If your pathology report names a mesothelioma “cell type,” or you’re trying to understand a report for someone in your family, this page is where to start. Cell type is common patient-facing shorthand. Pathologists more precisely call it the histologic subtype, meaning the tumor’s microscopic pattern.
For invasive diffuse mesothelioma, the three main histologic subtypes are epithelioid, sarcomatoid, and biphasic. Epithelioid and sarcomatoid describe two different malignant cell-growth patterns. Biphasic means that both patterns are present in the sampled tumor. Histology can provide important information about diagnosis, prognosis, and treatment, but it doesn’t tell you the tumor’s anatomical site, stage, grade, or likely course by itself.1,2
Evidence scope. Mesothelioma can arise in the pleura, peritoneum, pericardium, or tunica vaginalis—the membrane around a testicle. The same broad histologic subtypes are recognized across these sites, but the grading, prognosis, and treatment evidence attached to them is not necessarily interchangeable. Most numerical outcomes and clinical-trial results in the mesothelioma literature concern diffuse pleural disease. Peritoneal mesothelioma has a smaller, separate evidence base and different treatment pathways. Primary pericardial mesothelioma and mesothelioma of the tunica vaginalis are exceptionally rare, so evidence for those sites comes mainly from registries, small series, and case reports. Unless stated otherwise, pleural evidence should not be assumed to apply to another site.1,3,4
At a broad population level, about 6 in 10 mesotheliomas are epithelioid, about 1 in 10 are sarcomatoid, and the remaining 3 in 10 are biphasic.1 Those figures are useful site-wide shorthand, not fixed biological constants. Reported proportions vary with anatomical site, study population, specimen type, and whether classification is based on a small biopsy or a larger resection.
You don’t need to be an expert in pathology to understand the central questions. The purpose of this page is to explain what the three subtype labels mean, what a biopsy can miss, how to read additional pathology terms, and what questions may be worth raising with your medical team.
How each subtype looks under the microscope, with broad overall frequencies; the drawings are simplified illustrations, not microscope images
Frequencies: American Cancer Society. They are broad estimates and vary by anatomical site, study population, and whether the sample is a small biopsy or a larger resection. Subtype is one factor among many; it does not by itself set treatment or predict an individual outcome.1,3,7
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Quick answers
| What are the three main mesothelioma cell types? | The more precise term is histologic subtypes: epithelioid, sarcomatoid, and biphasic. Epithelioid and sarcomatoid describe different malignant microscopic patterns. Biphasic means both patterns are present. |
|---|---|
| Which subtype is most common, and which is rarest? | As a broad overall estimate, about 6 in 10 mesotheliomas are epithelioid, 3 in 10 are biphasic, and 1 in 10 is sarcomatoid. The proportions vary by anatomical site and the type of specimen studied.1 |
| Which subtype has the best or worst prognosis? | In studies of pleural mesothelioma, epithelioid is generally associated with the most favorable outcomes and sarcomatoid with the least favorable outcomes; biphasic is usually intermediate and more variable. These are population-level associations, not an individual forecast. Location, stage, lymph nodes, disease burden, grade where applicable, overall health, treatment, and actual response also matter.5 |
| Does histology determine treatment? | It influences the discussion but does not choose a treatment by itself. For pleural disease, histology affects whether radical surgery is considered and how clinicians weigh systemic options. Anatomical site, stage, resectability, performance status, organ function, other medical conditions, prior treatment, and the patient’s goals remain essential.6 |
| Is histology the same as stage or grade? | No. Histology describes microscopic appearance. Stage describes how far disease has spread. Grade describes additional features associated with biological behavior. The current WHO low-grade/high-grade framework applies to epithelioid diffuse pleural mesothelioma, not to an entire sarcomatoid or biphasic tumor. |
| Can a biopsy miss the true subtype? | Yes. A biopsy samples only part of a heterogeneous tumor. This is especially important for biphasic disease, because one biopsy may capture only the epithelioid or only the sarcomatoid component. A later subtype change does not necessarily mean the first pathologist made an error.7–9 |
| Does my subtype change my legal options? | Usually not by itself. Potential claims and benefit programs depend on the diagnosis, exposure evidence, responsible entities, program requirements, jurisdiction, and filing deadlines. Histology may affect medical proof or practical case planning, but it does not guarantee that every recovery channel applies. |
| How long do I have to file a claim? | It depends on the applicable law, type of claim, and whether the matter involves personal injury or wrongful death. Some deadlines are short. Prompt review is the safest way to identify which deadlines may apply. |
| Can a spouse or adult child call for the patient? | Yes. A spouse, adult child, or other family member can ask questions and begin gathering information. Formal authority may be required for later decisions or filings, but it is not ordinarily needed simply to make an initial inquiry. |
“I started this firm after learning that the cancers that killed my father and grandparents were caused by asbestos, something we did not realize until it was too late to hold anyone accountable. That experience is why we built this site: so that no other family misses their chance to ask the questions, hear the options, and decide for themselves what to do.”
