Peritoneal Mesothelioma

Peritoneal Mesothelioma
Critical facts for diagnosis and treatment
% of All Cases	10–20%
Gender Ratio	1.2:1 (M:F)
CRS/HIPEC 5-Yr OS	41–59%
Chemo-Only Median	~12 months
PCI Scoring Range	0–39
BAP1 Mutations	38–45%
Women 5-Yr OS	68% vs 39% men
Median Age	50–65 years
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Peritoneal Mesothelioma: Treatment Advances, Survival Data, and Emerging Therapies

Executive Summary

Malignant peritoneal mesothelioma (MPM) represents the second most common site of mesothelioma, accounting for 10–20% of all mesothelioma diagnoses in the United States, with approximately 315–800 new cases diagnosed annually. Unlike pleural mesothelioma, which predominantly affects men with occupational asbestos exposure, peritoneal mesothelioma demonstrates a near-equal gender distribution (1.2:1 male-to-female ratio), with approximately 20–40% of cases occurring without documented asbestos exposure—a distinction that reflects higher rates of secondary/paraoccupational exposure and germline genetic factors such as BAP1 mutations. Cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC) has become the standard of care for eligible patients, extending median overall survival from approximately 12 months with chemotherapy alone to 34–92 months in selected cohorts, with 5-year survival rates of 41–59%. Notably, women experience superior outcomes compared to men—68% five-year survival versus 39% in men—an advantage attributed to younger age at diagnosis, better performance status, and potential hormonal protective factors. Emerging therapies including pressurized intraperitoneal aerosol chemotherapy (PIPAC) for palliative care and downstaging, immunotherapy with checkpoint inhibitors, and targeted approaches addressing BAP1 mutations (present in 38–45% of cases) are expanding treatment options beyond traditional CRS/HIPEC for unresectable and relapsed disease. The peritoneal cancer index (PCI) scoring system, which stratifies disease burden on a 0–39 scale, remains the most important prognostic indicator—patients with PCI ≤20 achieve five-year survival rates exceeding 50%, while those with PCI ≥30 experience dramatically reduced survival.

Key Facts

Key Facts: Peritoneal Mesothelioma

Disease Burden: 10–20% of all mesothelioma cases; approximately 315–800 new cases annually in the United States Near-Equal Gender Distribution: 1.2:1 male-to-female ratio, dramatically different from pleural mesothelioma (1:7) Asbestos Attribution: 60–80% of cases have documented asbestos exposure; 20–40% occur without identifiable exposure Standard Treatment: CRS/HIPEC achieves 5-year survival of 41–59%, compared to ~12 months with chemotherapy alone Peritoneal Cancer Index (PCI): Divides abdomen into 13 regions on 0–39 scale; PCI ≤20 predicts 5-year OS >50% Gender-Based Outcomes: Women achieve 68% 5-year survival vs. 39% in men after CRS/HIPEC BAP1 Mutations: Present in 38–45% of peritoneal mesothelioma cases; associated with more inflamed tumor microenvironment Emerging PIPAC Therapy: Minimally invasive pressurized aerosol chemotherapy enables downstaging of unresectable disease Immunotherapy Data: Growing evidence of checkpoint inhibitor activity in unresectable peritoneal mesothelioma Misdiagnosis Risk: Frequently misdiagnosed as ovarian cancer in women due to overlapping presentation and imaging findings

What Is Peritoneal Mesothelioma?

Peritoneal mesothelioma is a rare, aggressive cancer arising from the mesothelial cells that line the peritoneum—the serosal membrane of the abdominal cavity. It is the second most common site of mesothelioma development after the pleura (lung lining). The disease typically presents with non-specific abdominal symptoms including pain, distension, ascites (fluid accumulation), and weight loss, often resulting in delayed diagnosis by 4–6 months from initial symptom presentation.

