Peritoneal Mesothelioma: Symptoms, Diagnosis, Treatment and Prognosis

Peritoneal Mesothelioma at a Glance

What Is Peritoneal Mesothelioma?

The peritoneum is a thin membrane that lines the inside of the abdomen and covers many abdominal organs.

Peritoneal mesothelioma begins when mesothelial cells in this lining become malignant. Rather than forming one isolated mass, invasive disease commonly spreads across multiple peritoneal surfaces.

The term diffuse malignant peritoneal mesothelioma, or DMPM, has traditionally been used for the invasive form of the disease. Modern pathology also recognizes several other mesothelial lesions of the abdomen that have very different behavior and should not be managed as invasive mesothelioma.

Peritoneal and pleural mesothelioma share some biological characteristics, but they are not interchangeable diseases. Evidence from pleural-mesothelioma trials should therefore not automatically be assumed to apply to tumors arising in the abdomen.

How Common Is Peritoneal Mesothelioma?

Peritoneal mesothelioma is extremely rare.

Current U.S. consensus literature estimates an incidence of approximately 0.1 cases per 100,000 people, corresponding to roughly 300–400 new diagnoses each year in the United States. Pleural mesothelioma remains substantially more common.

Because relatively few patients develop peritoneal disease, much of the evidence guiding treatment comes from specialized referral centers, retrospective cohorts, and expert consensus rather than large randomized phase III trials.

This limitation should be considered when interpreting treatment outcomes and survival statistics.

What Causes Peritoneal Mesothelioma?

Asbestos exposure

Asbestos is the best-established environmental risk factor for peritoneal mesothelioma.

Asbestos refers to a group of naturally occurring mineral fibers that were historically used in construction, insulation, shipbuilding, and many industrial products.

When asbestos-containing materials are disturbed, microscopic fibers may become airborne and be inhaled. Some fibers can remain in the body for many years and contribute to biological changes associated with mesothelioma development.

However, the association with asbestos is less consistently documented in peritoneal mesothelioma than in pleural mesothelioma. Some people diagnosed with peritoneal disease have no recognized history of asbestos exposure.

A history of exposure may inform epidemiologic understanding, but it alone cannot confirm or exclude the diagnosis. Pathologic diagnosis relies on morphologic, immunohistochemical, and, when needed, molecular findings. Our asbestos exposure guide covers this topic in more detail.

Does smoking cause peritoneal mesothelioma?

Smoking is not considered an established cause of mesothelioma.

While smoking is a significant risk factor for several cancers, especially lung cancer, it is not considered a cause of peritoneal mesothelioma.

Genetic susceptibility

Inherited cancer-predisposition variants contribute to mesothelioma risk in some patients.

The best-known example involves BAP1, a tumor-suppressor gene associated with BAP1 tumor predisposition syndrome.

Current ASCO guidance recommends that all patients with mesothelioma be offered germline testing. Patients offered testing should also be offered pretest genetic counseling with a qualified health professional.

Symptoms of Peritoneal Mesothelioma

Symptoms can develop gradually and depend partly on the amount and location of disease within the abdomen.

Common presentations may include:

• increasing abdominal size or swelling;
• abdominal discomfort or pain;
• ascites, or fluid accumulation in the abdomen;
• reduced appetite;
• feeling full after eating only a small amount;
• unintended weight loss;
• nausea;
• constipation or other changes in bowel function;
• fatigue.

Some patients are first evaluated after an imaging study performed for another reason reveals ascites, peritoneal thickening, nodules or abnormal omental tissue.

These findings are not specific to mesothelioma. Gastrointestinal disorders, liver disease, inflammatory conditions and other cancers can produce similar symptoms.

Persistent or unexplained abdominal swelling, pain, appetite changes, or weight loss should be evaluated by a healthcare professional rather than attributed to mesothelioma based solely on symptoms. Our symptoms guide covers the broader range of possible signs across all mesothelioma types.

