Epithelioid Mesothelioma: Subtypes, Staging, and Treatment

Epithelioid mesothelioma is the most common subtype of malignant mesothelioma, a cancer that arises from the thin lining surrounding the lungs, abdomen, or heart. Among the three major histologic subtypes, epithelioid carries the best prognosis, with a median survival of about 14 months compared with 10 months for biphasic and 4 months for sarcomatoid disease. That relative advantage shapes nearly every decision about treatment, from whether surgery is worth pursuing to which drug combinations are offered first. But “best prognosis among mesotheliomas” is still a grim benchmark, and understanding what makes epithelioid tumors behave differently is useful for anyone navigating this diagnosis.

How Epithelioid Mesothelioma Differs From Other Subtypes

The World Health Organization classifies diffuse malignant mesothelioma into three major histologic subtypes: epithelioid, biphasic, and sarcomatoid.¹ Epithelioid tumors are made up of cells that still resemble the normal mesothelial cells lining body cavities. They tend to grow in organized patterns and divide more slowly. Sarcomatoid tumors, by contrast, consist of spindle-shaped cells that look and act more like connective-tissue cancers. They invade surrounding structures aggressively and respond poorly to most treatments. Biphasic tumors contain a mixture of both cell types, and their behavior depends largely on how much of the sarcomatoid component is present.

A large analysis of U.S. registry data found that patients with epithelioid mesothelioma had a median survival of 14 months. Those with biphasic disease survived a median of 10 months, and those with sarcomatoid disease survived just 4 months. Sarcomatoid histology carried roughly two and a half times the risk of death compared with epithelioid, even after adjusting for age and stage.² The subtype distinction matters so much that it drives eligibility for certain clinical trials, surgical approaches, and newer combination therapies.

Causes and the Long Road From Exposure to Diagnosis

Asbestos exposure remains the dominant cause of malignant mesothelioma across all subtypes. Inhaled or ingested asbestos fibers lodge in mesothelial tissue and set off decades of chronic inflammation. Research has clarified some of the molecular steps involved, including the roles of a protein called HMGB1 and an inflammatory signaling complex called the inflammasome in driving cells toward malignancy.³ Other mineral fibers such as erionite and fluoroedenite, as well as inherited mutations in genes like BAP1, also play a role in some cases.⁴

One of the most striking features of mesothelioma is its latency period. An Italian registry study found a median gap of about 45 years between first asbestos exposure and mesothelioma diagnosis, and that gap has been growing in more recent cases. People exposed through their jobs had a somewhat shorter latency (around 43 years) than those exposed environmentally or through household contact (around 48 years).⁵ Timing of exposure matters as well: people who received high doses of asbestos early in their work history were at greater risk than those who accumulated the same total dose more gradually over time.⁶

Industrial knowledge about asbestos hazards grew slowly, with early recognition in the early 1900s that high chronic exposure posed a health threat, but the carcinogenic nature of asbestos was not widely characterized until the mid-1950s. Improved sampling methods in the late 1960s and 1970s allowed more precise exposure assessment and eventually led to meaningful occupational limits.⁷ Meanwhile, industry groups actively worked to downplay the threat in order to slow regulation.⁸ That decades-long delay in protective action helps explain why mesothelioma cases continued to climb long after the science was clear.

Diagnosing Epithelioid Mesothelioma

Getting the diagnosis right is harder than it might seem, because epithelioid mesothelioma can look remarkably similar to lung adenocarcinoma under the microscope. Both cancers form glandular-looking structures, and a wrong call sends a patient down an entirely different treatment path. Pathologists rely on panels of staining markers to distinguish the two. A comprehensive study found that calretinin, cytokeratin 5/6, and WT1 are the best “positive” markers for mesothelioma, meaning nearly all epithelioid mesotheliomas stain positive for them while very few lung adenocarcinomas do. On the flip side, markers like CEA, MOC-31, and Ber-EP4 light up in virtually all adenocarcinomas but rarely in mesotheliomas.⁹

A newer marker called glypican-1 has shown promise as well, achieving 100% sensitivity and 97% specificity for distinguishing epithelioid mesothelioma from lung adenocarcinoma in one study. That performance compared favorably to established markers: WT1, for example, has perfect specificity but catches only about four in five mesotheliomas, while calretinin catches nearly all mesotheliomas but has somewhat lower specificity.¹⁰

Molecular Tests That Aid Difficult Cases

When tissue samples are small or ambiguous, molecular tests can tip the diagnostic balance. Loss of BAP1 protein expression and deletion of a gene region called CDKN2A/p16 are two changes strongly associated with mesothelioma. One study found that morphologic examination alone caught only about 30% of mesotheliomas in fluid samples, but adding BAP1 and p16 testing more than doubled that sensitivity to nearly 69%.¹¹ A separate study confirmed that combining BAP1 loss with p16 deletion could push sensitivity to about 81% with specificity around 95%, making this combination particularly useful for confirming malignancy in equivocal pleural fluid specimens.¹²