The three main mesothelioma histologic subtypes
The 2021 WHO classification retains epithelioid, sarcomatoid, and biphasic as the three main histologic subtypes of diffuse mesothelioma. It also places greater emphasis on architectural patterns, cytologic and stromal features, epithelioid tumor grade, and ancillary molecular tests.2
| Histologic subtype | What the pathologist sees | Broad overall frequency | What the label generally means | Detailed page |
|---|---|---|---|---|
| Epithelioid | Relatively cohesive polygonal, cuboidal, or columnar cells that may form tubules, papillae, trabeculae, microcysts, or solid sheets. | About 6 in 10 cases.1 | The most favorable subtype on average and the one most often considered for the broadest range of treatment options. Diffuse pleural epithelioid tumors may also receive a final low or high WHO tumor grade. | Epithelioid mesothelioma |
| Sarcomatoid | Elongated or spindle-shaped malignant cells growing in infiltrating sheets, fascicles, or fibrous patterns. | About 1 in 10 cases.1 | The least favorable subtype on average. It can resemble reactive fibrosis, sarcoma, or sarcomatoid carcinoma, so diagnosis often requires a carefully selected panel and adequate tissue. Desmoplastic mesothelioma is a sarcomatoid pattern. | Sarcomatoid mesothelioma |
| Biphasic | Both malignant epithelioid and malignant sarcomatoid components in the same tumor specimen. | About 3 in 10 cases.1 | A heterogeneous subtype. The estimated proportions can add prognostic context, but they are specimen-dependent and do not create fixed treatment or survival categories. | Biphasic mesothelioma |
Epithelioid mesothelioma is the most common subtype. The word alone doesn’t mean that the tumor is early-stage, surgically removable, or low-grade. Within epithelioid diffuse pleural disease, architectural patterns, mitotic activity, nuclear atypia, necrosis, and the final WHO low/high tumor grade can add prognostic information. The epithelioid mesothelioma page explains those findings and their limits.
Sarcomatoid mesothelioma contains malignant spindle-cell morphology and can be particularly difficult to distinguish from non-mesothelial spindle-cell cancers or benign fibrous processes. Traditional mesothelial markers are less consistently expressed than in epithelioid disease, so a negative stain that would be unusual in epithelioid mesothelioma may not exclude sarcomatoid disease. The sarcomatoid mesothelioma page covers desmoplastic and transitional morphology, diagnostic mimics, and the subtype-specific treatment discussion.
Biphasic mesothelioma contains both malignant epithelioid and malignant sarcomatoid morphology. The components may be sharply separated, intermingled, or unevenly distributed. A percentage such as “70% epithelioid and 30% sarcomatoid” is an estimate from the material examined, not a measurement of every part of the tumor. The biphasic mesothelioma page explains the specimen-specific 10% rule, sampling limits, and why no particular percentage automatically selects a treatment.
Why histology matters, and what it can’t decide
Histology matters at diagnosis, prognosis, and treatment planning. Its value is greatest when it is interpreted with the anatomical site, specimen type, imaging, stage, and the patient’s overall clinical condition.
Diagnosis
Microscopic appearance helps the pathologist decide which diagnostic possibilities need to be tested. Epithelioid mesothelioma can resemble metastatic adenocarcinoma or another epithelioid cancer. Sarcomatoid mesothelioma can resemble reactive pleuritis, sarcoma, sarcomatoid carcinoma, or another spindle-cell tumor. Biphasic mesothelioma requires convincing evidence that both microscopic components are malignant mesothelial tumor rather than an epithelioid mesothelioma beside reactive spindle-cell stroma.7
A subtype label is therefore not based on shape alone. The pathologist integrates morphology, evidence of invasion when represented, immunohistochemistry (staining that identifies proteins in the cells), ancillary tests when useful, imaging, and the clinical setting.
