Peritoneal cancer (peritoneal carcinomatosis) is a condition where the peritoneum, a thin membrane that surrounds and protects the organs in the abdominal cavity, is affected by malignant tumor cells. This type of cancer is usually diagnosed in advanced stages and most often develops as a result of another organ cancer (such as ovarian, stomach, or bowel cancer) spreading to the peritoneum. However, rarely, primary peritoneal cancer can also occur, originating directly in the peritoneum without being linked to another primary cancer.