The peritoneum is a thin, slippery membrane that lines the abdominal and pelvic cavities, providing a protective barrier around organs and allowing them to move freely. When asbestos fibers reach the peritoneum and become lodged in this lining, they trigger chronic inflammation and malignant transformation of mesothelial cells. The disease remains largely confined to the abdominal cavity, distinguishing it from pleural mesothelioma, which can metastasize to distant sites. Unlike pleural mesothelioma's male predominance, peritoneal mesothelioma occurs with nearly equal frequency in men and women—a distinction with significant clinical and legal implications.

What Causes Peritoneal Mesothelioma?

Asbestos Exposure Pathways

Asbestos is the primary known risk factor for peritoneal mesothelioma, though the pathways by which inhaled or ingested fibers reach the peritoneum are incompletely understood. The most widely accepted mechanism involves mucociliary clearance and gastrointestinal translocation: inhaled asbestos fibers trapped in the respiratory tract are cleared upward by the mucociliary escalator, swallowed, and enter the gastrointestinal tract, where they may penetrate the intestinal wall and lodge in the mesentery or peritoneal cavity. Alternative mechanisms include direct translocation through diaphragmatic lymphatics and hematogenous or lymphatic spread.

Once lodged in the peritoneum, asbestos fibers irritate mesothelial cells, causing chronic inflammation, chromosomal instability, and eventually malignant transformation. Both amphibole fibers (crocidolite, amosite) and chrysotile demonstrate association with peritoneal mesothelioma development. Cumulative exposure thresholds vary by fiber type: >16.4 f/cc-years for crocidolite, 23.6 f/cc-years for amosite, and higher thresholds for chrysotile variants.

Secondary and Paraoccupational Exposure

A distinctive feature of peritoneal mesothelioma—particularly in women—is that secondary (paraoccupational) exposure is responsible for a significant proportion of cases. This includes:

• Laundering contaminated work clothing
• Direct physical contact with asbestos-exposed family members
• Exposure to contaminated household dust and environments
• Residential proximity to asbestos-using industries

Lung fiber burden analysis demonstrates that women with paraoccupational exposure accumulated fiber concentrations comparable to men with moderate direct occupational exposure, explaining the higher proportion of peritoneal mesothelioma in women relative to pleural disease. This has major implications for legal causation arguments.

Non-Asbestos Risk Factors

Approximately 20–40% of peritoneal mesothelioma cases occur without documented asbestos exposure. Contributing factors include:

• BAP1 mutations (38–45% somatic, ~12% germline): BRCA1-Associated Protein 1 mutations significantly increase mesothelioma susceptibility, sometimes with dramatically shortened latency periods (as brief as 8 years). Germline BAP1 mutation carriers can develop mesothelioma with minimal or absent asbestos exposure.
• Prior abdominal radiation: Documented as an independent risk factor
• Chronic peritoneal inflammation: Conditions like Mediterranean familial fever and chronic peritonitis increase risk
• Genetic predisposition: Beyond BAP1, germline BRCA mutations have been described in select cases

What Are the Symptoms of Peritoneal Mesothelioma?

Peritoneal mesothelioma presents with non-specific abdominal symptoms that frequently delay diagnosis by months. Common presenting symptoms include:

• Abdominal pain and distension/bloating (>30–50% of cases)
• Ascites (fluid accumulation in abdomen)—often the presenting finding
• Weight loss and early satiety (feeling full quickly)
• Nausea, vomiting, and bowel obstruction
• Fatigue and decreased energy
• Palpable abdominal mass (occasional)
• Fever and temperature fluctuations
• New-onset hernia

The non-specific nature of these symptoms and the rarity of the disease contribute to significant diagnostic delay. Median time from initial presentation to diagnosis is typically 4–6 months.

