Mesothelioma Cell Types: Epithelioid, Sarcomatoid, and Biphasic Explained

Mesothelioma is classified into three histological cell types: epithelioid (about 60 to 80 percent of pleural cases), sarcomatoid (about 10 to 20 percent), and biphasic (about 10 to 20 percent) [1][4][5]. Cell type, also called histology, is one of the most important pieces of information in a mesothelioma diagnosis because it shapes how the disease behaves, how it responds to treatment, and what the broader prognosis range looks like [1][2]. This article explains each subtype in plain language, describes how pathologists distinguish them, and outlines why cell type matters in the larger diagnostic picture.

This page is written for patients, family members, and caregivers who have received a mesothelioma diagnosis or who are trying to understand a pathology report. It builds on the broader overview at What is mesothelioma, which covers the disease at a higher level. If you have not yet read that introduction, it may help to start there.

Key takeaways

• Mesothelioma has three main cell types: epithelioid, sarcomatoid, and biphasic, classified by how tumor cells appear under a microscope [4][6].
• Epithelioid is the most common subtype, accounting for roughly 60 to 80 percent of pleural cases, and published studies generally report more predictable responses to standard chemotherapy than for non-epithelioid subtypes [2][5].
• Sarcomatoid is the rarest pattern (about 10 to 20 percent of cases) and is generally associated in published studies with more aggressive clinical behavior than epithelioid disease [4][5].
• Biphasic tumors contain both epithelioid and sarcomatoid components, with at least 10 percent of each component required per WHO 2021 criteria [6].
• Pathologists confirm cell type using microscopy plus immunohistochemistry markers such as calretinin, WT-1, cytokeratin 5/6, and BAP1 [4][7].

What mesothelioma cell types are and why they matter

Mesothelioma cell types are the three patterns (epithelioid, sarcomatoid, and biphasic) that tumor cells form under a microscope, and they classify the disease for diagnosis and treatment planning [4][6]. Mesothelioma is a cancer that arises from the mesothelium, the thin layer of cells that lines the lungs, abdomen, heart, and testicles [3]. When that layer becomes cancerous, the tumor cells take on one of these three recognizable patterns. Pathologists, doctors who diagnose disease by examining tissue, classify the tumor by the cell pattern they see [4].

The three patterns are:

• Epithelioid: rounded or polygonal cells that resemble normal epithelial tissue
• Sarcomatoid: spindle-shaped cells that resemble connective tissue
• Biphasic: a tumor that contains a mix of both epithelioid and sarcomatoid components

Cell type matters for several reasons. It can influence how rapidly the cancer tends to grow, whether surgery or other treatments may be discussed, how the tumor may respond to systemic therapy, and how clinicians interpret published outcome data for each subtype [1][5]. The World Health Organization (WHO) classification of pleural tumors, most recently updated in 2021, formally recognizes these three patterns as the core histological categories [6].

A few terms worth defining before going further:

• Histology: the study of tissue structure under a microscope
• Subtype: a recognized variation within a disease category
• Mitosis: the process of cell division (high mitotic counts indicate rapidly dividing tumor cells)
• Pleomorphism: variation in size and shape among tumor cells

Epithelioid mesothelioma: the most common subtype

Epithelioid mesothelioma is the most frequent histological subtype, accounting for roughly 60 to 80 percent of pleural mesothelioma cases according to clinical reviews [4][5]. It is also generally associated with the broadest published survival ranges among the three patterns, though individual outcomes vary significantly based on stage, treatment, and overall health [1].

How epithelioid cells appear under the microscope

Under a microscope, epithelioid mesothelioma cells look rounded or polygonal, with relatively uniform nuclei and visible cytoplasm. They often grow in patterns that pathologists describe as tubular, papillary, solid, or trabecular [4]. Some of these growth patterns have their own named variants, including tubulopapillary, solid, and the rarer deciduoid pattern. These finer distinctions are usually mentioned in a pathology report but do not always change treatment planning.