Prognosis
Histologic subtype is a strong population-level prognostic factor. In older pleural registry data, epithelioid disease was associated with longer survival than biphasic disease, and sarcomatoid disease with the shortest survival.5 Those historical differences remain clinically important, but they should not be converted into a personal countdown. Registry cohorts include patients with different stages, ages, health conditions, treatments, and time periods.
Within the same subtype, outcomes can still vary substantially. For example, two patients with epithelioid disease may have different final tumor grades, stages, lymph-node findings, anatomical sites, and treatment options. Two biphasic biopsy specimens may show different component estimates without establishing a precise whole-tumor ratio. A patient’s actual response to treatment can ultimately matter more than any baseline population estimate.
Treatment
For diffuse pleural mesothelioma, histology influences, but doesn’t independently determine, the treatment discussion. Current ASCO (American Society of Clinical Oncology) guidance confines possible radical cytoreductive surgery (the surgical removal of visible tumor) to a narrow group of comprehensively staged patients with early-stage epithelioid disease at experienced centers. Non-epithelioid histology generally shifts the discussion away from radical surgery and toward systemic therapy. The exact regimen still depends on contraindications, disease burden, symptoms, organ function, prior treatment, and patient preferences.6
Peritoneal mesothelioma follows a different treatment pathway, including possible evaluation for cytoreductive surgery and HIPEC (heated chemotherapy delivered into the abdomen during surgery) in selected patients.10 Pleural treatment recommendations should not be transferred automatically to peritoneal, pericardial, or tunica-vaginalis disease.
For the regimen details, clinical-trial results, adverse effects, radiation, tumor treating fields, later-line therapy, and costs, see the pleural mesothelioma treatment page.
Sampling and interpretation
A pathology report describes the material the laboratory received. It does not prove that every part of the tumor looks the same. Mesothelioma can be heterogeneous, and a small biopsy may:
- Show mesothelioma but miss a second histologic component
- Underrepresent necrosis or an unfavorable architectural pattern
- Provide too little tissue for confident grading
- Contain crush artifact or treatment effect that limits interpretation
- Miss the tissue interface needed to demonstrate invasion
Studies comparing pleural biopsies with later resections confirm that subtype and component estimates can change when more tissue is examined. Expert pathologists also do not always assign exactly the same percentage to borderline biphasic or transitional areas.8,9 A revised diagnosis may therefore reflect better sampling or updated classification rather than negligence or an obvious initial mistake.
Histology vs. location vs. stage vs. grade
A pathology report and an oncology note may describe the same mesothelioma in several different ways. These terms answer different questions.
| Term | What it describes | Examples and limitations |
|---|---|---|
| Anatomical site | Where the tumor begins | Pleural, peritoneal, pericardial, or tunica vaginalis. Site changes the staging system, differential diagnosis, treatment pathway, and relevance of published evidence. |
| Histologic subtype | The main microscopic growth pattern | Epithelioid, sarcomatoid, or biphasic. This is what many patient-facing pages call “cell type.” |
| Stage | How far the disease has extended anatomically | Tumor extent, lymph-node involvement, and distant spread. Formal TNM staging (the standard tumor-node-metastasis system for measuring how far cancer has spread) is best established for pleural mesothelioma; other sites require site-specific assessment. |
| Tumor grade | Additional microscopic features associated with biological behavior | For epithelioid diffuse pleural mesothelioma, nuclear atypia and mitotic activity produce a three-tier nuclear grade, which is combined with necrosis to produce a final low or high WHO tumor grade. This framework does not assign a low/high grade to an entire sarcomatoid or biphasic tumor.2,3 |
| Architectural, cytologic, or stromal feature | A more detailed description within or alongside the main subtype | Examples include tubulopapillary architecture, rhabdoid cytology, transitional morphology, desmoplasia, or myxoid stroma. Most are not separate fourth or fifth subtypes. |
| Biomarker or molecular finding | A protein-expression or genetic result | Examples include BAP1 loss, MTAP loss, CDKN2A deletion, or PD-L1 expression. These results answer specific diagnostic, hereditary, research, or treatment-context questions; they do not replace the subtype. |
The same broad histologic subtype can occur at more than one anatomical site, but that does not make the evidence interchangeable. An epithelioid peritoneal tumor may share morphology and several immunohistochemical markers with an epithelioid pleural tumor while following a different staging and treatment pathway. Likewise, the WHO pleural epithelioid grading framework should not be presented as a universal grading system for every mesothelioma site.