Not All Epithelioid Tumors Behave the Same

Even within the epithelioid category, pathologists can identify growth patterns and features that meaningfully affect outcomes. One study grouped epithelioid mesotheliomas into three tiers based on their microscopic architecture: tumors with tubulopapillary or microcystic patterns survived significantly longer (median around 732 days) than those with solid or trabecular patterns (about 397 days). The worst performers were pleomorphic tumors, which survived a median of only 173 days.¹³

Nuclear grade, a measure of how abnormal the tumor cells look under the microscope, carries independent prognostic weight as well. A study comparing long-term survivors with typical patients found that 90% of long survivors had the lowest nuclear grade, and the presence of an unusual exophytic polypoid growth pattern was also independently linked to prolonged survival.¹⁴ The practical takeaway: asking your pathologist about the specific architectural subtype and nuclear grade within an epithelioid diagnosis gives a more precise picture than the subtype label alone.

Treatment Options for Epithelioid Mesothelioma

Treatment typically involves some combination of surgery, chemotherapy, immunotherapy, and radiation. Epithelioid histology is generally a prerequisite for the most aggressive multimodality approaches, since the tumor’s relatively better behavior makes the risks of intensive treatment more justifiable.

Surgery

Two main operations are used for pleural mesothelioma. Extrapleural pneumonectomy removes the affected lung, the pleural lining, the diaphragm on that side, and part of the pericardium. Pleurectomy/decortication strips away the diseased pleural lining but spares the lung. A systematic review found that pleurectomy/decortication had a lower 30-day mortality (median around 2%) and better overall survival (median about 21 months) than extrapleural pneumonectomy (about 6% mortality and roughly 18 months of overall survival).¹⁵ An earlier analysis of over 660 patients confirmed that epithelioid histology was among the strongest predictors of surgical benefit, and noted that the choice between the two procedures should be tailored to the extent of disease and the planned multimodality approach.¹⁶

Chemotherapy

The backbone of systemic therapy for mesothelioma is a platinum drug combined with pemetrexed. Epithelioid tumors respond better to this regimen than non-epithelioid tumors. A real-world cohort study found an objective response rate of 38% in epithelioid patients versus 22% in non-epithelioid patients. Median overall survival for epithelioid patients receiving cisplatin-pemetrexed was about 31 months, compared with roughly 17 months for non-epithelioid patients on the same regimen.¹⁷ For patients who are candidates for surgery, combining this chemotherapy with surgery achieves pathological complete response in about 5% of cases, and ongoing trials are testing whether adding immunotherapy to that combination can improve that rate further.¹⁸

Immunotherapy

The CheckMate 743 trial reshaped the treatment landscape in 2020 by showing that the combination of nivolumab plus ipilimumab, two immune checkpoint inhibitors, significantly extended survival compared with standard platinum-pemetrexed chemotherapy. Median overall survival was about 18 months with immunotherapy versus about 14 months with chemotherapy, and the two-year survival rate was 41% versus 27%.¹⁹ Three-year follow-up confirmed that this benefit held across histologic subtypes.²⁰

An important nuance: the immunotherapy benefit was initially most striking in non-epithelioid patients, who traditionally had the fewest effective options. Epithelioid patients still benefited, but the relative advantage over chemotherapy was narrower for them, likely because they already respond relatively well to platinum-pemetrexed. This has created an ongoing debate about whether epithelioid patients should start with chemotherapy, immunotherapy, or some combination. Recent trials are exploring adding pembrolizumab to chemotherapy as a potential way to capture the benefits of both approaches.

Tumor Treating Fields

A more unconventional treatment called Tumor Treating Fields (TTFields) uses a wearable device to deliver low-intensity electric fields to the thorax, disrupting cell division. The STELLAR trial led to approval for this device in unresectable mesothelioma when combined with pemetrexed and a platinum drug. Overall survival in STELLAR was about 18 months, with further analysis showing extended survival specifically in patients with epithelioid histology. The main side effect was mild-to-moderate skin reactions where the device pads contact the chest.²¹ Laboratory research suggests the fields work in part by increasing how much drug reaches the inside of tumor cells: mouse experiments showed roughly a 30% increase in drug concentration within mesothelioma tumors when TTFields were applied alongside chemotherapy.²²

Radiation

Radiation in mesothelioma typically plays a supporting role rather than a primary one. Intensity-modulated radiation therapy (IMRT) directed at the entire affected hemithorax can be given after surgery to reduce local recurrence. A phase II trial found that combining IMRT with chemotherapy and lung-sparing surgery produced a median overall survival of about 24 months with an acceptable rate of lung inflammation.²³ Whether radiation is given before or after surgery does not appear to make a major survival difference, though neoadjuvant (pre-surgical) radiation has been explored in highly selected patients at specialized centers.²⁴ Proton therapy offers a technical advantage by delivering almost no radiation beyond the target area, which helps protect the opposite lung.²⁵

The Immune Microenvironment and Why It Matters

One reason epithelioid and non-epithelioid tumors respond differently to immunotherapy lies in their immune microenvironment. Research has found that sarcomatoid and biphasic mesotheliomas tend to have more immune-activating features within the tumor itself: higher levels of a type of immune cell called CD8+ T cells and greater expression of PD-L1 on tumor cell surfaces. Epithelioid tumors, on the other hand, show higher levels of CD4+ T cells and B cells at the tumor’s edges.²⁶ This difference helps explain why checkpoint inhibitors can produce dramatic responses in some non-epithelioid patients whose tumors are already inflamed and “primed” for immune attack, while epithelioid tumors may be somewhat less responsive to this approach even though they carry a better overall prognosis.

