up to 70% of all mesothelioma cases.

Pathology to Determine Mesothelioma Cell Type Icon

Pathology to Determine Mesothelioma Cell Type

Identifying the cell type as part of the diagnostic process is called histology, which is the microscopic study of tissue samples for the presence of cancer. Once mesothelioma histology tests are completed, the results will be given in the form of a pathology report. This is the most important and conclusive factor in determining a patient’s diagnosis.

Pathologists, who study the tissue samples, look at tissue cells under a microscope to identify abnormalities. The appearance of the cells and how they react to lab stains can help the pathologist to classify and identify cancer and to determine the mesothelioma cell type.

After determining that the cells are cancerous, pathologists distinguish between the cell types.

They identify the cell type based on the cells’:

  • Behavior
  • Size and shape
  • Nuclei
  • Pattern of the sample
What Do Epithelioid Mesothelioma Cells Look Like? Icon

What Do Epithelioid Mesothelioma Cells Look Like?

Epithelioid mesothelioma cells are easier to identify than sarcomatoid mesothelioma cells due to the well-defined shape and visible nuclei of epithelioid cells. They also clump together and grow quickly, but the cells spread slowly to other areas of the body.

Sarcomatoid mesothelioma cells, by contrast, have a less-defined, irregular shape. They look elongated and stretched out, with undefined nuclei. Sometimes these cells have multiple nuclei. They don’t clump together and usually spread quickly to other areas.

Knowing the differences between mesothelioma cell types is how pathologists can determine if your cancer is made up of epithelioid cells. The pathology report will provide the definitive diagnosis of epithelioid mesothelioma cancer.

What Defines an Epithelioid Cell?

epithelioid cell image

  • Clearly defined clump of cells
  • Pink cytoplasm
  • Easier to identify and remove during surgery
  • Cell type with best prognosis

Staining for Epithelioid Mesothelioma Cell Proteins

Aside from viewing the cells under a microscope with a trained medical eye for appearance and other characteristics, doctors can also perform tests, called immunohistochemistry, on the cells. This testing can provide further evidence of mesothelioma and epithelioid cells.

Immunohistochemistry requires immunohistochemical staining, which involves introducing a foreign substance to the tissue samples. The foreign substance is an antibody that reveals the presence of mesothelioma cells and determines the cell type.

The immunohistochemical markers include:


Demonstrated in nearly all cases

Cytokeratin 5 or 5/6

Expressed in between 75% and 100% of cases

Wilms’ tumor-I antigen (WT1)

Shown in between 70% and 95% of cases

Podoplanin (D2-40)

Demonstrated in between 90% and 100% of cases

Steps of an Epithelioid Mesothelioma Diagnosis

There are important steps that should be taken if you are experiencing symptoms that are consistent with mesothelioma. These steps are crucial in determining the exact cause of your symptoms and getting an accurate mesothelioma diagnosis.

The steps of an epithelioid mesothelioma diagnosis are:

  1. Patient meets with a doctor to report symptoms
  2. Patient undergoes imaging tests, such as an X-ray, CT scan or MRI scan
  3. If the imaging show signs of mesothelioma, the doctor will recommend a biopsy procedure to remove tissue from either the lung cavity or abdominal cavity (depending on where the mesothelioma is believed to be)
  4. Following the tissue biopsy, pathologists review the tissue samples under a microscope to look for distinct attributes of mesothelioma cancer
  5. If mesothelioma is present, pathologists will determine the cell type (epithelioid, sarcomatoid, or biphasic) present in the tissue samples

Stages of Epithelioid Mesothelioma

When you receive a diagnosis of epithelioid mesothelioma, you’ll also likely learn which stage your cancer is in. The stages of mesothelioma are the same regardless of the cell type. The main difference between the cell types is epithelioid mesothelioma cases are more likely to slowly advance from stage to stage compared to sarcomatoid mesothelioma cases.

