Many of the organs in the abdomen are enveloped by a thin membrane of mesothelial cells, known as the peritoneum.

Peritoneal mesothelioma is a tumor of this membrane. Its only known cause is previous exposure to asbestos, but it can be many years after exposure before the disease appears. Peritoneal mesotheliomas account for about one-fifth of all mesotheliomas.

Like pleural mesothelioma, peritoneal mesothelioma can be either benign or malignant. This discussion is only about malignant peritoneal mesothelioma.

Mesothelioma is sometimes diagnosed by coincidence, before any symptoms have appeared. For example, the tumor is sometimes seen on a routine abdominal x-ray for a check-up or before surgery.

When the symptoms of peritoneal mesothelioma appear, they typically include abdominal pains, weakness, weight loss, loss of appetite, nausea, and abdominal swelling. Fluid often accumulates in the peritoneal space, a condition known as ascites. Over time the wasting symptoms can become more and more severe.

The growing tumor can exert increasing pressure on the organs in the abdomen, leading to bowel obstruction and distention. If the tumor presses upward, it can impair breathing capacity. If the tumor pushes against areas with many nerve fibers, and the bowel distends, the amount of pain can increase.

X-rays and CT scans are, typically, the first step towards detecting peritoneal mesothelioma. The actual diagnosis is typically achieved by obtaining a piece of tissue. The medical procedure of looking at the peritoneum is known as a peritoneoscopy. It is a hospital procedure and requires anesthesia. If an abnormality is seen, the doctor will attempt to obtain a tissue sample - this is known as a biopsy. The tissue sample will be examined by a pathologist who makes a diagnosis using microscopic analysis of specialized stains.

There are at least two explanations for how asbestos fibers can get into the peritoneum. The first is that fibers caught by the mucus of the trachea and bronchi end up being swallowed. Some of them lodge in the intestinal tract and from there they can move through the intestinal wall into the peritoneum. The second explanation is that fibers that lodge in the lungs can move into the lymphatic system and be transported to the peritoneum.

Medical science does not know exactly how or why, at a cellular level, a carcinogen like asbestos causes a cell to become malignant (cancerous.) Thus it is not known whether only one fiber can cause a tumor to develop or whether it takes many fibers, or what the exact conditions and predispositions are for this change to happen.

At this time there are treatments, but no known cure, for peritoneal mesothelioma. The prognosis depends on various factors, including the size and stage of the tumor, its extent, the cell type, and whether or not the tumor responds to treatment.

However, the options for relief and treatment of people with peritoneal mesothelioma have improved, especially for those whose cancer is diagnosed early and treated vigorously. Many people receive a combination of therapies, sometimes known as multimodal therapy.