How Peritoneal Mesothelioma Is Diagnosed

Diagnosis usually requires several complementary steps. Our diagnosis guide covers this process in more detail.

Medical history and examination

The initial evaluation typically includes symptoms, medical and surgical history, overall health, and possible occupational or environmental exposure to asbestos.

A known exposure history may raise clinical suspicion, but lack of a recalled exposure does not rule out the disease.

CT imaging

Contrast-enhanced CT of the abdomen and pelvis is commonly used when a peritoneal malignancy is suspected.

Possible findings include:

• ascites;
• peritoneal thickening;
• peritoneal nodules or masses;
• omental involvement;
• bowel or mesenteric involvement;
• enlarged lymph nodes;
• disease outside the abdominal cavity.

Imaging also assists specialists in assessing disease distribution and determining whether complete or near-complete surgical cytoreduction is technically feasible.

However, CT findings alone cannot definitively diagnose peritoneal mesothelioma.

MRI and PET/CT

MRI may provide additional anatomical detail in selected patients.

PET/CT can sometimes help evaluate metabolically active disease or possible disease beyond the peritoneal cavity.

Neither test substitutes for a tissue diagnosis. Our imaging tests guide covers CT, PET, and MRI in more detail.

Ascitic fluid

Some patients undergo paracentesis to remove abdominal fluid.

The procedure may relieve symptoms and allows the fluid to be examined for malignant cells.

However, cytology alone may be insufficient to distinguish peritoneal mesothelioma from other malignancies or to determine the full histologic pattern. Obtaining adequate tissue is often necessary.

Biopsy

Tissue may be obtained through:

• image-guided core biopsy;
• laparoscopy;
• or another surgical biopsy procedure.

Laparoscopy can allow direct inspection of peritoneal surfaces, collection of multiple tissue samples and, in selected cases, assessment of disease distribution before major cytoreductive surgery.

The optimal biopsy method depends on imaging findings, overall health, and the planned treatment strategy. Our tests and biopsy guide covers these procedures in more detail.

How Pathologists Confirm Peritoneal Mesothelioma

Peritoneal mesothelioma can resemble other tumors, particularly cancers that have spread to the peritoneum from the ovaries, gastrointestinal tract or other organs.

Diagnosis therefore depends on the microscopic appearance of the tumor together with a carefully selected panel of immunohistochemical markers that establish mesothelial lineage and help exclude competing diagnoses. IMIG guidance emphasizes that diagnosis should be based on adequate tissue and appropriate morphologic, immunohistochemical, and clinical-radiologic correlation.

BAP1, MTAP and CDKN2A

Additional tests may help distinguish malignant mesothelioma from benign or reactive mesothelial proliferations.

These can include:

• BAP1 immunohistochemistry;
• MTAP immunohistochemistry;
• CDKN2A fluorescence in situ hybridization, or FISH.

Loss of nuclear BAP1 can be particularly useful in peritoneal mesothelial lesions.

An important peritoneal-specific distinction is that MTAP loss and CDKN2A homozygous deletion occur much less often in peritoneal than in pleural mesothelioma. IMIG reports MTAP loss in roughly 5%–10% and CDKN2A homozygous deletion in approximately 10%–15% of peritoneal mesotheliomas. A normal result therefore does not exclude malignancy.

These tests should be interpreted within the context of a comprehensive pathology assessment, not as standalone cancer tests.

Not Every Peritoneal Mesothelial Tumor Is Malignant Mesothelioma

This distinction is especially important in abdominal pathology.

Other mesothelial lesions include well-differentiated papillary mesothelial tumor, or WDPMT, and peritoneal inclusion cysts. These conditions have different biological behavior and treatment approaches from diffuse invasive peritoneal mesothelioma.

Pathology reports should specify the exact tumor entity rather than using the broad term “mesothelioma” without proper qualification.

When the diagnosis is uncertain, specialist pathology review is particularly valuable before major surgery or systemic treatment.