Stages of Epithelioid Pleural Mesothelioma Icon

Stages of Epithelioid Pleural Mesothelioma

The TNM (tumor, lymph node, metastasis) staging system is used when determining the stages of epithelioid pleural mesothelioma. There are four stages of epithelioid pleural mesothelioma.

epithelioid pleural mesothelioma stage 1

Stage 1

Stage 1 is the earliest stage, defined by tumors localized to the pleura, which is the thin lining of the lungs and where pleural mesothelioma forms.

epithelioid pleural mesothelioma stage 2

Stage 2

Stage 2 is defined by tumors found in the pleura on one side of the chest with possible tumor growth into the diaphragm or the lung tissue.

epithelioid pleural mesothelioma stage 3

Stage 3

Stage 3 is defined by tumors that have extended beyond the pleura to the lung, chest wall, diaphragm and mediastinum.

epithelioid pleural mesothelioma stage 4

Stage 4

Stage 4 is the latest stage, defined by tumors expanding throughout the lung cavity and into other areas of the body, such as the abdominal cavity, the spinal cord or the brain.

Stages of Epithelioid Peritoneal Mesothelioma Icon

Stages of Epithelioid Peritoneal Mesothelioma

The stages of peritoneal mesothelioma are determined by the peritoneal cancer index (PCI), which measures how far the cancer has spread in the abdominal cavity. The abdomen is divided into 13 sections, and each section of the abdominal cavity receives a score of 0-3. The scores of each section are added up to determine the stage of the peritoneal mesothelioma.

The Peritoneal Cancer Index (PCI) defines each stage as:

epithelioid peritoneal cancer index score chart

PCI Score


1 - 10

11 - 20

21 - 30

31 and higher

Stage 1

Stage 2

Stage 3

Stage 4

Frequently Asked Questions About Epithelioid Mesothelioma Diagnosis

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What is the Test to Diagnose Epithelioid Mesothelioma?

A biopsy is the procedure used to diagnose epithelioid mesothelioma. Doctors remove a tissue sample from the patient’s lung cavity or abdominal cavity. This tissue sample will be tested for the presence of mesothelioma or another cancer. The process of testing the tissue sample is called histology, or histopathology.

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What Are the Markers for Diagnosing Epithelioid Mesothelioma?

The most reliable markers for diagnosing epithelioid mesothelioma cancer are calretinin, cytokeratin 5 or 5/6, WT1, and podoplanin. Pathologists study the patient’s tissue under a microscope to officially determine if mesothelioma is present.

Sources & Author

  1. Guidelines for Pathologic Diagnosis of Malignant Mesothelioma. International Mesothelioma Interest Group. Retrieved from: https://www.archivesofpathology.org/doi/pdf/10.5858/arpa.2017-0124-RA. Accessed: 10/04/19.
  2. Immunohistochemistry. National Cancer Institute. Retrieved from: https://www.cancer.gov/publications/dictionaries/cancer-terms/def/immunohistochemistry. Accessed: 10/04/19.
  3. The mesothelial cell. U.S. National Library of Medicine. Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/14592528. Accessed: 04/17/19.
  4. Epithelioid Cell. ScienceDirect. Retrieved from: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/epithelioid-cell. Accessed: 10/04/19.
  5. New Immunohistochemical Markers for Pleural Mesothelioma Subtyping. Diagnostics. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/37761312/. Accessed: 10/02/2023.
  6. Malignant Mesothelioma Stages. American Cancer Society. Retrieved from: https://www.cancer.org/cancer/types/malignant-mesothelioma/detection-diagnosis-staging/staging.html. Accessed: 05/04/2023.
Devin Golden

About the Writer, Devin Golden

Devin Golden is the senior content writer for Mesothelioma Guide. He produces mesothelioma-related content on various mediums, including the Mesothelioma Guide website and social media channels. Devin's objective is to translate complex information regarding mesothelioma into informative, easily absorbable content to help patients and their loved ones.