Still, occasional epithelioid cases show remarkable responses to immunotherapy. A case report described an epithelioid mesothelioma patient who achieved an impressive clinical response to anti-PD-1 therapy, with PD-L1 expression varying from 10% in the original pleural biopsy to 100% in a metastatic brain lesion, illustrating how PD-L1 levels can differ dramatically even within the same patient’s disease.²⁷

Epithelioid Peritoneal Mesothelioma

Though most discussion of mesothelioma focuses on the pleural form, mesothelioma also arises in the peritoneum, the lining of the abdominal cavity. Epithelioid histology dominates here even more heavily than in pleural disease, and the treatment approach is quite different. The standard for peritoneal mesothelioma is cytoreductive surgery combined with heated intraperitoneal chemotherapy (CRS/HIPEC), where chemotherapy drugs are heated and circulated directly through the abdomen during surgery. Median overall survival with this approach is around 50 months, substantially longer than what is typically seen in pleural disease.²⁸

Completeness of surgical removal is one of the most powerful prognostic factors. Australian data showed that epithelioid peritoneal mesothelioma patients who achieved a complete cytoreduction had a median survival of over 87 months, well over seven years.²⁹ A recent multisocietal panel unanimously recommended CRS/HIPEC for carefully selected peritoneal mesothelioma patients, issuing a strong recommendation despite limited formal evidence, based on the life-threatening nature of the disease and the limited benefit of systemic chemotherapy alone.³⁰

Blood Biomarkers for Monitoring

Following mesothelioma over time currently depends heavily on imaging, but blood-based biomarkers are becoming clinically relevant. Soluble mesothelin-related peptides (SMRP) can be measured via a simple blood draw, and levels correlate with tumor burden as measured on imaging. A pilot study found that serial SMRP measurements tracked closely with imaging response scores, suggesting that this blood test could help guide how often patients need CT scans.³¹ Other biomarkers under investigation include osteopontin, fibulin-3, HMGB1, and microRNAs, though none has yet achieved enough validation for routine standalone use.³² For epithelioid mesothelioma specifically, mesothelin-based markers tend to be most useful since the protein is highly expressed by epithelioid cells.

CAR T Cells and the Mesothelin Target

The same mesothelin protein that makes a useful blood biomarker also makes an attractive target for a newer type of immunotherapy: chimeric antigen receptor (CAR) T cell therapy. In this approach, a patient’s own T cells are engineered in a laboratory to recognize mesothelin on the surface of mesothelioma cells, then infused back to attack the tumor. Early-phase clinical trials have confirmed that this approach is relatively safe in mesothelioma patients. The challenge is efficacy. The tumor microenvironment in mesothelioma is powerfully immunosuppressive, reducing the ability of CAR T cells to infiltrate the tumor, persist, and kill cancer cells.³³ Research is ongoing to overcome these barriers, including modifying the CAR T cells to resist immune suppression and combining them with checkpoint inhibitors. Given that mesothelin is abundantly expressed in the majority of epithelioid mesotheliomas and barely present in healthy tissue, this subtype remains the prime candidate for mesothelin-directed therapies as the field matures.

Staging, Imaging, and the Cost of Care

Staging mesothelioma relies primarily on CT scanning, but MRI and PET-CT with a radioactive glucose tracer can add valuable information in select cases, particularly for assessing whether the tumor has spread to the chest wall or diaphragm. There is a practical caveat: PET-CT results can be less reliable in patients who have already undergone pleurodesis, a procedure to fuse the pleural layers and prevent fluid buildup.³⁴

The cost dimension of mesothelioma treatment is worth acknowledging. A cost-effectiveness analysis of the CheckMate 743 immunotherapy combination found that among patients with epithelioid histology, the cost per quality-adjusted life year gained was around $761,000, far exceeding a commonly used willingness-to-pay threshold. The probability that immunotherapy represented an economic advantage for epithelioid patients specifically was estimated at less than 1%.³⁵ This does not mean the treatment should never be used in this population. It does mean that access, insurance coverage, and conversations about goals of care all become part of the decision, especially since epithelioid patients may get comparable benefit from the less expensive chemotherapy backbone.

A multidisciplinary approach to symptom management is considered critical throughout the course of the disease, including proactive handling of pleural effusions, which cause breathlessness and significantly affect quality of life.³⁶ For many patients, the comfort and function gained from controlling fluid buildup and managing pain may matter more day-to-day than the choice of systemic therapy.