The cellular pattern resembles normal epithelial tissue (the cell type that lines body surfaces and cavities), which is where the name comes from. Because epithelioid cells are more uniform, some studies report more predictable responses to standard chemotherapy than with non-epithelioid subtypes, although response varies by person and treatment plan [2].

Frequency and general prognosis range

Published studies generally associate epithelioid mesothelioma with more favorable group-level outcomes than sarcomatoid or biphasic disease, especially when diagnosis occurs earlier and patients can receive appropriate multimodal care [1][5]. These are aggregate observations, not predictions for any specific person. Stage at diagnosis, the patient's age and overall health, where the tumor is located (pleural versus peritoneal), and what treatments are pursued all influence individual outcomes [4].

Peritoneal mesothelioma, which forms in the abdominal lining, is more often epithelioid. Some studies of selected patients with epithelioid peritoneal disease treated with cytoreductive surgery and heated intraperitoneal chemotherapy (HIPEC) report more favorable group-level outcomes than many pleural mesothelioma cohorts [5]. Whether that approach is appropriate is an individualized surgical and oncology decision.

Sarcomatoid mesothelioma: the rarest and most aggressive subtype

Sarcomatoid mesothelioma is the least common cell type, representing roughly 10 to 20 percent of cases, and published studies generally describe it as the pattern most often associated with aggressive clinical behavior and less consistent response to standard platinum-based chemotherapy [2][4][5].

How sarcomatoid cells appear under the microscope

Sarcomatoid mesothelioma cells are spindle-shaped and elongated, resembling cells from connective tissue rather than from a surface lining. They often grow in dense, disorganized sheets or fascicles (bundle-like patterns) [4]. Because they lack the orderly architecture of epithelioid cells, sarcomatoid tumors can be harder to distinguish from other spindle-cell cancers, including sarcomas of the chest wall or pleura. This is one reason expert pathology review is often valuable [7].

Several recognized variants fall under the sarcomatoid umbrella, including desmoplastic mesothelioma (characterized by dense fibrous tissue) and lymphohistiocytoid mesothelioma (characterized by an unusual immune-cell-rich background). These variants are diagnostically important because they can mimic non-cancerous conditions on initial review [4].

Why sarcomatoid mesothelioma is harder to treat

Several biological features contribute to sarcomatoid mesothelioma being harder to treat. Sarcomatoid cells tend to divide more rapidly, invade surrounding tissue more aggressively, and respond less predictably to platinum-based chemotherapy than epithelioid cells [2]. Surgery is less commonly discussed for sarcomatoid disease in many cohorts, but surgical candidacy depends on the full clinical picture and cannot be determined by cell type alone [5].

That said, the treatment landscape has shifted. The combination of nivolumab and ipilimumab is an FDA-approved first-line option for some patients with unresectable malignant pleural mesothelioma, and clinical trial subgroup data suggested a relative benefit in non-epithelioid disease compared with chemotherapy alone [2]. Whether immunotherapy is appropriate depends on an individualized oncology evaluation.

Biphasic mesothelioma: a mixed-cell subtype

Biphasic mesothelioma is a tumor that contains both epithelioid and sarcomatoid components in the same lesion, with at least 10 percent of each component required for the classification per WHO 2021 criteria [6]. Biphasic disease accounts for roughly 10 to 20 percent of pleural mesothelioma cases [4].

The clinical behavior of biphasic mesothelioma generally falls between the two pure subtypes, and the proportion of each component within the tumor matters. Tumors that are predominantly epithelioid (for example, 80 percent epithelioid and 20 percent sarcomatoid) tend to behave more like pure epithelioid disease, while tumors with a higher sarcomatoid fraction behave more aggressively [5].

One practical issue with biphasic diagnoses is sampling. Small biopsies may capture only one component of a heterogeneous tumor, leading to a pure epithelioid or pure sarcomatoid diagnosis that later changes when more tissue is examined. This is one reason clinicians may discuss whether additional tissue or expert pathology review is needed when initial sampling is limited [7].