Histologic Types of Peritoneal Mesothelioma

The major invasive histologic categories are:

IMIG notes that sarcomatoid histology accounts for less than 5% of peritoneal mesotheliomas, while biphasic tumors account for roughly 10%–20%.

Histology influences prognosis and treatment strategy, but it is only one aspect of the clinical assessment. Disease burden, lymph-node involvement, extra-abdominal disease, potential for cytoreduction, performance status, and overall health are also critical.

Genetic Counseling and Germline Testing

Genetics has become an increasingly important part of modern mesothelioma care.

The 2025 ASCO guideline recommends that all patients with mesothelioma be offered germline testing and that patients offered testing also be offered pretest genetic counseling by a qualified health professional.

Testing may identify inherited pathogenic variants involving BAP1 or other genes associated with hereditary cancer risk.

A positive result may have implications for:

• assessment of additional cancer risks;
• future cancer surveillance;
• biological relatives who may carry the same inherited variant;
• prognosis in selected circumstances;
• and potentially clinical-trial eligibility.

Germline testing differs from tumor-only genomic sequencing. If tumor sequencing indicates a possible inherited alteration, confirmatory germline testing and genetic counseling are recommended.

How Is Peritoneal Mesothelioma Staged?

Peritoneal mesothelioma should not simply be staged with the AJCC pleural mesothelioma Stage I–IV system.

A research-based TNM model has been proposed for diffuse malignant peritoneal mesothelioma using:

• peritoneal tumor burden;
• lymph-node involvement;
• and distant metastases.

However, this system has not become a universally used clinical equivalent of the AJCC pleural staging system. Our mesothelioma stages guide covers how the AJCC system is used for pleural disease.

For practical treatment planning, specialists often focus instead on:

• histology;
• distribution of disease;
• the Peritoneal Cancer Index;
• lymph-node involvement;
• extra-abdominal disease;
• surgical fitness;
• and the likelihood of achieving complete cytoreduction.

Understanding the Peritoneal Cancer Index

The Peritoneal Cancer Index, or PCI, is a surgical-oncology tool used to describe the extent and distribution of peritoneal tumor.

It considers disease in different regions of the abdomen together with the size of tumor deposits.

In general, increasing PCI reflects a greater volume or broader distribution of peritoneal disease.

However, PCI is not a sole determinant for treatment decisions.

Two patients with similar PCI values may have very different surgical options depending on:

• where the tumor is located;
• involvement of the small bowel or mesentery;
• histology;
• lymph-node disease;
• disease outside the abdomen;
• performance status;
• and whether complete cytoreduction appears achievable.

How Specialists Assess Treatment Risk

Treatment of Peritoneal Mesothelioma

There is no single treatment plan appropriate for every patient.

Treatment planning may consider:

• histologic subtype;
• peritoneal tumor burden;
• likelihood of complete cytoreduction;
• lymph-node disease;
• disease outside the abdomen;
• performance status;
• medical fitness for major surgery;
• previous treatment;
• symptoms;
• patient preferences;
• and clinical-trial availability.

Current consensus emphasizes multidisciplinary assessment and recognizes that several key questions, especially regarding the optimal timing of systemic therapy in relation to surgery, remain unresolved. Our treatment guide covers the full range of approaches used across all mesothelioma types.

Cytoreductive Surgery

Cytoreductive surgery, or CRS, aims to remove all or nearly all visible peritoneal tumor that can be safely removed.

It is not one standardized operation.

Depending on where the disease is located, surgery may involve removal of affected peritoneal surfaces and, when necessary, portions of involved abdominal organs.

The goal is commonly described as complete or near-complete macroscopic cytoreduction.

Completeness of cytoreduction is one of the strongest prognostic factors consistently identified in specialist surgical series.

However, CRS is a major operation and may not be appropriate for all patients.

What Is HIPEC?

HIPEC stands for hyperthermic intraperitoneal chemotherapy.