How pathologists determine the histologic subtype
Mesothelioma diagnosis is an integrated process rather than a single-stain test. The exact workup varies with the anatomical site, specimen type, microscopic appearance, imaging, and the cancers or benign conditions that remain in the differential diagnosis.7
1. Tissue and microscopic review
A pathologist first evaluates the specimen’s source, size, depth, viability, and morphology. In suspected pleural disease, thoracoscopic biopsy often permits direct inspection and sampling from multiple abnormal areas when it is medically feasible. An image-guided core biopsy can also be appropriate when it provides a safe, representative target. Cytology from pleural or peritoneal fluid may be helpful in selected cases but has important limitations for invasion, sarcomatoid morphology, and whole-tumor subtyping.7
The pathologist looks for:
- Cell shape and cohesiveness
- Architectural growth patterns
- Spindle-cell or fibrous areas
- Tissue invasion
- Necrosis and mitotic activity
- Reactive changes and treatment effects
- Whether two morphologically distinct malignant components are present
2. Immunohistochemistry
Immunohistochemistry uses antibodies to identify proteins in the cells. The panel is chosen to answer the actual differential diagnosis, not applied as one universal checklist.
For an epithelioid tumor, the pathologist commonly uses more than one mesothelial-lineage marker—such as calretinin, WT1, D2-40, or cytokeratin 5/6—and more than one marker selected to exclude carcinoma, often including claudin-4 and organ-specific markers. For a spindle-cell tumor, broad-spectrum cytokeratins and a broader exclusion panel may be needed, because traditional mesothelial markers can be focal or negative in sarcomatoid areas.7
No single positive marker proves mesothelioma, and no single negative marker excludes it in every case. A mesothelial staining pattern establishes lineage; it does not, by itself, prove that the proliferation is malignant.
3. Ancillary tests that support malignancy
After mesothelial differentiation has been established, ancillary tests can help distinguish malignant mesothelioma from a benign or reactive mesothelial proliferation:
- BAP1 loss by immunohistochemistry strongly supports malignancy in the proper morphologic and mesothelial context.11
- MTAP loss by immunohistochemistry can support malignancy and often serves as a practical surrogate for deletion involving the nearby CDKN2A region, a use validated mainly in pleural mesothelioma.7,12
- Homozygous CDKN2A deletion by FISH (fluorescence in situ hybridization, a test that detects missing genes) strongly supports malignancy and can be particularly useful when BAP1 is retained.7
These tests are complementary, not interchangeable. Retained BAP1 or MTAP and a negative CDKN2A FISH result do not rule out mesothelioma. The result must also be assigned to the correct cell population. In a possible biphasic tumor, an abnormal result confined to epithelioid cells does not automatically prove that nearby spindle cells are malignant.
Tumor BAP1 loss does not establish that an alteration was inherited. Germline testing requires a separate non-tumor sample, usually blood or saliva, and a separate interpretation. Current ASCO guidance recommends offering germline genetic testing to patients with mesothelioma.6
4. Correlation and specialist review
The final interpretation should fit the imaging, clinical presentation, and specimen source. When the morphology and stains do not fit together, the correct response may be additional stains, molecular testing, review of deeper sections, retrieval of another tissue block, specialist consultation, or, when the existing material can’t answer the question, additional sampling.
Review of the existing slides and blocks is often the appropriate first step before assuming that another invasive procedure is necessary. A mesothelioma-specialized pathologist may confirm the original diagnosis, add a subtype or grade, identify a sampling limitation, or occasionally change the diagnosis.
Understanding percentages in a biphasic report
A biphasic report may estimate the percentage of epithelioid and sarcomatoid morphology. That estimate can be useful, but it must be read with the specimen type.
