“Peritoneal carcinomatosis is mostly caused by metastasis of cancer cells to the peritoneum, which is due to the blockage of subplasma lymphatic vessels or capillaries and the increase of capillary permeability, and some primary peritoneal carcinomatosis may also lead to ascites in patients. Ascites formed when cancer cells originate from or metastasize to peritoneum is also called carcinomatous ascites, which is mostly caused by primary peritoneal cancer, lung cancer, gastric cancer, hepatocellular carcinoma and other malignant tumors with implantation metastasis to peritoneum. The peritoneal lymphatic vessels or capillaries are blocked by cancer plugs (emboli formed by cancer cells), the permeability of capillaries increases, blood leaks out, and hemorrhage caused by tumor destroying blood vessels accumulates in the peritoneal cavity, resulting in the formation of effusion. Cancer ascites has varying amounts of cancer cells, and cytology of certain ascites can be done to detect malignant tumor cells and is one of the means to help diagnose malignant tumors.