The Ovarian Cancer Misdiagnosis Problem

A particularly important diagnostic challenge is that peritoneal mesothelioma in women is frequently misdiagnosed as ovarian cancer or primary peritoneal carcinoma due to overlapping clinical presentation (ascites, omental caking, elevated CA-125) and similar radiographic appearance. Documented cases exist where women's peritoneal mesothelioma was initially assumed to be ovarian cancer, with the correct diagnosis only established during surgery or on final pathology review. This misdiagnosis can delay appropriate multimodality treatment and affects legal causation narratives. Immunohistochemistry demonstrating mesothelial markers (calretinin+, CK5/6+, WT-1+) and absence of adenocarcinoma markers (BerEP4−) is critical for distinguishing these entities.

How Is Peritoneal Mesothelioma Diagnosed?

Imaging and Cross-Sectional Evaluation

Diagnosis begins with clinical suspicion based on diffuse abdominal symptoms and/or ascites. CT imaging is the first-line diagnostic modality, revealing diffuse omental masses, mesenteric nodules, peritoneal thickening, and ascites. Favorable CT findings (ascites with minimal soft tissue disease and preserved small bowel anatomy) predict better outcomes and increased likelihood of complete cytoreduction. Unfavorable findings include absent ascites with large diffuse nodular thickening and marked bowel distortion.

MRI with specialized diffusion-weighted imaging protocols can accurately predict peritoneal cancer index (PCI) preoperatively. PET/CT shows FDG uptake along the peritoneum and in regional lymph nodes but remains of unclear initial diagnostic value. Enteral contrast is recommended to delineate small bowel involvement.

Diagnostic Biopsy

Diagnostic laparoscopy has become the preferred approach for obtaining tissue, offering the added advantages of:

• Direct visualization of tumor burden and distribution
• Assessment of peritoneal cancer index
• Identification of patients eligible for CRS/HIPEC
• Avoidance of unnecessary exploratory laparotomy in unresectable cases

Paracentesis (fluid aspiration) has limited diagnostic utility because malignant cells are typically sparse in ascites. However, fine-needle aspiration of peritoneal tumor implants combined with immunohistochemistry can establish diagnosis.

Immunohistochemistry Confirmation

Definitive diagnosis requires a panel of immunohistochemical antibodies:

Mesothelial markers (positive in mesothelioma):

• Calretinin (most sensitive)
• Cytokeratin 5/6 (CK5/6)
• WT-1 (Wilms Tumor-1)
• D2-40 (podoplanin)
• HBME-1
• Vimentin

Adenocarcinoma markers (negative—used to exclude ovarian/GI cancers):

• BerEP4
• CEA
• B72.3
• MOC-31
• TTF-1

Two or more mesothelial markers confirm diagnosis. Loss of BAP1 nuclear expression is highly specific for distinguishing mesothelioma from benign mesothelial proliferation. Loss of MTAP expression serves as a surrogate for CDKN2A deletion and supports malignancy.

Serum Biomarkers

Biomarkers assist in diagnosis and prognosis:

• Mesothelin: Elevated in up to 71% of cases with 84.6% sensitivity
• CA-125: Elevated in 53% of cases; more useful for monitoring recurrence
• CA 15-3: Elevated in approximately 48.5% of cases
• Osteopontin: Shows promise as complementary biomarker

What Is CRS/HIPEC and Why Is It the Standard Treatment?

Procedure Overview and Technical Details

Cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy (CRS/HIPEC) is the established standard of care for selected peritoneal mesothelioma patients. The procedure occurs in two distinct phases:

Phase 1: Cytoreductive Surgery (CRS): Complete macroscopic tumor removal through peritonectomies (removal of affected peritoneal surfaces) and visceral resections. A complete parietal peritonectomy removes all peritoneum regardless of gross appearance, because microscopic disease may involve 54% of normal-appearing peritoneum. Visceral resections may include partial or total gastrectomy, colectomy, small bowel resection, splenectomy, or gynecologic organ resection depending on extent of disease. The goal is CC-0 or CC-1 cytoreduction (no residual disease or <2.5 mm residual).

