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Histological Subtypes of Mesothelioma

From WikiMesothelioma — Mesothelioma Knowledge Base
Histological Subtypes of Mesothelioma
Classification by tumor cell morphology
Subtype Share / median survival
Epithelioid ~60–70% · 14.7 mo
Biphasic ~10–20% · 8.8 mo
Sarcomatoid ~10–20% · 4.8 mo
Transitional Rare · 6.7 mo
Strongest prognostic axis Cell type (independent factor)
Shared cause Asbestos exposure (all subtypes)

Executive Summary

Histological subtype is one of the strongest independent predictors of survival and treatment response in mesothelioma. Under the World Health Organization classification, malignant mesothelioma is divided into three primary cell types — epithelioid (roughly 60–70% of cases), biphasic or mixed (about 10–20%), and sarcomatoid (about 10–20%, the most aggressive).[1][2] A SEER database analysis of 1,183 patients found epithelioid in 69%, biphasic in 12%, and sarcomatoid in 19% of cases.[3] The MESOPATH Reference Center cohort of more than 7,600 patients reported median overall survival of 14.7 months for epithelioid, 8.8 months for biphasic, and 4.8 months for sarcomatoid disease.[4]

This page is the technical companion to the broader Mesothelioma Types overview. Where that page surveys mesothelioma by tumor location and gives a plain-language summary of the cell types, this page focuses on the pathology itself: how the subtypes are diagnosed, how they differ at the molecular level, what the 2021 WHO classification changed, and how cell type steers the choice between surgery, chemotherapy, and immunotherapy. Two distinctions matter most clinically. First, epithelioid tumors respond far better to platinum/pemetrexed chemotherapy and are the main candidates for aggressive surgery. Second, non-epithelioid tumors — historically resistant to chemotherapy — derive a dramatically larger benefit from immunotherapy, a reversal demonstrated in the CheckMate 743 trial.[5][6]

All histological subtypes share the same primary cause: asbestos exposure. Cell type does not change why the disease occurred; it changes how the disease behaves and how it is treated. Because subtype is established only after a pathologist examines biopsy or surgical tissue, an accurate diagnosis — including expert pathology review — is the foundation for both the medical treatment plan and any legal claim for compensation.

At-a-Glance

Mesothelioma histological subtypes at a glance:

  • Three primary subtypes — epithelioid, biphasic, and sarcomatoid, defined by the WHO classification of pleural tumors.[1][7]
  • Epithelioid is most common — roughly 60–70% of cases and carries the most favorable prognosis of the three.[3][8]
  • Sarcomatoid is most aggressive — spindle-shaped cells, the least common subtype, with median survival of about 4.8 months in large cohorts.[4]
  • Biphasic requires ≥10% of each component — a mixed tumor is diagnosed only when at least 10% epithelioid and 10% sarcomatoid elements are present on a resection specimen.[2]
  • Cell type drives surgical eligibility — epithelioid histology is the strongest predictor of benefit from surgery; sarcomatoid patients generally are not surgical candidates.[2]
  • Immunotherapy benefit is largest in non-epithelioid disease — in CheckMate 743, nivolumab plus ipilimumab reduced the risk of death by 54% in non-epithelioid patients (hazard ratio 0.46).[6]
  • The 2021 WHO update added nuclear grading — high-grade epithelioid tumors carry a hazard ratio of about 3.09 for overall survival.[7]
  • Transitional mesothelioma is now classed under sarcomatoid — its median survival (6.7 months) and molecular profile cluster with non-epithelioid disease.[4]
  • Biopsy can underestimate non-epithelioid disease — because mixed tumors are sampled unevenly, a tumor called epithelioid on biopsy can prove biphasic on full resection.[2]

Key Facts

Subtype Approx. frequency Median overall survival Primary treatment signal
Epithelioid ~69% (SEER, n=1,183)[3] 14.7 months (MESOPATH, n=7,621)[4] Best chemotherapy response; main surgical candidates[2]
Biphasic ~12% (SEER)[3] 8.8 months (MESOPATH)[4] Outcome tracks the sarcomatoid fraction[2]
Sarcomatoid ~19% (SEER)[3] 4.8 months (MESOPATH)[4] Poor chemo response; largest immunotherapy benefit[5][6]
Transitional Rare (subset of sarcomatoid)[4] 6.7 months (MESOPATH panel)[4] Managed as non-epithelioid disease[4]

What Are the Histological Subtypes of Mesothelioma?