After cytoreductive surgery, heated chemotherapy is circulated inside the abdominal cavity while the patient remains in the operating room.

The goal is to expose microscopic residual tumor cells within the peritoneal cavity to chemotherapy after as much visible tumor as possible has been removed.

HIPEC therefore works as part of a combined CRS-HIPEC strategy rather than as a substitute for cytoreductive surgery.

Specialist centers may use varying chemotherapy drugs and protocols. No universally standardized HIPEC regimen exists for peritoneal mesothelioma.

Who May Be Considered for CRS-HIPEC?

Patient selection is critical.

Factors considered by specialist teams can include:

• whether disease is confined mainly to the abdomen;
• epithelioid versus non-epithelioid histology;
• amount and distribution of tumor;
• bowel and mesenteric involvement;
• lymph-node disease;
• distant metastases;
• likelihood of complete cytoreduction;
• functional status;
• cardiovascular and general surgical fitness;
• nutrition;
• and patient preferences.

Patients not eligible for CRS-HIPEC may still have options including systemic therapy, clinical trials, and symptom management.

Unresectable disease does not mean untreatable disease.

How Strong Is the Evidence for CRS-HIPEC?

CRS-HIPEC has become the major specialist locoregional treatment for appropriately selected patients with diffuse peritoneal mesothelioma.

A multisocietal consensus supports CRS-HIPEC for carefully selected patients, while also emphasizing that the certainty of evidence is limited because randomized trials are lacking and much of the survival evidence comes from observational specialist-center studies.

This distinction matters.

CRS-HIPEC should neither be dismissed as an unproven experimental procedure nor presented as a treatment proven by randomized trials to benefit every patient.

Selection is central to the outcome.

What Are the Risks of CRS-HIPEC?

CRS-HIPEC is a major abdominal treatment and can involve significant complications.

In a multi-institutional series of 401 patients summarized by the NCI, 31% experienced grade 3–4 complications and 2% died in the perioperative period. The same cohort included highly selected patients treated at centers experienced with cytoreductive surgery.

Potential risks vary based on the extent of surgery and individual health, and should be discussed with the surgical team.

These risks highlight the need to balance potential long-term benefits with operative morbidity, rather than focusing solely on tumor removal.

Systemic Chemotherapy

Systemic therapy can be important when:

• surgery is not appropriate;
• complete cytoreduction is unlikely;
• disease is higher risk;
• cancer has spread outside the abdomen;
• disease recurs;
• or systemic treatment forms part of a multimodality strategy.

For epithelioid peritoneal mesothelioma, current consensus guidance includes pemetrexed plus a platinum drug, usually cisplatin or carboplatin, with consideration of bevacizumab in appropriately selected patients.

Evidence for chemotherapy specific to peritoneal mesothelioma is much less extensive than for many common cancers.

The optimal timing of chemotherapy in relation to CRS-HIPEC remains uncertain.

Immunotherapy

Immune-checkpoint inhibitors have become increasingly important in mesothelioma treatment, but the evidence base for peritoneal mesothelioma is smaller than for pleural disease.

The pivotal CheckMate 743 trial that established nivolumab plus ipilimumab enrolled patients with pleural mesothelioma, not peritoneal mesothelioma. Its survival results therefore should not simply be transferred to patients with disease arising in the abdomen.

Nevertheless, newer peritoneal-specific consensus guidance incorporates immunotherapy into systemic-treatment pathways. For epithelioid disease, nivolumab plus ipilimumab is listed among preferred first-line options alongside platinum-pemetrexed-based treatment and clinical-trial enrollment. For biphasic or sarcomatoid peritoneal mesothelioma, the consensus favors nivolumab plus ipilimumab as a preferred first-line systemic approach.

The important limitation is that these recommendations partly rely on extrapolation from pleural mesothelioma and smaller real-world peritoneal datasets rather than a dedicated randomized phase III peritoneal-mesothelioma trial.