Phase 2: Hyperthermic Intraperitoneal Chemotherapy (HIPEC): After complete resection, heated chemotherapy is circulated directly within the abdominal cavity at 41–42°C for 60–90 minutes to address microscopic residual disease. The abdominal cavity is temporarily closed with a temporary plastic barrier (Redon technique), heated saline is circulated, and chemotherapy is delivered. Cisplatin-based regimens (cisplatin alone or cisplatin + doxorubicin) are preferred, as cisplatin-based HIPEC is associated with superior survival compared to mitomycin C protocols. The entire procedure typically lasts 6–10 hours.

Patient Selection Criteria

Not all patients are candidates for CRS/HIPEC. Selection criteria include:

• Peritoneal Cancer Index (PCI) ≤20: Generally favorable for complete cytoreduction; PCI 21–30 is moderate; PCI >30 indicates high-burden disease with significantly worse outcomes
• Performance Status: Adequate physiological health to tolerate a major abdominal operation (ECOG 0–2 preferred)
• Cell Type: Epithelioid histology preferred; sarcomatoid and biphasic subtypes have worse outcomes and require careful patient selection
• CT Imaging Criteria: Absence of >5 cm tumor in epigastric region and preserved small bowel/mesentery anatomy predict 94% probability of complete cytoreduction
• Absence of Extra-Abdominal Disease: Peritoneal mesothelioma rarely metastasizes distantly, making distant disease a relative contraindication

Completeness of Cytoreduction (CC) Scoring

The completeness of cytoreduction is the strongest independent predictor of survival:

CC Score	Definition	Prognosis
CC-0	No residual disease	Best outcomes
CC-1	Residual tumor <2.5 mm	Good outcomes
CC-2	Residual tumor 2.5 mm – 2.5 cm	Significantly worse
CC-3	Residual tumor >2.5 cm	Poorest outcomes

Complete cytoreduction (CC-0 or CC-1) is achieved in approximately 67% of patients undergoing CRS/HIPEC and is the most consistent independent predictor of improved survival across all major studies.

CRS/HIPEC Survival Data by Study

Study	Year	Patients	Median OS	5-Year OS
Yan et al. (multi-institutional)	2009	401	53 months	47%
Alexander et al. (3 US centers)	2013	211	38.4 months	41%
Feldman et al. (NCI)	2003	49	92 months	59%
Baratti et al. (Milan)	2013	108	63.2 months	52.4%
Sugarbaker (CRS+HIPEC+NIPEC)	2022	35	Not reported	~80%
Helm et al. (meta-analysis)	2015	1,047	—	42%

CRS/HIPEC extends median survival to 34–92 months depending on patient selection, compared to approximately 12 months with chemotherapy alone and 6 months without treatment.

Treatment Centers of Excellence

Major referral centers specializing in peritoneal mesothelioma include:

• Washington Cancer Institute / Paul Sugarbaker: Pioneer of CRS/HIPEC; reported ~80% 5-year survival with CRS+HIPEC+NIPEC approach
• National Cancer Institute (NCI): Feldman et al. reported 92-month median survival in selected patients
• MD Anderson Cancer Center: Major referral for peritoneal surface malignancies
• Moffitt Cancer Center: Active in treatment and clinical trials
• National Cancer Institute Milan (Deraco/Baratti): Published outcomes in 108–116 patients with 49–52.4% 5-year survival
• University of Pittsburgh: Prognostic factor analysis and multimodality treatment

"CRS/HIPEC represents the most significant advance in peritoneal mesothelioma treatment in decades. We have seen patients who were told they had months to live survive for 10, 15, even 20 years with aggressive surgery and heated chemotherapy followed by systemic therapy and careful surveillance."

— Paul Danziger, Founding Partner, Danziger & De Llano

How Does Cell Type Affect Treatment and Survival?