The 10% rule is different for resections and biopsies
For a definitive pleural resection specimen, both the epithelioid and sarcomatoid components generally must make up at least 10% of the tumor examined for classification as biphasic. A resection showing 95% epithelioid and 5% sarcomatoid morphology would ordinarily be classified by the dominant epithelioid subtype, although the minor sarcomatoid focus may still be described. Peritoneal disease is treated more cautiously: because some evidence links even a focal sarcomatoid component to a less favorable outlook in peritoneal mesothelioma, pathology guidance advises noting sarcomatoid growth in a peritoneal resection even when it is under 10%.2,3,7
For a biopsy, there is no 10% minimum. If convincing malignant epithelioid and malignant sarcomatoid components are both present, a pathologist can diagnose biphasic mesothelioma even when one component occupies less than 10% of the sampled tissue. A thoracoscopic biopsy is still a biopsy for this purpose; larger biopsy fragments do not become a definitive resection.2,3,7
The 10% figure is a classification convention, not a biological cliff. It doesn’t mean that 9% sarcomatoid morphology is harmless, or that 10% automatically changes treatment.
What the estimate can and can’t tell you
A larger sarcomatoid component has generally been associated with less favorable outcomes in retrospective studies, but the percentage is not a validated treatment formula. No prospective trial establishes 15%, 25%, 30%, 40%, or 50% as an automatic cutoff for surgery, chemotherapy, immunotherapy, or an individual survival estimate.6,7
The estimate is also subject to two kinds of uncertainty:
- Sampling uncertainty. A biopsy may capture one region of a heterogeneous tumor. A later specimen from another location may show a different ratio.
- Interpretive uncertainty. Experts can disagree about borderline spindle-cell or transitional areas and about the exact percentage, especially away from the extremes.8,9
For that reason, wording such as “approximately 70% epithelioid and 30% sarcomatoid in this specimen” is more accurate than implying that the complete tumor has been measured. If a report confirms biphasic mesothelioma but does not provide percentages, it is reasonable to ask whether an estimate can be made. Sometimes the sample is too small, crushed, necrotic, or otherwise unrepresentative; that limitation should be stated rather than hidden, and care should not automatically be delayed solely to obtain a more precise number.
Other pathology terms you may see
Most additional terms describe an architectural pattern, a cytologic feature, a stromal feature, or a related mesothelial diagnosis. They usually do not replace the main epithelioid, sarcomatoid, or biphasic subtype.2,3,7
| Term or group of terms | Category | How to understand it |
|---|---|---|
| Tubulopapillary, trabecular, adenomatoid, microcystic, solid, micropapillary | Architectural patterns | These describe how epithelioid tumor cells are arranged. Some patterns are associated with more or less favorable outcomes across groups, but no pattern guarantees an individual result or independently selects treatment. |
| Desmoplastic | Sarcomatoid pattern or variant | A dense fibrous form of sarcomatoid mesothelioma that can mimic fibrous pleuritis or another benign fibrosing process. In a small biopsy, the report may use “sarcomatoid mesothelioma with desmoplastic features” because the full-tumor proportion cannot be assessed. |
| Transitional morphology | Sarcomatoid cytologic/morphologic pattern | Relatively cohesive, elongated but plump cells with an appearance between classic epithelioid and classic spindle-cell morphology. Current WHO guidance classifies convincing transitional morphology with sarcomatoid disease. It should not be described literally as cells observed transforming from one subtype into another. |
| Rhabdoid, deciduoid, clear cell, small cell, signet-ring, pleomorphic | Cytologic features | These describe the appearance of individual cells and can create diagnostic or prognostic questions. They are not separate fourth, fifth, or sixth main subtypes. |
| Lymphohistiocytoid | Cytologic and inflammatory feature | A rare appearance involving tumor cells and a prominent inflammatory background. It can occur in more than one main subtype. Limited series have suggested prognostic associations, but it is not a dependable stand-alone predictor of treatment response. |
| Myxoid stroma | Stromal feature | A loose, gel-like supporting matrix around tumor cells. It is not a separate subtype. Its significance depends on the accompanying architecture and the proportion present. |
| Heterologous elements | Sarcomatoid-associated feature | Rare bone-, cartilage-, or muscle-like malignant elements within mesothelioma. The term usually appears in specialist pathology reports and requires exclusion of other sarcomas. |
| Mesothelioma in situ | Preinvasive mesothelial diagnosis | A recognized preinvasive category, not one of the three invasive histologic subtypes. Diagnosis requires the proper clinicopathologic setting and may be supported by BAP1 or MTAP loss or CDKN2A deletion. A marker result alone is not enough to relabel an arbitrary specimen as mesothelioma in situ. |
| Well-differentiated papillary mesothelial tumor (WDPMT) | Related but distinct mesothelial tumor | The 2021 WHO classification renamed the entity formerly called well-differentiated papillary mesothelioma because it often behaves much more indolently than diffuse mesothelioma. It should not be presented as a fourth diffuse mesothelioma subtype. |
The word desmoplasia also deserves care. Desmoplasia can refer generally to dense fibrous stroma around a tumor, while desmoplastic mesothelioma is a specific sarcomatoid pattern with defined diagnostic considerations. The two phrases should not be treated as interchangeable without context.