Malignant mesothelioma is classified by the appearance of its tumor cells under the microscope. The WHO classification of pleural tumors recognizes three primary histological subtypes — epithelioid, sarcomatoid, and biphasic — and this subclassification is the single most important pathological feature a diagnosing physician records.[1][7]

Epithelioid mesothelioma is by far the most common, accounting for roughly 60–70% of cases. Its cells are polygonal or oval and form clusters, sheets, or gland-like (tubulopapillary) structures that loosely resemble normal epithelial tissue. The FDA's review of the CheckMate 743 trial population noted that epithelioid disease comprises approximately 60% of cases overall, while registry series often report figures closer to 70%.[8][3]

Sarcomatoid mesothelioma is the least common and most aggressive form. Its spindle-shaped cells resemble those of soft-tissue sarcomas and grow in disordered bundles. Several uncommon variants fall under this heading, including the desmoplastic variant, which carries an especially poor prognosis, and the lymphohistiocytoid variant, an unusual exception associated with notably longer survival.[5][4]

Biphasic (mixed) mesothelioma contains both epithelioid and sarcomatoid components. Under standard criteria, the diagnosis requires at least 10% of each component to be present in a resection specimen, and the proportion of sarcomatoid tissue strongly influences behavior — sarcomatoid-predominant biphasic tumors fare significantly worse than epithelioid-predominant ones.[2] Because biphasic tumors are, by definition, heterogeneous, a small biopsy may sample only the epithelioid region. As a result, a tumor reported as epithelioid on biopsy can be reclassified as biphasic once the full surgical specimen is examined, meaning non-epithelioid disease is sometimes underdiagnosed at the biopsy stage.[2]

Reported percentages vary by registry and by how cases are ascertained. A SEER database study of 1,183 patients found epithelioid in 69%, biphasic in 12%, and sarcomatoid in 19%, while clinical-trial populations and reference sources commonly cite an approximate 70/20/10 split.[3][1]

How Does Cell Type Affect Mesothelioma Survival?

Across virtually every published series, histological subtype is an independent prognostic factor — meaning it predicts survival even after accounting for stage, age, and treatment.[2] The largest single dataset, the MESOPATH Reference Center cohort of 7,621 patients, reported median overall survival of 14.7 months for epithelioid, 8.8 months for biphasic, and 4.8 months for sarcomatoid disease.[4]

Real-world treatment data show the same gradient. Registry and cohort series consistently report that patients with epithelioid tumors live substantially longer than those with non-epithelioid disease when treated with the same first-line chemotherapy, reflecting the greater chemosensitivity of epithelioid histology.[2] National Cancer Database and SEER figures for surgically treated patients likewise show better two- and five-year survival for epithelioid than for biphasic disease.[3]

Two points temper a purely subtype-based prognosis. First, survival ranges are wide and depend heavily on stage at diagnosis and on whether the patient is eligible for surgery or immunotherapy. Second, the boundaries between subtypes are not always crisp — interobserver agreement among expert pathologists is only moderate for difficult cases, which is one reason specialist pathology review is strongly recommended before treatment decisions are finalized.[4]

How Does Histology Determine Treatment?

Cell type shapes all three pillars of mesothelioma treatment.