How pathologists identify mesothelioma cell type

Pathologists identify mesothelioma cell type by examining biopsy tissue under a microscope and interpreting immunohistochemistry (IHC) staining, a panel of antibody-based stains that helps distinguish mesothelial cells from other cancers [4][7]. Mesothelioma diagnosis is challenging because the disease can resemble several other cancers, particularly lung adenocarcinoma and metastatic carcinomas that have spread to the pleura or peritoneum.

IHC uses antibodies that bind to specific proteins inside or on the surface of cells, producing a visible stain when the protein is present. For mesothelioma, the panel of stains generally includes:

• Calretinin: a protein typically positive in mesothelioma cells
• WT-1 (Wilms tumor 1): another mesothelial marker
• Cytokeratin 5/6: often positive in mesothelioma
• BAP1 loss: loss of BAP1 nuclear staining is highly suggestive of mesothelioma and helps distinguish it from benign reactive tissue
• TTF-1 and napsin A: typically negative in mesothelioma (positive in lung adenocarcinoma, which helps rule that out)

Patients sometimes ask why their biopsy is being re-reviewed or sent to a specialist pathologist. The most common reason is that mesothelioma is uncommon, the histology can mimic other diseases, and getting the subtype right has real treatment implications. Second-opinion pathology review at a center that handles a high volume of mesothelioma cases is recognized as good practice [7]. Patients can ask their oncologist whether such a review is appropriate for their situation.

Comparison of the three cell types at a glance

The table below summarizes the three subtypes side by side. These descriptions reflect aggregate published data and are not predictions for any individual patient or recommendations for treatment.

Cell type	Approximate frequency	Cell appearance	General prognosis context	Treatment planning context
Epithelioid	60 to 80 percent of pleural cases	Rounded, polygonal, uniform nuclei	Generally associated with more favorable group-level outcomes than other subtypes	Clinicians may discuss surgery, chemotherapy, or other options depending on the full case [2][5]
Sarcomatoid	10 to 20 percent of cases	Spindle-shaped, disorganized sheets or fascicles	Generally associated with more aggressive clinical behavior in published studies	Clinicians may discuss systemic therapy, immunotherapy, trials, or supportive care based on individualized factors [2][5]
Biphasic	10 to 20 percent of cases	Mix of both, at least 10 percent of each component	May be influenced by the proportion of epithelioid and sarcomatoid components	Planning is individualized and informed by the full pathology and clinical picture [4][6]

How cell type fits into the broader diagnostic and staging picture

Cell type is one of several pieces of information that together describe a mesothelioma diagnosis. The other major pieces include:

• Anatomic site: pleural (around the lungs), peritoneal (in the abdomen), pericardial (around the heart), or testicular [3]
• Stage: the extent of disease, typically scored using the TNM system (tumor size, lymph nodes, metastasis) for pleural mesothelioma [1]
• Molecular features: including BAP1 status, NF2 alterations, and other markers that may influence treatment trial eligibility [4]
• Patient factors: age, performance status, lung function, comorbidities, and personal preferences

A complete pathology report typically includes the cell type, the location of the biopsy, the IHC panel results, any notable variants (such as desmoplastic features), and a pathologist's overall impression. Patients have the right to request a copy of their pathology report from the institution that performed the biopsy, and reviewing it with the treating oncologist can help clarify questions. The American Cancer Society publishes patient guides to interpreting cancer pathology reports that some patients find useful [2].

It is worth noting what cell type does not determine. It does not on its own tell a patient how long they will live, whether they will respond to a specific treatment, or whether they should pursue a specific clinical trial. Those questions involve the broader clinical picture and are best discussed with an oncologist familiar with mesothelioma.

Frequently Asked Questions

What is the most common cell type of mesothelioma?

