The peritoneum is a thin, smooth membrane that lines the inside of the abdomen and covers most of the organs within it. It is not an organ itself. Think of it as a slippery wrapping that allows the bowel, stomach, and other organs to slide against one another as you move and digest food.
The peritoneum has two parts. The part lining the abdominal wall is called the parietal peritoneum. The part covering the organs is called the visceral peritoneum, and where it wraps an organ it is usually called the serosa. Between the two lies the peritoneal cavity, a narrow space containing a small amount of lubricating fluid.
What does the peritoneum look like under the microscope?
The peritoneum is much thinner than most people expect. It is made of a single layer of flat cells called mesothelial cells, resting on a thin band of supporting tissue containing small blood vessels and lymphatic channels.
That single cell layer produces the fluid that keeps the surface slippery. When infection, surgery, or a tumor irritates the peritoneum, the mesothelial cells enlarge and multiply. Pathologists call this a reactive change, and it can look worrying under the microscope even though it is a normal response to injury.
Why does the peritoneum matter on a pathology report?
The peritoneum appears on reports for several different reasons.
- It is a staging boundary. For tumors of the stomach, colon, appendix, and other abdominal organs, reaching the peritoneal surface changes the tumor stage. It usually raises the stage to T4a. Once a tumor reaches this surface, cells can shed directly into the abdominal cavity.
- It is a site of spread. Tumor cells that reach the peritoneum can settle anywhere along it and form deposits. Because the peritoneum is a surface rather than an organ, this kind of spread is called peritoneal dissemination or peritoneal carcinomatosis, and it is counted as metastasis.
- It produces fluid that can be tested. When fluid builds up in the abdomen, some can be drawn off and examined for tumor cells. Surgeons also sometimes rinse the cavity with saline and send the fluid for examination, a test called a peritoneal wash.
- It can be sampled directly. Small pieces of peritoneum are often biopsied during abdominal surgery to check for deposits that are too small to see clearly.
Does peritoneal involvement mean my margins are positive?
No, and this is a common source of confusion. A margin is a surface created by the surgeon when tissue is cut. The peritoneum is a natural surface that was already there.
When a report says a tumor reaches the peritoneal surface, it is describing how far the tumor grew, not whether the surgeon removed all of it. The two findings appear in different parts of the report and mean different things.
Conditions involving the peritoneum
- Peritoneal metastasis. Deposits from cancers of the ovary, stomach, colon, appendix, or pancreas.
- Pseudomyxoma peritonei. Mucin accumulating in the abdominal cavity, most often from a tumor of the appendix.
- Mesothelioma. An uncommon cancer arising from the mesothelial cells of the peritoneum itself.
- Adhesions. Bands of scar tissue that form after surgery or inflammation and can join surfaces that should slide freely.
- Peritonitis. Inflammation of the peritoneum, usually caused by infection or a perforation of the bowel.
Questions to ask your doctor
- Does my report say the tumor reached the peritoneal surface?
- If it did, what does that mean for my stage?
- Was any peritoneal tissue removed and examined?
- Was fluid from my abdomen tested for tumor cells?
- Were any deposits found on the peritoneum, and where were they?
- Is peritoneal involvement the same thing as a positive margin in my case?
- Does this finding change the recommended treatment?
- Will I need imaging to follow the peritoneum over time?
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