In the United States, the FDA-labeled mesothelioma indication for nivolumab plus ipilimumab is specifically unresectable malignant pleural mesothelioma, not peritoneal mesothelioma.

Pembrolizumab: Peritoneal-Specific Evidence

There is also limited evidence specific to pembrolizumab in peritoneal disease.

A retrospective two-center study evaluated 24 adults with diffuse peritoneal mesothelioma treated with pembrolizumab.

Among evaluable patients, approximately:

• 21% achieved a partial response;
• 53% had stable disease;
• median progression-free survival was 4.9 months;
• median overall survival from pembrolizumab initiation was 20.9 months.

Most patients had prior systemic chemotherapy, and the study lacked a randomized control group.

These findings show clinical activity but do not establish pembrolizumab as the universally preferred treatment.

The FDA approval of pembrolizumab with pemetrexed and platinum chemotherapy is specifically for unresectable advanced or metastatic malignant pleural mesothelioma, so that pleural indication should not be presented as a peritoneal-specific approval.

Is Radiation Therapy Used?

Radiation therapy has a limited role in diffuse peritoneal mesothelioma.

Treating large areas of the abdomen can be difficult because sensitive organs—including the bowel, liver and kidneys—may be nearby.

Radiation may nevertheless be considered in selected situations for localized symptom control or treatment of a specific disease site.

The use of radiation therapy should be individualized based on disease distribution and prior treatments.

Managing Ascites and Other Symptoms

Cancer-directed treatment and symptom-directed care should work together.

Ascites

Ascites can cause:

• increasing abdominal size;
• pressure or discomfort;
• reduced appetite;
• early fullness;
• and sometimes shortness of breath.

Paracentesis can remove fluid and provide symptom relief in selected patients.

When ascites repeatedly returns, additional management depends on the underlying disease, anticipated treatment and individual circumstances.

Pain, nutrition and digestive symptoms

Supportive treatment can also address:

• abdominal pain;
• nausea;
• constipation or diarrhea;
• poor appetite;
• unintended weight loss;
• fatigue;
• sleep problems;
• functional limitations;
• emotional distress.

Nutritional support is especially important for patients whose abdominal disease or treatment impairs their ability to eat.

Palliative Care Can Be Used During Active Treatment

Palliative care is not the same as hospice.

Palliative-care specialists focus on symptoms, quality of life, communication and support for patients and families.

Palliative care may be provided alongside ongoing anticancer treatments, including systemic therapy or surgery.

Current peritoneal-mesothelioma consensus guidance also emphasizes establishing patient support services and using palliative care when appropriate.

What Happens if Peritoneal Mesothelioma Comes Back?

Recurrence does not necessarily mean all previous treatment options are permanently unavailable.

Specialists may consider:

• location of recurrent disease;
• time since previous treatment;
• previous CRS-HIPEC;
• current tumor burden;
• histology;
• general health;
• previous systemic therapies;
• whether another meaningful cytoreduction could be achieved;
• and available clinical trials.

Some carefully selected patients may be considered for additional locoregional treatment, while others may receive systemic therapy or symptom-focused care.

Given the limited evidence for recurrence strategies, multidisciplinary specialist review is particularly important.

Clinical Trials

Clinical trials are particularly important in peritoneal mesothelioma because relatively few prospective trials have focused specifically on this disease.

The current Peritoneal Surface Malignancies Consortium consensus reached unanimous support for considering clinical trial enrollment, and its systemic treatment recommendations encourage trial consideration throughout the treatment pathway.

Research areas include:

• immune-checkpoint inhibitors;
• new immunotherapy combinations;
• targeted treatments;
• biomarker-directed therapy;
• intraperitoneal therapies;
• optimization of CRS-HIPEC;
• treatment sequencing.

Eligibility depends on factors such as histology, previous treatment, disease distribution, molecular findings, organ function, and performance status.