Reading your pathology report
A useful report should identify not only the final diagnosis but also what material was examined and any limitations that affect confidence. Questions worth raising with the treating physician or pathologist include:
- What anatomical site and specimen are being described? Is this pleural fluid, a needle core, a thoracoscopic biopsy, a resection, a lymph node, or a metastatic site?
- What is the final diagnosis and histologic subtype? Is it epithelioid, sarcomatoid, biphasic, or not yet classifiable?
- What supports mesothelial lineage and malignancy? Which immunohistochemical markers were used, was invasion seen, and were BAP1, MTAP, or CDKN2A tests performed when appropriate?
- If the diagnosis is biphasic, are the component percentages estimated? Does the report make clear that the estimate applies to the specimen examined?
- If the diagnosis is epithelioid diffuse pleural mesothelioma, can the final WHO low/high tumor grade be assigned? If not, is the limitation due to specimen size, absent viable tumor, or another reason?
- Are architectural patterns, necrosis, transitional morphology, desmoplastic features, or other cytologic or stromal findings described?
- If this is a resection, are lymph nodes, margins, and pathological stage addressed?
- Does the report identify uncertainty or sampling limitations? Phrases such as “atypical mesothelial proliferation,” “cannot exclude,” “favor,” or “limited for classification” may signal an unresolved question.
- Would review by a pathologist with mesothelioma expertise add useful information before an irreversible treatment decision?
If you do not have the report, asking the hospital or doctor’s office for the pathology report—and, when needed, the operative report and imaging reports—is a reasonable first step. Patients may also need help arranging transfer of slides or tissue blocks for an outside review.
Want help organizing the questions raised by a pathology report? A member of our litigation team, not a medical screener, can review the document with you for case-intake purposes, help identify where subtype, grade, percentages, BAP1, MTAP, CDKN2A, PD-L1, or other terms appear, and help you prepare questions for your medical team. We do not diagnose disease or recommend treatment. The conversation is free and creates no obligation.
Call 833-4-ASBESTOS or request a free case review.
How histology connects to a potential claim
A pathology report can help establish that mesothelioma is present and describe its subtype. It can’t, by itself, identify when or where exposure occurred, which products or premises were involved, which entities may be legally responsible, which jurisdiction applies, or whether a particular trust, lawsuit, workers’ compensation system, or benefit program is available.
Histologic subtype ordinarily does not create or eliminate a claim by itself. A legal or benefit review may instead require:
- The complete diagnosis and medical record
- Occupational, military, household, environmental, and product-exposure history
- Evidence connecting the exposure to particular companies, worksites, or service
- The rules of any trust or benefit program
- Applicable statutes of limitation and repose
- Jurisdiction-specific medical and legal causation standards
- Consideration of liens, reimbursement rights, offsets, releases, and other coordination issues
Many mesotheliomas—especially pleural mesotheliomas—are associated with asbestos exposure, but an individual matter still requires evidence that satisfies the applicable medical, factual, and legal standards. A subtype does not create extra time under a statute of limitation, and population survival figures should not be used to assume how much time any person has to participate in a case.
Potential compensation, when available, may help address uncovered medical expenses, specialty-care travel, lost income, household assistance, or other losses. Neither recovery nor its amount or timing can be guaranteed, and medical care should not be delayed while a legal matter is evaluated.
References
- American Cancer Society. Mesothelioma. Last revised February 3, 2026. Accessed October 27, 2026. https://www.cancer.org/cancer/types/malignant-mesothelioma.html
- Sauter JL, Dacic S, Galateau-Salle F, et al. The 2021 WHO Classification of Tumors of the Pleura: advances since the 2015 classification. J Thorac Oncol. 2022;17(5):608-622. doi:10.1016/j.jtho.2021.12.014.
