Peritoneal mesothelioma is a cancer that forms in the peritoneum, the thin membrane lining the inside of your abdomen and covering most of your abdominal organs. It accounts for a smaller share of all mesothelioma cases compared to the more common pleural (lung lining) form, but it behaves differently, is staged differently, and has its own treatment approach. Almost all cases are linked to asbestos exposure, though the cancer typically doesn’t appear until decades after that exposure.
Where It Starts and Why
The peritoneum is a two-layered membrane. One layer lines the abdominal wall, the other wraps around the organs inside it. These layers are made of mesothelial cells, which produce a small amount of fluid that lets organs glide smoothly against each other. Peritoneal mesothelioma begins when these mesothelial cells start growing out of control.
The overwhelming driver is asbestos. When asbestos fibers are inhaled, they don’t necessarily stay in the lungs. Research on fiber translocation shows they can cross the lung’s air sacs into surrounding tissue, get picked up by lymph fluid, enter the bloodstream, and eventually filter into the abdominal cavity through tiny blood vessels in the peritoneal lining. Another route runs through the diaphragm, which is packed with lymphatic openings on both its chest and abdominal sides that drain into a shared network. This migration is slow, unfolding over decades, helped along by the fact that asbestos fibers are extremely durable and resist being broken down by the body.
Once fibers reach the peritoneum, they cause chronic irritation. The mesothelial cells go through repeated cycles of damage and repair, triggering inflammation that releases molecules capable of damaging DNA. At the same time, asbestos fibers physically interfere with cell division by disrupting the structures that pull chromosomes apart. The combination of DNA damage, chromosomal instability, and persistent signals telling cells to keep dividing eventually tips normal cells toward cancer.
The Long Gap Between Exposure and Diagnosis
One of the most striking features of this disease is its latency period. The median time between first asbestos exposure and a mesothelioma diagnosis is roughly 40 years across most registries, though individual cases range widely, from under 10 years to over 70. Some occupational cohorts have documented peritoneal cases with latencies as short as 7 to 8 years, but these are unusual. The Helsinki Criteria, a widely used framework for attributing mesothelioma to asbestos, require a minimum 10-year gap between exposure and diagnosis.
This long delay means many patients are diagnosed in their 60s or 70s, often long after they’ve left the jobs or environments where they encountered asbestos. Peritoneal mesothelioma also has a somewhat different gender pattern than its pleural counterpart. In pleural mesothelioma, women make up a small fraction of cases (roughly 1 woman for every 7 men among those with occupational exposure). In the peritoneal form, the ratio narrows: about 1 woman for every 3 men with occupational exposure. The reasons aren’t fully understood, but this higher relative proportion in women is a consistent finding.
Common Symptoms
Peritoneal mesothelioma tends to announce itself with vague abdominal complaints that mimic many less serious conditions. The most common symptoms include:
- Fluid buildup in the abdomen (ascites), which causes visible swelling and a feeling of fullness
- Diffuse abdominal pain that feels spread out rather than pinpointed to one spot
- Swelling or bulging of the belly
- Constipation or bowel obstruction
- Nausea and vomiting
- Fever and night sweats
- Unexplained weight loss and loss of appetite
Because these symptoms overlap with so many other conditions, from irritable bowel syndrome to ovarian cancer, peritoneal mesothelioma is often not the first diagnosis considered. Patients frequently go through weeks or months of testing before a biopsy confirms it.
How It’s Diagnosed
Imaging scans like CT can reveal fluid in the abdomen, thickening of the peritoneal lining, or masses on organ surfaces, but a tissue biopsy is essential for a definitive diagnosis. Under a microscope, pathologists use a panel of protein markers to confirm the tumor came from mesothelial cells rather than another cancer that spread to the abdomen.
The standard approach tests for at least two mesothelial markers and two markers associated with other cell types. Calretinin is one of the most reliable: it stains positive in 80 to 100 percent of the most common subtype (epithelioid). Other useful markers include CK5/6, WT-1, and podoplanin. To distinguish cancer from benign mesothelial growths, pathologists look for loss of a protein called BAP1, which is virtually 100 percent specific for malignancy in mesothelial tissue, with sensitivity of 55 to 67 percent in peritoneal cases. When BAP1 results are inconclusive, a genetic test for deletion of the CDKN2A gene can help, though its sensitivity in peritoneal cases is lower, around 25 to 29 percent.
Histologic Subtypes
The tumor’s microscopic appearance matters enormously for prognosis. Epithelioid is the most common and least aggressive subtype. Sarcomatoid is rarer and more aggressive. Biphasic tumors contain a mix of both. Finer distinctions within epithelioid tumors also matter: those with less than 10 percent solid growth patterns carry a significantly better outlook than those with more solid components.
Staging With the Peritoneal Cancer Index
Unlike many cancers that use a simple stage I through IV system, peritoneal mesothelioma is typically assessed using the Peritoneal Cancer Index, or PCI. This scoring system divides the abdomen into 13 regions and assigns a score of 0 to 3 in each based on the size of the largest tumor deposit, producing a total from 0 to 39.
PCI scores of 1 to 10 correspond to early-stage disease, while scores above 19 indicate extensive spread. This score directly influences treatment decisions: patients with a PCI of 10 or less, epithelioid histology with minimal solid components, and lower levels of a blood marker called CA-125 tend to have the best outcomes. On the other end, patients with biphasic or sarcomatoid tumors and PCI scores above 19 face a much more difficult prognosis. Imaging findings that suggest surgery won’t be able to remove all visible disease, like small bowel obstruction or tumor nodules larger than 5 centimeters on the bowel surface, also factor into treatment planning.
Treatment Options
The most effective treatment for eligible patients combines two procedures performed together. First, surgeons remove all visible tumor from the abdominal cavity, a process called cytoreductive surgery. This can be extensive, sometimes involving removal of portions of the peritoneum, sections of bowel, the spleen, or other affected structures. Immediately after, while the abdomen is still open, heated chemotherapy solution is circulated directly inside the abdominal cavity for a period of time. This approach, commonly referred to as CRS with HIPEC, delivers chemotherapy at high concentrations directly where the cancer lives while minimizing the side effects of systemic treatment.
Not everyone is a candidate. Eligibility depends on overall health and physical fitness, the tumor’s subtype, and whether surgeons believe they can remove all or nearly all visible disease. Patients with sarcomatoid histology or very high PCI scores are less likely to benefit. For those who aren’t surgical candidates, systemic chemotherapy remains an option to slow disease progression and manage symptoms.
Immunotherapy is also being explored. A major clinical trial is testing atezolizumab, a drug that helps the immune system recognize and attack cancer cells, in combination with a drug that cuts off blood supply to tumors. These are being added to standard chemotherapy to see if outcomes improve for peritoneal mesothelioma specifically.
Prognosis and What Influences It
Peritoneal mesothelioma is a serious cancer, but outcomes have improved meaningfully with the adoption of combined surgery and heated chemotherapy. Three factors stand out as the strongest predictors of survival: the detailed histologic subtype, the PCI score before surgery, and the pre-treatment CA-125 level. Patients in the most favorable category across all three factors have substantially longer survival than those in the least favorable group.
The completeness of surgical removal also plays a critical role. When surgeons can remove all visible tumor, long-term survival becomes a realistic possibility for a subset of patients, particularly those with epithelioid tumors. When complete removal isn’t achievable, the goal shifts toward controlling symptoms and slowing progression. The gap in outcomes between complete and incomplete surgical removal is one of the largest in oncology, which is why careful patient selection matters so much.