Chemotherapy. Epithelioid mesothelioma responds significantly better to platinum/pemetrexed chemotherapy than non-epithelioid disease. Registry and cohort data consistently show longer survival for epithelioid patients on the same regimen, reflecting greater chemosensitivity.[2] Sarcomatoid tumors have historically shown poor response to standard chemotherapy, which is part of why other modalities have become so important for non-epithelioid patients.[5]

Surgery. Aggressive surgery — extrapleural pneumonectomy or pleurectomy/decortication — is generally offered to patients with epithelioid histology and favorable stage. Epithelioid subtype is the strongest single predictor of potential surgical benefit, whereas sarcomatoid patients typically do not benefit from surgery and are not considered surgical candidates.[2]

Immunotherapy. The relationship reverses for immune checkpoint inhibitors. Sarcomatoid tumors express PD-L1 more frequently and carry higher numbers of tumor-infiltrating lymphocytes than epithelioid tumors, and a systematic review found that sarcomatoid mesotheliomas almost universally expressed PD-1.[5] A meta-analysis of PD-1/PD-L1 inhibitors in pretreated advanced mesothelioma reported an overall response rate of 16% and a disease control rate of 57%, with outcomes comparable or superior to second-line chemotherapy.[9] The decisive evidence came from a first-line randomized trial, discussed next.

What Did CheckMate 743 Show by Cell Type?

CheckMate 743 was the first phase 3 trial to show an overall survival benefit for immunotherapy over chemotherapy in first-line unresectable pleural mesothelioma. It randomized 605 patients to nivolumab plus ipilimumab or to platinum plus pemetrexed chemotherapy; 75% had epithelioid and 25% non-epithelioid histology. Overall median survival was 18.1 months with the immunotherapy doublet versus 14.1 months with chemotherapy (hazard ratio 0.74, 95% CI 0.61–0.89).[6][8]

The subgroup analysis by histology was striking. In non-epithelioid patients, nivolumab plus ipilimumab reduced the risk of death by 54% (hazard ratio 0.46, 95% CI 0.31–0.68), and median survival roughly doubled versus chemotherapy. In epithelioid patients the benefit was more modest and not statistically significant as a subgroup (hazard ratio 0.85, 95% CI 0.68–1.06), in part because chemotherapy already produces reasonable outcomes in epithelioid disease.[6]

Subgroup Nivolumab + ipilimumab vs chemotherapy Overall survival hazard ratio
Non-epithelioid Survival roughly doubled 0.46 (95% CI 0.31–0.68)[6]
Epithelioid More modest, not significant as a subgroup 0.85 (95% CI 0.68–1.06)[6]

Two mechanisms explain the differential benefit. Non-epithelioid tumors respond poorly to chemotherapy, so the chemotherapy comparator performs worse in that subgroup; and sarcomatoid tumors more often express PD-L1, which may enhance the response to checkpoint blockade.[6] The practical result is that nivolumab plus ipilimumab is now widely regarded as a standard first-line option for non-epithelioid mesothelioma, a shift reflected in how later trials have been designed around histology.[8]

What Changed in the 2021 WHO Classification?

The 2021 WHO Classification of Tumours of the Pleura introduced several refinements beyond the traditional three-subtype framework.[7]

  • Nuclear grading for epithelioid mesothelioma was formally incorporated, based on nuclear atypia and mitotic count, separating low-, intermediate-, and high-grade tumors. High-grade epithelioid tumors carry a hazard ratio of approximately 3.09 for overall survival, giving pathologists a way to stratify prognosis within the epithelioid group.[7]
  • Architectural and cytologic patterns within epithelioid disease — including tubular, tubulopapillary, solid, trabecular, micropapillary, and pleomorphic patterns — are now recognized for prognostic significance; tubulopapillary patterns tend to track with better outcomes and pleomorphic with worse.[7][2]
  • Mesothelioma in situ was added as a recognized diagnostic category, identified through loss of BAP1 and/or MTAP staining on immunohistochemistry and CDKN2A deletion by FISH testing of the tumor tissue.[7]
  • Well-differentiated papillary mesothelioma was renamed the well-differentiated papillary mesothelial tumor and reclassified as a preinvasive lesion, and the prefix "malignant" was dropped from the standard nomenclature.[7]

These molecular markers — BAP1, MTAP, and CDKN2A — are read from the patient's own tumor tissue as diagnostic and classification tools. They describe what the established cancer looks like at the molecular level, helping pathologists confirm mesothelioma and distinguish it from benign mimics; they are part of diagnosis, not a statement about why the disease arose.[7]