- Klebe S, Judge M, Brcic L, et al. Mesothelioma in the pleura, pericardium and peritoneum: recommendations from the International Collaboration on Cancer Reporting (ICCR). Histopathology. 2024;84(4):633-645. doi:10.1111/his.15106.
- Marinaccio A, Consonni D, Mensi C, et al.; ReNaM Working Group. Association between asbestos exposure and pericardial and tunica vaginalis testis malignant mesothelioma: a case-control study and epidemiological remarks. Scand J Work Environ Health. 2020;46(6):609-617. doi:10.5271/sjweh.3895.
- 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. doi:10.1016/j.jss.2015.01.043.
- Kindler HL, Ismaila N, Bazhenova L, et al. Treatment of pleural mesothelioma: ASCO guideline update. J Clin Oncol. 2025;43(8):1006-1038. doi:10.1200/JCO-24-02425.
- Husain AN, Chapel DB, Attanoos R, et al. Guidelines for pathologic diagnosis of mesothelioma: 2023 update of the consensus statement from the International Mesothelioma Interest Group. Arch Pathol Lab Med. 2024;148(11):1251-1271. doi:10.5858/arpa.2023-0304-RA.
- Chirieac LR, Hung YP, Foo WC, et al. Diagnostic value of biopsy sampling in predicting histology in patients with diffuse malignant pleural mesothelioma. Cancer. 2019;125(23):4164-4171. doi:10.1002/cncr.32416.
- Dacic S, Le Stang N, Husain A, et al. Interobserver variation in the assessment of the sarcomatoid and transitional components in biphasic mesotheliomas. Mod Pathol. 2020;33(2):255-262. doi:10.1038/s41379-019-0320-y.
- PDQ Adult Treatment Editorial Board. Malignant mesothelioma treatment (PDQ®): health professional version. National Cancer Institute. Updated May 12, 2025. Accessed October 27, 2026. https://www.cancer.gov/types/mesothelioma/hp/mesothelioma-treatment-pdq
- Cigognetti M, Lonardi S, Fisogni S, et al. BAP1 (BRCA1-associated protein 1) is a highly specific marker for differentiating mesothelioma from reactive mesothelial proliferations. Mod Pathol. 2015;28(8):1043-1057. doi:10.1038/modpathol.2015.65.
- Chapel DB, Schulte JJ, Berg K, et al. MTAP immunohistochemistry is an accurate and reproducible surrogate for CDKN2A fluorescence in situ hybridization in diagnosis of malignant pleural mesothelioma. Mod Pathol. 2020;33(2):245-254. doi:10.1038/s41379-019-0310-0.
Frequently asked questions
My report lists percentages of epithelioid and sarcomatoid morphology. Does that always mean biphasic mesothelioma?
It usually means the pathologist has identified both components, but the specimen type matters. In a definitive pleural resection, both components generally must reach 10% for classification as biphasic. In a biopsy, convincing malignant epithelioid and sarcomatoid components can support a biphasic diagnosis even when one component is below 10%. The report should be read together with the specimen description and any comment explaining how the classification rule was applied.2,3,7
The percentage is an estimate from the material examined. It does not establish an exact whole-tumor ratio or a fixed treatment category. See the biphasic mesothelioma page for the detailed discussion.
My first biopsy named one subtype, but a later review or resection named another. Was the first diagnosis wrong?
Not necessarily. Mesothelioma can be heterogeneous, and a small biopsy may sample only one region. A later, larger specimen can reveal a second component, different architecture, necrosis, or transitional morphology that was not present in the first material. A classification may also change after expert review or application of current WHO criteria.8,9
The important next step is to have the treating team compare the reports, specimen types, and slides and decide whether the revised classification changes staging, treatment, or trial eligibility. A later change can reflect improved information rather than an obvious error.
My report mentions a rare feature such as rhabdoid, deciduoid, or clear cell. Is that another cell type?
Usually not. Those are generally cytologic descriptors within a main subtype. The same is true of small-cell, signet-ring, pleomorphic, and lymphohistiocytoid features. Architectural terms such as tubulopapillary or solid describe cell arrangement, while stromal terms such as myxoid or desmoplastic describe supporting tissue or a particular fibrous pattern.
The exact significance depends on the main subtype, proportion, specimen type, and whether the feature creates a diagnostic mimic. The detailed subtype page is the best place to interpret the term in context.
