Epithelioid mesothelioma is the most common form of mesothelioma, accounting for roughly 70% of all cases. It develops in the thin tissue lining the lungs, abdomen, or heart, and gets its name from the shape of its cancer cells, which resemble the flat epithelial cells that normally line these surfaces. Compared to the other two main types of mesothelioma (sarcomatoid and biphasic), epithelioid carries the best prognosis and responds most favorably to treatment.
How Epithelioid Cells Differ From Other Types
Mesothelioma is classified into three cell types based on how the tumor looks under a microscope. Epithelioid cells are polygonal, oval, or cube-shaped, with round nuclei and visible internal structures called nucleoli. They tend to cluster in recognizable patterns, sometimes forming berry-like tissue fragments with clear spaces between cells. This orderly growth is part of why epithelioid tumors are slower to spread than their counterparts.
Sarcomatoid mesothelioma, by contrast, features elongated spindle-shaped cells that behave more aggressively and resist treatment. Biphasic mesothelioma contains a mix of both cell types, and its prognosis depends on which type dominates. When a biopsy shows predominantly epithelioid cells, treatment options expand significantly.
Subtypes Within Epithelioid Mesothelioma
Not all epithelioid mesotheliomas behave the same way. Pathologists identify several growth patterns within this category, and the specific pattern influences outcomes. The most common are solid, tubulopapillary, and trabecular. Among these, tubulopapillary and trabecular patterns tend to carry the most favorable prognosis, as do tumors with a myxoid (gel-like) stroma making up more than half the tissue.
On the other end of the spectrum, the pleomorphic subtype behaves much more like sarcomatoid mesothelioma, with poor survival rates despite being technically classified as epithelioid. The transitional pattern, where cells sit somewhere between epithelioid and spindle-shaped, also carries a similarly aggressive course. The small cell variant is rare and has a reported mean survival of just over 8 months. Deciduoid mesothelioma, first described in 1985, is extremely uncommon (fewer than 50 pleural cases in the medical literature) but tends to have outcomes closer to typical epithelioid disease.
These distinctions matter because your pathology report may specify a subtype that gives your oncology team a clearer picture of how the disease is likely to progress.
Causes and Latency Period
Asbestos exposure is the primary cause. When microscopic asbestos fibers are inhaled or swallowed, they embed in the mesothelial lining and cause chronic irritation and DNA damage over decades. The latency period is unusually long, sometimes up to 40 years between first exposure and diagnosis. Many people diagnosed today were exposed to asbestos in the 1970s or 1980s through construction, shipbuilding, automotive work, or military service.
Because the disease develops so slowly, it is almost always diagnosed in adults over 60. There is no reliable way to screen for it early, which is why most cases are found at an advanced stage.
Symptoms and How It’s Found
Pleural mesothelioma (the most common site) typically presents with chest pain and shortness of breath. The shortness of breath usually signals a pleural effusion, a buildup of fluid between the lung and chest wall that occurs in about 90% of patients at initial diagnosis. Other symptoms are less specific: unintentional weight loss, loss of appetite, persistent cough, fatigue, or a noticeable mass along the chest wall.
As the disease progresses, it can invade the chest wall, diaphragm, or the space around the heart. Because these early symptoms overlap with many common conditions, mesothelioma is frequently misdiagnosed as pneumonia, COPD, or even heart failure before imaging and biopsy reveal the true cause.
How It’s Diagnosed
A definitive diagnosis requires a tissue biopsy, not just imaging. Under the microscope, pathologists use a panel of protein stains to confirm epithelioid mesothelioma and rule out cancers that can look similar, particularly lung adenocarcinoma. Epithelioid mesothelioma cells test positive for markers like calretinin, podoplanin, and mesothelin, while testing negative for markers typically found in lung cancer or cancers that have spread from other organs. This staining panel is essential because treatment for mesothelioma differs substantially from treatment for lung cancer, and a misdiagnosis could mean the wrong therapy.
Treatment Options
Treatment depends on the stage at diagnosis, overall health, and the specific subtype. Epithelioid histology opens the widest range of options among all mesothelioma types.
Surgery
The best surgical candidates have early-stage, low-volume epithelioid disease and good overall fitness. Surgery typically involves removing the visible tumor along with the affected pleural lining, and it is most effective when performed by a surgeon with specific mesothelioma expertise as part of a multidisciplinary team. Patients with biphasic tumors that are predominantly sarcomatoid, widespread disease, or poor functional status are generally not considered good surgical candidates. Notably, a recent clinical trial (MARS 2) found that patients who received chemotherapy alone actually had longer median survival (24.8 months) and better quality of life than those who had surgery plus chemotherapy (19.3 months), raising important questions about when surgery truly helps.
Chemotherapy
For epithelioid mesothelioma specifically, the standard chemotherapy combination is pemetrexed with a platinum-based drug (typically cisplatin or carboplatin). Adding bevacizumab, a drug that starves tumors of their blood supply, has been shown to push median survival to about 18.8 months. This chemotherapy backbone remains a cornerstone of treatment, either alone or combined with immunotherapy.
Immunotherapy
Immunotherapy has changed the treatment landscape. Current guidelines from ASCO recommend two immunotherapy-based approaches as first-line options. The combination of ipilimumab and nivolumab, two drugs that help your immune system recognize and attack cancer cells, improved median survival to 18.1 months compared to 14.1 months with chemotherapy alone in a major trial with over three years of follow-up.
A newer option combines pembrolizumab (another immune checkpoint drug) with standard chemotherapy. This combination received FDA approval in September 2024 for unresectable advanced mesothelioma. In patients with epithelioid histology, median overall survival reached 19.8 months with the combination versus 18.2 months with chemotherapy alone. The benefit was more dramatic in nonepithelioid patients, but the combination remains a strong option for epithelioid disease as well.
For patients with epithelioid mesothelioma, the choice between immunotherapy alone and chemotherapy (with or without immunotherapy) is typically individualized based on other health conditions, kidney function, and tolerance for different side effect profiles.
Prognosis and Survival
Epithelioid mesothelioma has the longest survival of the three cell types, but the numbers are still sobering. Advanced-stage disease treated with systemic therapy alone carries a median survival of approximately one year. With more aggressive approaches combining chemotherapy, immunotherapy, or surgery, median survival stretches to roughly 17 to 19 months in clinical trials.
Several factors influence individual prognosis beyond cell type. Early-stage disease, younger age, good functional status, and favorable histological subtypes (like tubulopapillary) all correlate with longer survival. The presence of a pleomorphic or transitional growth pattern within an epithelioid tumor, conversely, can shift the prognosis closer to that of sarcomatoid disease.
Patients with epithelioid histology also show statistically significant improvements in progression-free and overall survival when treated with immunotherapy, a benefit not consistently seen in nonepithelioid patients. This makes accurate pathological classification at diagnosis critically important for choosing the right treatment path.