Beyond the markers formalized in the 2021 update, the Hippo tumor-suppressor pathway is among the most frequently disrupted in mesothelioma, and its key genes track with cell type. In a 266-tumor series, mutations in NF2 and LATS2 were more common in non-epithelioid tumors and were associated with a more sarcomatoid molecular profile, with NF2 alteration also linked to poorer overall survival.[10] A whole-exome study of pleomorphic mesothelioma — a pattern now grouped with the sarcomatoid variant — similarly found NF2 mutated in about 13% and LATS2 in about 8% of cases.[11] As with BAP1, MTAP, and CDKN2A, these are somatic alterations within the established tumor; they help explain why a given subtype behaves as it does, not where the disease came from.

What Is Transitional Mesothelioma?

Transitional mesothelioma is a morphologic pattern in which the tumor cells have lost some epithelioid features — appearing plump and elongated rather than round — without becoming overtly sarcomatoid. It was historically difficult to place, and even expert panels disagreed on its classification.[4]

The defining evidence came from the MESOPATH International Mesothelioma Panel, which evaluated 49 transitional cases with 16 expert pathologists using both molecular profiling and a deep-learning approach.[4] The panel found:

  • Median survival of 6.7 months — not significantly different from sarcomatoid mesothelioma (4.8 months), and significantly worse than epithelioid (14.7 months) or biphasic (8.8 months).[4]
  • Distinct molecular profile — CDKN2A homozygous deletion in 73% of transitional cases (the highest of any subtype) and BAP1 loss in 44%, measured on tumor tissue.[4]
  • RNA-sequencing clustering with sarcomatoid disease rather than with epithelioid disease, supporting its classification as a non-epithelioid variant.[4]

On the strength of these data, the most recent WHO classification places the transitional pattern under sarcomatoid mesothelioma as a cytologic feature. The clinical takeaway is practical: patients whose tumors show a transitional pattern should generally be managed as non-epithelioid disease — favoring immunotherapy over surgery — rather than treated as epithelioid.[4]

What Emerging Therapies Target Specific Subtypes?

Because non-epithelioid disease responds poorly to chemotherapy, research has focused on alternative mechanisms that may work across or specifically within subtypes. As reviewed by Obacz and colleagues, arginine-depleting therapy with ADI-PEG20 combined with pemetrexed and cisplatin produced a high disease-control rate in biphasic and sarcomatoid tumors in early-phase testing, and CDK4/6 inhibitors are under investigation given the frequency of CDKN2A deletion in mesothelioma.[12] Separately, the HDAC inhibitor FK228 (romidepsin) suppressed the growth of both epithelioid and non-epithelioid mesothelioma cells in preclinical models, suggesting a mechanism independent of histology.[13] A further epigenetic target is EZH2 (enhancer of zeste homolog 2), a histone-modifying enzyme whose activity is a therapeutic vulnerability in mesotheliomas that have lost the BAP1 tumor suppressor.[14] In a multicentre phase 2 trial, the oral EZH2 inhibitor tazemetostat achieved disease control in 54% of patients with relapsed, BAP1-inactivated mesothelioma at 12 weeks, although confirmed tumor responses were rare.[14] Because EZH2 also helps tumors evade immune recognition, it is being investigated in combination with immune checkpoint inhibitors rather than as a single agent.[15] Patients with EZH2-overexpressing tumors may be eligible for investigational studies — see mesothelioma clinical trials for current recruiting protocols. These remain investigational; standard first-line care continues to be chemotherapy or immunotherapy chosen according to subtype.[6]

Every histological subtype of mesothelioma shares the same primary cause — asbestos exposure — and an accurate subtype diagnosis strengthens, rather than complicates, a patient's claim by documenting the specific disease and its prognosis.[1] Cell type matters to a claim in two practical ways. It shapes the expected course of treatment and its costs, which inform the value of a claim; and the more aggressive non-epithelioid subtypes, with their shorter median survival, often make timely filing especially important.[4]

Pathology review is the foundation. Because subtypes can be misread on a limited biopsy and because expert agreement is only moderate in difficult cases, patients and families benefit from specialist confirmation of the diagnosis before major treatment or legal decisions.[2][4] For an overview of how compensation works across all mesothelioma types, see Mesothelioma Compensation and the Asbestos Trust Funds pages.