Epithelioid Peritoneal Mesothelioma

Epithelioid is the most common subtype, comprising 79–91% of surgical cases. This cell type carries the best prognosis:

• Median age-standardized survival: 16.6 months in population-based data
• Epithelioid histology is an independent predictor of improved survival in CRS/HIPEC patients on multivariate analysis
• Preferred for aggressive surgical intervention
• In surgical series with CRS/HIPEC, epithelioid patients achieve median survivals of 50–60 months

Sarcomatoid Peritoneal Mesothelioma

Sarcomatoid is the rarest and most aggressive subtype, representing only 1–9.5% of surgical cases:

• Median survival: Only 2.0 months in population-based data
• Hazard ratio of 2.85 relative to epithelioid histology
• Generally considered poor candidates for aggressive surgical intervention
• May benefit from palliative PIPAC or chemotherapy
• Even with CRS/HIPEC, outcomes are significantly worse than epithelioid

Biphasic/Mixed Peritoneal Mesothelioma

Biphasic contains both epithelioid and sarcomatoid components:

• Represents 8–13% of surgical cases
• Intermediate prognosis (HR 1.49 relative to epithelioid)
• Treatment decisions depend on proportion of sarcomatoid component
• Patients with predominantly epithelioid features may be candidates for CRS/HIPEC

Well-Differentiated Papillary Mesothelioma (WDPM)

WDPM is a rare subtype comprising 0.3–5% of all mesothelioma:

• Most common in young women
• Generally considered of low malignant potential—most cases are benign
• Excellent prognosis with 5+ year and sometimes decades-long survival
• Conservative surgical resection typically adequate; aggressive CRS/HIPEC not routinely recommended
• 2024 Memorial Sloan Kettering series: No WDPM-related mortality
• Premenopausal presentation associated with best outcomes

What Emerging Therapies Show Promise?

Pressurized Intraperitoneal Aerosol Chemotherapy (PIPAC)

PIPAC is an emerging minimally invasive treatment delivering chemotherapy as a pressurized aerosol directly into the abdominal cavity during laparoscopy. Key applications in peritoneal mesothelioma:

• Palliative Treatment: For patients with unresectable disease not eligible for CRS/HIPEC
• Downstaging/Conversion Therapy: Can convert initially unresectable disease (PCI >30) to resectable disease enabling subsequent CRS/HIPEC. A 2025 case series reported two patients with PCI >30 successfully downstaged by PIPAC to achieve complete cytoreduction.
• Long-Term Survival: One remarkable case achieved 7-year survival with 24 consecutive PIPAC procedures using cisplatin and doxorubicin
• Safety Profile: ISSPP PIPAC database (2020–2024) reported 3,224 PIPAC treatments in 1,126 patients with only 0.7% major complication rate and 57–75% complete or major response rates
• Combined Approach: Patients receiving PIPAC combined with CRS/HIPEC survived 33.5 months on average

Immunotherapy and Checkpoint Inhibitors

Checkpoint inhibitors are showing emerging activity in peritoneal mesothelioma, though the disease was largely excluded from the landmark CheckMate 743 trial (only 18 of 571 patients):

• 2024 Real-World Analysis: 221 peritoneal mesothelioma patients sequenced; 20% received immunotherapy
• Case Reports: Major and sustained responses to first-line nivolumab + ipilimumab documented in BAP1-negative tumors
• 2025 French Cohort: 22 unresectable peritoneal mesothelioma patients with ICI-based therapy showed median OS of 16.8 months and 30% objective response rate
• NCI Phase II Trial (NCT05041062): Currently investigating perioperative nivolumab + ipilimumab combined with CRS/HIPEC
• MIST5 Trial (2024): Niraparib (PARP inhibitor) + dostarlimab (anti-PD-1) showed clinical activity in platinum-sensitive relapsed mesothelioma

BAP1 loss in peritoneal mesothelioma is associated with a more inflamed tumor microenvironment, potentially making these tumors more responsive to immune checkpoint inhibitors.