Eligibility for clinical trials must be determined by a healthcare professional; a website cannot assess individual eligibility. Talk to your oncology team and check official registries like ClinicalTrials.gov.

Peritoneal Mesothelioma Prognosis

Prognosis varies considerably between patients.

Important factors can include:

• histologic subtype;
• peritoneal tumor burden;
• lymph-node involvement;
• extra-abdominal disease;
• ability to achieve complete or near-complete cytoreduction;
• performance status;
• overall health;
• and response to treatment.

Epithelioid histology, absence of lymph-node metastases and successful cytoreduction have consistently been associated with more favorable outcomes in surgical series.

Survival after CRS-HIPEC

One of the largest multi-institutional datasets summarized by the NCI included 401 patients treated with cytoreductive surgery; 92% also received HIPEC.

In that selected surgical population:

• median overall survival was 53 months;
• estimated 3-year survival was 60%;
• estimated 5-year survival was 47%.

Better outcomes were associated with epithelioid histology, absence of lymph-node metastases, complete or near-complete cytoreduction and use of HIPEC.

Our mesothelioma prognosis guide covers survival data and terminology in more detail.

Why Specialist Care Matters

Peritoneal mesothelioma sits at the intersection of several specialties.

A multidisciplinary team may include:

• surgical oncology;
• medical oncology;
• gastrointestinal oncology;
• abdominal radiology;
• pathology;
• genetics;
• nutrition;
• palliative care.

Specialist evaluation can help answer critical questions:

• Has an experienced pathologist confirmed the diagnosis?
• Is this invasive mesothelioma or another mesothelial lesion?
• What is the histologic subtype?
• How extensive is the disease?
• Is it confined primarily to the abdomen?
• What is the expected likelihood of complete cytoreduction?
• Is CRS-HIPEC appropriate?
• Should systemic therapy be used?
• Is immunotherapy appropriate in this setting?
• Should treatment be given before or after surgery?
• Is a clinical trial available?

Obtaining a second opinion does not require transferring all future care to another hospital. A specialist center can review pathology, imaging, and treatment plans while some care continues locally.

Questions to Ask Your Medical Team

1. Has my biopsy been reviewed by a pathologist experienced in mesothelioma?
2. Is this diffuse invasive peritoneal mesothelioma or another mesothelial tumor?
3. What histologic subtype do I have?
4. Is disease confined mainly to the abdomen?
5. Are lymph nodes or distant organs involved?
6. How extensive is the peritoneal disease?
7. Has my Peritoneal Cancer Index been assessed or estimated?
8. Do you believe complete or near-complete cytoreduction is achievable?
9. Am I a candidate for CRS-HIPEC?
10. How much experience does this center have treating peritoneal mesothelioma?
11. What are the major risks of CRS-HIPEC in my situation?
12. Should systemic treatment be considered before or after surgery?
13. If surgery is not appropriate, which systemic treatments should we discuss?
14. How strong is the evidence for immunotherapy specifically in peritoneal mesothelioma?
15. Should I have germline genetic testing?
16. Are there clinical trials relevant to my diagnosis?
17. What can be done now to manage ascites, pain, appetite problems, or other symptoms?
18. Would a second opinion at a peritoneal-surface malignancy center be useful?

Frequently Asked Questions

References

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6. Marmarelis ME, Wang X, Roshkovan L, et al. Clinical Outcomes Associated With Pembrolizumab Monotherapy Among Adults With Diffuse Malignant Peritoneal Mesothelioma. JAMA Network Open. 2023;6(3):e232526. doi:10.1001/jamanetworkopen.2023.2526.
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9. U.S. Food and Drug Administration. FDA approves nivolumab and ipilimumab for unresectable malignant pleural mesothelioma. October 2, 2020.
10. U.S. Food and Drug Administration. FDA approves pembrolizumab with chemotherapy for unresectable advanced or metastatic malignant pleural mesothelioma. September 17, 2024. FDA.gov.
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