Should every pathology report include a tumor grade?
No. The current WHO final low/high tumor-grade framework applies to epithelioid diffuse pleural mesothelioma. It is not assigned to an entire sarcomatoid or biphasic tumor. Even in epithelioid pleural disease, a small or poorly preserved biopsy may not provide enough representative tissue to assess mitotic activity, necrosis, and grade confidently.2,3,7
It is reasonable to ask whether grade can be assigned or whether specialist review would help. Treatment or symptom management should not be delayed solely to obtain a grade when the medical team believes care needs to begin.
Should I get a pathology second opinion even if I trust my doctor?
A second opinion can be reasonable without implying distrust. Mesothelioma is uncommon, its mimics are numerous, and treatment decisions may depend on accurate lineage, subtype, grade, and recognition of uncommon features. Review is particularly worth discussing when:
- The report uses uncertain or hedged language
- The sample was small, superficial, crushed, or cytology-only
- The clinical and imaging findings do not fit the pathology
- A possible sarcomatoid or desmoplastic tumor could be reactive fibrosis or another spindle-cell cancer
- A biphasic diagnosis or percentage depends on a small sample
- Surgery, a high-risk procedure, or clinical-trial enrollment is being considered
- A receiving specialty center requires its own review
The original laboratory can usually send slides, blocks, or digital images to the consulting center. The review may confirm the original diagnosis, add detail, or occasionally change it.7
What does it mean if the report mentions BAP1, MTAP, or CDKN2A?
These are commonly used ancillary tests when the pathologist is deciding whether a mesothelial proliferation is malignant. BAP1 or MTAP loss by immunohistochemistry, or homozygous CDKN2A deletion by FISH, strongly supports malignancy in the correct context. A normal or retained result does not rule out mesothelioma, and the tests do not establish mesothelial lineage by themselves.7,11,12
Tumor BAP1 loss does not show whether an alteration was inherited. Germline testing uses a separate blood or saliva sample and should be discussed with the medical and genetics team. A tumor or germline result also does not, by itself, prove or disprove the role of asbestos in an individual legal matter.
My loved one has already died. Can the family still investigate a claim?
Potential trust, estate, survival, and wrongful-death claims depend on the applicable jurisdiction, whether a claim was filed during life, the legally authorized representative, exposure evidence, and filing deadlines. Some deadlines begin at death and can be short. Prompt review is the safest way to determine which options may remain available; no particular claim or recovery can be assumed from the diagnosis alone.
I do not remember product brands or have old employment records. Can a review still begin?
Yes. An initial review can begin with what the patient or family remembers: employers, job titles, worksites, military service, trades nearby, tasks performed, approximate dates, household contacts, and places lived. Records, coworker evidence, product identification, military documents, union or pension materials, and trust or corporate records may be investigated later. The ability to establish a claim depends on the evidence ultimately available, not on arriving at the first call with a complete file.
What happens when I call, and what does it cost?
A member of our litigation team, not a medical screener, takes the initial call. We listen to the diagnosis and exposure history, review a pathology report if it is available, identify potential legal or benefit questions that may warrant investigation, and discuss the likely process without requiring a commitment on that call.
There is no charge for the initial review. Representation is offered on a contingency-fee basis when a matter is accepted, which means attorney fees are paid from a recovery rather than in advance. The exact agreement, costs, and allocation of any recovery are governed by the written representation agreement and applicable law. No recovery is guaranteed.
Talk with an asbestos lawyer about a mesothelioma diagnosis
A pathology report is one part of a larger medical and legal record. Whether the report says epithelioid, sarcomatoid, or biphasic, the next legal questions usually concern the exposure history, responsible entities, available evidence, jurisdiction, and deadlines, not whether one subtype is more or less favorable medically.
A member of our litigation team, not a screener, will take your call. We can review the terminology in the pathology report for case-intake purposes, listen to the work, service, household, and environmental history, and explain which potential claims or benefit routes may warrant investigation. We do not diagnose disease or recommend treatment, and no medical or legal result is promised.
Related resources
This page is provided for general informational purposes and does not constitute legal advice. Every asbestos case turns on its own facts, exposure history, diagnosis, deadlines, available evidence, and applicable law. Reviewing this page does not create an attorney-client relationship. For advice about your specific situation, call 833-4-ASBESTOS for a free case review.