Other Emerging Approaches

• Targeted BAP1 Therapies: While no BAP1-specific therapy is approved, EZH2 inhibitors are under investigation as BAP1 loss leads to EZH2 dependence
• Anti-VEGF Therapy: Bevacizumab + chemotherapy showed promise in pleural mesothelioma and is being tested in peritoneal disease
• Bispecific Antibodies: Volrustomig (anti-PD-1/CTLA-4 bispecific) under investigation in phase 3 trials
• Telomerase-Targeting: TERT promoter mutations occur in ~12% of mesothelioma; therapeutic approaches in preclinical development

Why Are Women's Outcomes Significantly Better?

One of the most striking features of peritoneal mesothelioma is the superior survival in women compared to men—a pattern not observed in pleural mesothelioma:

Survival Metric	Women	Men
1-Year Survival	89%	77%
3-Year Survival	76%	50%
5-Year Survival	68%	39%

Female sex is an independent predictor of improved survival (HR 0.66; 95% CI: 0.58–0.76) on multivariate analysis. Contributing factors include:

• Younger age at diagnosis: Women typically present 5–10 years younger than men
• Better performance status: Women often have fewer comorbidities and better baseline health
• Epithelioid predominance: Women are more likely to have epithelioid histology
• Potential hormonal protection: Premenopausal women have significantly better outcomes than postmenopausal women
• Selection bias in surgical population: Women selected for CRS/HIPEC may represent a more favorable risk group

The superior outcomes in women underscore the importance of age and performance status as prognostic factors, independent of disease biology.

What Legal Options Exist for Peritoneal Mesothelioma Patients?

Unique Causation Challenges

The lower asbestos attribution rate for peritoneal mesothelioma (60–80% vs. ~96% for pleural) creates distinct legal challenges. Defense attorneys may argue weaker causal linkage, particularly when:

• The claimant is female with no direct occupational exposure
• No documented occupational asbestos contact history exists
• Latency period is shorter than expected (20 years vs. 30–40 years for pleural)
• The claimant carries a BAP1 mutation

Establishing causation requires:

• Expert medical testimony linking asbestos to peritoneal mesothelioma
• Occupational and residential exposure history, including secondary/paraoccupational exposure documentation
• Fiber pathway evidence (ingestion, lymphatic translocation, peritoneal seeding)
• Lung fiber burden analysis from tissue samples
• Epidemiological evidence showing dose-response relationships and risk elevation even at low-dose exposures
• Genetic testing for BAP1 mutations—carriers with minimal asbestos exposure can develop mesothelioma

Compensation Mechanisms

Peritoneal mesothelioma patients have access to multiple compensation sources:

• Asbestos Bankruptcy Trust Funds: Approximately $25–30 billion remains available across 60+ active trusts. Average combined payouts are $300,000 to $400,000 across multiple funds, with individual trust payments ranging from $7,000 to $1.2 million. Trust fund claims require only documented exposure to manufacturers' products—no need to prove negligence.

• Lawsuit Settlements: Average settlement amounts fall between $1 million and $1.4 million, with trial verdicts averaging approximately $2.4 million. Notable verdicts include a 2022 California award of $53.3 million and a 2025 Boston jury verdict of $8 million to an 84-year-old woman with mesothelioma linked to talcum powder exposure.

• Simultaneous Claims: Patients can pursue both trust fund claims and lawsuits simultaneously to maximize total recovery, and trust fund payments do not reduce other compensation sources including settlement proceeds or VA benefits.

"Women with peritoneal mesothelioma often dismiss their exposure as 'just laundry' when in reality, secondary exposure through contaminated work clothing can deliver significant fiber doses. These cases deserve full investigation and aggressive prosecution—we have recovered substantial compensation for women whose exposure came entirely through paraoccupational means."

— Rod De Llano, Founding Partner, Danziger & De Llano

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