Peritoneal cancer is cancer that involves the peritoneum, the thin membrane lining the inside of your abdomen and covering your abdominal organs. It comes in two forms: primary peritoneal cancer, which starts in the membrane itself, and secondary peritoneal cancer, which spreads to the peritoneum from another organ like the colon, stomach, or ovaries. Secondary peritoneal cancer is far more common.
What the Peritoneum Does
The peritoneum is a two-layered membrane made of specialized cells called mesothelial cells. The outer layer attaches to your abdominal and pelvic walls, while the inner layer wraps around your organs. Between them sits a thin space containing roughly 50 to 100 milliliters of fluid, just a few tablespoons, that lets your organs glide against each other without friction.
Beyond acting as a lubricant, the peritoneum supports your abdominal organs and serves as a highway for blood vessels, nerves, and lymphatic channels. A fatty apron of tissue called the greater omentum hangs from it, acting as an insulating and protective layer. Because the peritoneum touches nearly every organ in the abdomen, cancer that reaches it can spread widely across multiple surfaces.
Primary vs. Secondary Peritoneal Cancer
Primary peritoneal cancer originates in the mesothelial cells of the peritoneum itself. One well-known type, called primary peritoneal carcinoma, develops through a process in which normal peritoneal cells transform into a tissue type similar to what’s found in the ovaries. This is why primary peritoneal cancer can look and behave almost identically to ovarian cancer under a microscope, and the two are often treated the same way. Primary peritoneal mesothelioma, a rarer form, also arises directly from the lining cells.
Secondary peritoneal cancer is more common. It happens when cancer cells from a tumor elsewhere, most often the colon, appendix, stomach, or ovaries, invade through the organ wall and seed onto peritoneal surfaces. Cancer cells can also spill into the peritoneal cavity during surgery to remove a tumor. Once loose in the abdomen, these cells follow a “seed and soil” pattern: they circulate and take root wherever the local environment supports their growth, forming new tumor deposits across the peritoneal lining.
Risk Factors
For primary peritoneal carcinoma, the strongest known risk factor is carrying an inherited change in the BRCA1 or BRCA2 genes. Women with a harmful BRCA1 change have a 39 to 58 percent lifetime risk of developing ovarian, fallopian tube, or primary peritoneal cancer. For BRCA2 carriers, that range is 13 to 29 percent. By comparison, the general population risk is about 1.1 percent. This is why some women with BRCA mutations choose preventive removal of their ovaries and fallopian tubes, though primary peritoneal cancer can still develop afterward since the peritoneum remains.
For secondary peritoneal cancer, the main risk factor is having an advanced cancer in a nearby organ. Colorectal cancers, appendiceal tumors, gastric cancers, and ovarian cancers are the most frequent sources. The more advanced the original tumor, the higher the chance it will seed the peritoneum.
Symptoms and Fluid Buildup
Peritoneal cancer is often difficult to detect early because its symptoms overlap with many common, less serious conditions. The most characteristic sign is ascites, an abnormal buildup of fluid in the abdomen. Cancer cells irritate the peritoneum and cause it to leak excess fluid into the space between its two layers. This fluid can accumulate to several liters, causing noticeable abdominal swelling, bloating, and a feeling of tightness or pressure.
Other symptoms include:
- Unexplained weight gain or clothes that suddenly feel tight around the waist
- Loss of appetite, nausea, or constipation
- Shortness of breath and coughing as fluid pushes up against the lungs
- Fatigue and general weakness
- Swelling in the ankles or legs
Because these symptoms develop gradually and mimic digestive problems, peritoneal cancer is frequently diagnosed at an advanced stage.
How It Is Diagnosed and Scored
Diagnosis typically involves imaging (CT scans or MRI), blood tests for tumor markers, and often a biopsy of peritoneal tissue or analysis of fluid drained from the abdomen. Once peritoneal cancer is confirmed, doctors use a scoring system called the Peritoneal Cancer Index (PCI) to map how far it has spread.
The PCI divides the abdomen into 13 sectors: nine regions of the abdominal cavity plus four segments of the small bowel. Each sector receives a score based on the size of tumor deposits found there, and the scores are added together. A lower PCI generally means the disease is more contained and more amenable to surgery. Studies show that patients with a PCI below 15 tend to respond better to treatment and have a more favorable outlook.
Surgical Treatment
The primary treatment for peritoneal cancer that hasn’t spread beyond the abdomen is cytoreductive surgery, sometimes called debulking. The goal is to remove every visible tumor deposit from the peritoneal surfaces, which can mean stripping sections of the peritoneum and removing parts of affected organs.
Surgeons grade the completeness of their work on a scale. A score of CC-0 means no visible disease remains. CC-1 means any leftover tumor nodules are smaller than 2.5 millimeters, small enough that chemotherapy delivered directly into the abdomen can penetrate them. Both CC-0 and CC-1 count as “complete” cytoreduction. Scores of CC-2 or CC-3 indicate larger residual disease and are considered incomplete. The completeness score is one of the strongest predictors of long-term outcomes across nearly every type of peritoneal cancer, including colorectal, ovarian, gastric, and appendiceal origins.
Heated Chemotherapy During Surgery
Immediately after cytoreductive surgery, many patients receive a procedure called HIPEC (hyperthermic intraperitoneal chemotherapy). While the abdomen is still open, chemotherapy solution warmed to about 106 to 109 degrees Fahrenheit is circulated throughout the abdominal cavity for 60 to 90 minutes. The heat improves the drug’s ability to penetrate tissue and destroy microscopic cancer cells that the surgeon couldn’t see or reach.
The combined operation, cytoreductive surgery plus HIPEC, is long and demanding. It typically takes 6 to 12 hours depending on the extent of cancer and how many organs are involved. Recovery is significant, and the number of bowel reconnections required during surgery has the biggest impact on complication rates. The procedure is offered at specialized cancer centers and is best suited for patients whose disease can be reduced to minimal or no visible residual tumor.
Managing Ascites
For many people with peritoneal cancer, fluid buildup is one of the most disruptive day-to-day challenges. The standard approach is paracentesis, a procedure in which a needle is inserted through the abdominal wall to drain the fluid. It provides rapid relief, but fluid typically reaccumulates within about 10 days, meaning repeated trips for drainage.
When ascites keeps returning, a semi-permanent drain can be placed under local anesthesia with ultrasound guidance, either in a radiology suite or at bedside. The thin catheter stays in place and is anchored to the skin with a single stitch. A nurse can then drain fluid at home whenever symptoms flare, usually removing 1 to 3 liters at a time. In studies, this type of drain improved disabling symptoms in over 92 percent of patients and allowed 60 percent to manage their care at home without additional hospital visits for drainage. Over time, some patients find the drainage intervals can be spaced further apart as fluid production slows.
In certain cases where the body is retaining sodium and contributing to fluid buildup, medications that block sodium reabsorption can help slow reaccumulation. Permanent drains are generally not recommended for patients receiving cancer treatments that suppress the immune system, because of the higher infection risk.