Epithelioid mesothelioma is the most common subtype, accounting for roughly 60 to 80 percent of pleural mesothelioma cases in published reviews. Under a microscope, the cells appear rounded or polygonal and may grow in tubular, papillary, or solid patterns. Some studies report that epithelioid tumors respond more predictably to standard chemotherapy than sarcomatoid or biphasic tumors, but individual treatment response varies. [4][5].

Which mesothelioma cell type has the best prognosis?

Epithelioid mesothelioma is generally associated with more favorable published outcomes than sarcomatoid or biphasic disease, but cell type alone cannot predict an individual prognosis. Stage, tumor location, age, overall health, pathology findings, and treatment course all matter. Prognosis questions should be reviewed with an oncology team familiar with the case. [1][5].

What is biphasic mesothelioma?

Biphasic mesothelioma is a tumor that contains both epithelioid cells and sarcomatoid cells in the same lesion. By World Health Organization criteria, at least 10 percent of each component must be present for the biphasic label to apply. Clinical behavior may be influenced by the proportion of each component, but the full diagnosis and treatment context matters. [6].

How is mesothelioma cell type diagnosed?

Pathologists determine cell type by examining biopsy tissue under a microscope and interpreting an immunohistochemistry stain panel. Markers such as calretinin, WT-1, cytokeratin 5/6, BAP1, TTF-1, and napsin A may be used together to support or rule out diagnoses. The panel must be interpreted by qualified pathology professionals in context. [4][7].

Does cell type change treatment options?

Cell type can influence which treatments clinicians may discuss, but it does not determine a treatment plan by itself. Surgery, chemotherapy, immunotherapy, clinical trials, or supportive care may be considered depending on stage, tumor location, performance status, goals of care, and other clinical factors. Specific decisions belong with a mesothelioma-experienced oncology team. [2].

Conclusion

Mesothelioma cell type, whether epithelioid, sarcomatoid, or biphasic, is one of the most consequential pieces of information in a mesothelioma diagnosis. It shapes how the tumor behaves, what treatments clinicians may discuss, and how the broader prognosis literature is interpreted. Epithelioid is the most common pattern, sarcomatoid is the rarest and is often described as more aggressive in published studies, and biphasic contains a mix of both. Pathologists rely on microscopic appearance and immunohistochemistry to make the call, and expert pathology review is often valuable when the diagnosis is uncertain.

For a broader picture of the disease, the What is mesothelioma overview describes how mesothelioma develops, where it forms, and how it is diagnosed and treated. Patients and families with questions about a specific pathology report should discuss them with the treating oncologist and, when appropriate, request a second-opinion pathology review at a mesothelioma-experienced center.

Last reviewed: 2026-05-22

Sources

1. National Cancer Institute. "Malignant Mesothelioma Treatment (PDQ) - Patient Version." https://www.cancer.gov/types/mesothelioma/patient/mesothelioma-treatment-pdq
2. American Cancer Society. "What Is Malignant Mesothelioma?" https://www.cancer.org/cancer/types/malignant-mesothelioma/about/what-is-malignant-mesothelioma.html
3. Centers for Disease Control and Prevention. "Asbestos and Cancer Risk." https://www.cdc.gov/cancer/asbestos/index.html
4. Panou V, Vyberg M, Weinreich UM, et al. "Malignant Mesothelioma." StatPearls, NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK519557/
5. Mesothelioma Applied Research Foundation. "Cell Types of Mesothelioma." https://www.curemeso.org/mesothelioma/cell-types/
6. Sauter JL, Dacic S, Galateau-Salle F, et al. "The 2021 WHO Classification of Tumors of the Pleura: Advances Since the 2015 Classification." Journal of Thoracic Oncology. https://www.jto.org/article/S1556-0864(21)03200-2/fulltext
7. Cleveland Clinic. "Mesothelioma: Types, Symptoms, Causes & Treatment." https://my.clevelandclinic.org/health/diseases/17834-mesothelioma

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