Frequently Asked Questions

Which mesothelioma cell type has the best prognosis? Epithelioid mesothelioma has the most favorable prognosis of the three primary subtypes, with median survival of about 14.7 months in the large MESOPATH cohort and longer with surgery and modern therapy. It also responds best to chemotherapy and is the main subtype eligible for aggressive surgery.[4][2]

Which subtype is the most aggressive? Sarcomatoid mesothelioma is the most aggressive, with median survival of about 4.8 months in large series. It responds poorly to chemotherapy and is generally not treated with surgery — but it derives the largest benefit from immunotherapy.[4][6]

What is biphasic mesothelioma? Biphasic mesothelioma contains both epithelioid and sarcomatoid components, with at least 10% of each required for the diagnosis on a resection specimen. Its prognosis falls between the two pure subtypes and depends on how much sarcomatoid tissue is present.[2]

Why does my biopsy result sometimes change after surgery? Biphasic tumors are heterogeneous, so a small biopsy may capture only the epithelioid portion. When the full surgical specimen is examined, a tumor first called epithelioid may be reclassified as biphasic — which is why specialist pathology review is valuable.[2]

Does cell type change how mesothelioma is treated? Yes. Epithelioid tumors are the main candidates for surgery and respond best to chemotherapy, while non-epithelioid tumors derive the largest benefit from immunotherapy. In CheckMate 743, nivolumab plus ipilimumab reduced the risk of death by 54% in non-epithelioid patients.[6]

Is transitional mesothelioma a separate subtype? No. The transitional pattern is now classified under sarcomatoid mesothelioma. Its survival and molecular profile resemble non-epithelioid disease, and it is managed accordingly.[4]

Do all subtypes come from asbestos exposure? Yes. All histological subtypes of mesothelioma share asbestos exposure as their primary cause. Cell type describes how the tumor looks and behaves, not a different origin.[1]

Quick Statistics

  • ~60–70% of mesothelioma cases are epithelioid, the most common subtype.[3][8]
  • 69% / 12% / 19% — epithelioid, biphasic, and sarcomatoid distribution in a 1,183-patient SEER study.[3]
  • 14.7 / 8.8 / 4.8 months — median survival for epithelioid, biphasic, and sarcomatoid in the 7,621-patient MESOPATH cohort.[4]
  • ≥10% of each component is required to diagnose biphasic mesothelioma on resection.[2]
  • Hazard ratio 0.46 — death-risk reduction with nivolumab plus ipilimumab in non-epithelioid CheckMate 743 patients.[6]
  • Hazard ratio 3.09 — overall-survival risk for high-grade epithelioid tumors under 2021 WHO nuclear grading.[7]
  • 73% — CDKN2A homozygous deletion rate in transitional mesothelioma, the highest of any subtype.[4]
  • 16% / 57% — overall response and disease-control rates for PD-1/PD-L1 inhibitors in pretreated mesothelioma.[9]
  • Independent prognostic factor — histologic subtype predicts survival even after accounting for stage, age, and treatment.[2]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Malignant Mesothelioma. StatPearls. National Center for Biotechnology Information (NCBI Bookshelf). https://www.ncbi.nlm.nih.gov/books/NBK519530/
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 Brcic L, Kern I. Clinical significance of histologic subtyping of malignant pleural mesothelioma. Transl Lung Cancer Res. 2020;9(3):924-933. PMID: 32676358. https://pubmed.ncbi.nlm.nih.gov/32676358/
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Meyerhoff RR, Yang CF, Speicher PJ, et al. Impact of mesothelioma histologic subtype on outcomes in the Surveillance, Epidemiology, and End Results database. J Surg Res. 2015;196(1):23-32. PMID: 25791825. https://pubmed.ncbi.nlm.nih.gov/25791825/
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 Galateau Salle F, Le Stang N, Tirode F, et al. Comprehensive Molecular and Pathologic Evaluation of Transitional Mesothelioma Assisted by Deep Learning Approach: A Multi-Institutional Study of the International Mesothelioma Panel from the MESOPATH Reference Center. J Thorac Oncol. 2020;15(6):1037-1053. PMID: 32165206. https://pubmed.ncbi.nlm.nih.gov/32165206/
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Mansfield AS, Symanowski JT, Peikert T. Systematic review of response rates of sarcomatoid malignant pleural mesotheliomas in clinical trials. Lung Cancer. 2014;86(2):133-6. PMID: 25217189. https://pubmed.ncbi.nlm.nih.gov/25217189/
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 Baas P, Scherpereel A, Nowak AK, et al. First-line nivolumab plus ipilimumab in unresectable malignant pleural mesothelioma (CheckMate 743): a multicentre, randomised, open-label, phase 3 trial. Lancet. 2021;397(10272):375-386. PMID: 33485464. https://pubmed.ncbi.nlm.nih.gov/33485464/
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 Mastromarino MG, Lenzini A, Aprile V, et al. New Insights in Pleural Mesothelioma Classification Update: Diagnostic Traps and Prognostic Implications. Diagnostics (Basel). 2022;12(12). PMID: 36552912. https://pubmed.ncbi.nlm.nih.gov/36552912/
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Nakajima EC, Vellanki PJ, Larkins E, et al. FDA Approval Summary: Nivolumab in Combination with Ipilimumab for the Treatment of Unresectable Malignant Pleural Mesothelioma. Clin Cancer Res. 2022;28(3):446-451. PMID: 34462287. https://pubmed.ncbi.nlm.nih.gov/34462287/
  9. ↑ 9.0 9.1 Zafar A, Rashid AA, Moeed A, et al. Safety and efficacy of PD-1/PD-L1 immune checkpoint inhibitors in patients with pre-treated advanced stage malignant mesothelioma: a systematic review and meta-analysis. BMC Cancer. 2024;24(1):1353. PMID: 39501196. https://pubmed.ncbi.nlm.nih.gov/39501196/
  10. ↑ Quetel L, Meiller C, Assié JB, et al. Genetic alterations of malignant pleural mesothelioma: association with tumor heterogeneity and overall survival. Mol Oncol. 2020;14(6):1207-1223. PMID: 32083805. https://pubmed.ncbi.nlm.nih.gov/32083805/
  11. ↑ Roy S, Galateau-Sallé F, Le Stang N, et al. Molecular characterization of pleomorphic mesothelioma: a multi-institutional study. Mod Pathol. 2022;35(1):82-86. PMID: 34531524. https://pubmed.ncbi.nlm.nih.gov/34531524/
  12. ↑ Obacz J, Yung H, Shamseddin M, et al. Biological basis for novel mesothelioma therapies. Br J Cancer. 2021;125(8):1039-1055. PMID: 34226685. https://pubmed.ncbi.nlm.nih.gov/34226685/
  13. ↑ Chan JM, Chang YC, Chan HC, et al. FK228 suppress the growth of human malignant pleural mesothelioma tumor independent to epithelioid or non-epithelioid histology. Mol Med. 2024;30(1):73. PMID: 38822233. https://pubmed.ncbi.nlm.nih.gov/38822233/
  14. ↑ 14.0 14.1 Zauderer MG, Szlosarek PW, Le Moulec S, et al. EZH2 inhibitor tazemetostat in patients with relapsed or refractory, BAP1-inactivated malignant pleural mesothelioma: a multicentre, open-label, phase 2 study. Lancet Oncol. 2022;23(6):758-767. PMID: 35588752. https://pubmed.ncbi.nlm.nih.gov/35588752/
  15. ↑ Lofiego MF, Cannito S, Fazio C, et al. Epigenetic Immune Remodeling of Mesothelioma Cells: A New Strategy to Improve the Efficacy of Immunotherapy. Epigenomes. 2021;5(4):27. PMID: 34968251. https://pubmed.ncbi.nlm.nih.gov/34968251/
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