Peritoneal cancer is difficult to diagnose, especially since it is often not possible to detect it using imaging studies. As a result, the incidence of peritoneal cancer has been underestimated, and has been considered a very rare disease. Even today, the reported incidence rates of peritoneal cancer vary widely, but increased awareness has led to the increase in the reported incidence of peritoneal cancer in the most recent studies. However, the "real" incidence is potentially higher because autopsy studies have found peritoneal metastasis unfailing in many cancer patients. Malignant mesothelioma is a truly rare disease since it is diagnosed in about one to three patients per 1,000,000 people per year. The disease is three times more common in men compared to women, and the risk of developing the disease increases with age. Malignant mesothelioma is mainly diagnosed in people 60 years and older, although younger patients have been described. The number of patients is expected to increase in the coming years due to exposure to asbestos. Peritoneal pseudomyxoma (PMP) is another rare subtype of peritoneal cancer. Studies of the incidence of PMP have been hampered by difficulties in the definition and nomenclature of this disease. Most reports suggest that the incidence of PMP is one to two patients per 1,000,000 people per year, and the incidence of PMP slightly higher in women than in men. In contrast to mesothelioma and PMP, peritoneal cancer that originates from a malignant tumor at a site other than the body is much more common and affects hundreds and thousands of patients each year. This form of peritoneal cancer should be considered a mestastatic disease of the primary tumor. Hence, this form is often referred to as "peritoneal metastasis." Virtually, every primary tumor in the human body can cause peritoneal cancer, but tumors of the gastrointestinal tract and ovaries in women are by far the most common sources. It is thought that at least 10% of patients with colon cancer will develop peritoneal cancer during their illness. Patients with gastric cancer even have a higher incidence of peritoneal cancer, which is thought to be approximately 30% of patients. More detailed information on the incidence of specific subtypes can be found in the "Knowledge Base”

In early stages, peritoneal cancer may not cause any symptoms. Frequently, peritoneal cancer is even discovered as a finding during surgery for a primary tumor. Even during the asymptomatic stage, the disease can already be widely disseminated and advanced, which supports its reputation for peritoneal cancer as a "silent killer" When the nodules begin to grow on the intestinal surface, they can cause progressive obstruction of the intestinal tract. This obstruction can result in an uncomfortable discomfort of the abdomen, loss of appetite and weight, nausea and constipation. Additionally, nonspecific symptoms, such as tiredness and pain, may occur. Peritoneal carcinomatosis can also result in the accumulation of large amounts of aqueous fluid in the abdominal cavity. This phenomenon is known as "malignant ascites" and eventually results in a grown abdominal cavity. For peritoneal pseudomyxoma, ascites typically consists of mucus, hence the name "jelly abdomen" is sometimes used to describe this specific subtype. Ascites can cause symptoms similar to those of intestinal obstruction, but ascites can also lead to shortness of breath due to pulmonary oppression. Complete intestinal obstruction, resulting in vomiting, abdominal pain and the inability to eat and drink, is often a severe and late symptom of peritoneal cancer and can result in the rapid deterioration of the patient's condition.

Peritoneal cancer is usually very difficult to diagnose, but it can be discovered during the examination or surgical treatment of the primary tumor or when a patient develops symptoms related to peritoneal awareness. There is no specific test that is considered ideal for the diagnosis of peritoneal cancer. Most of the time several of these are required. These approaches usually include the following:

• PHYSICAL EXAM - Sometimes nodules peritoneal tumors or ascites can be detected during physical examination of the abdomen, but this typically occurs only in advanced cases.
• DETECTION OF TUMOR MARKERS - These are blood studies that may indicate the presence of a malignant tumor in the body, but these studies are not very specific for most cancers, especially cancer peritoneal The most commonly used tumor markers are Embryonic Carcino Antigen (ACE) for colon cancer, CA 19-9 for pancreatic cancer and CA-125 for ovarian cancer.
• ULTRASOUND - This test uses sound waves to create a detailed image of the abdominal cavity on a computer monitor. This study is good for ascites detection, but it is usually not sensitive enough to detect peritoneal cancer or the primary tumor.
• COMPUTERIZED TOMOGRAPHY (CAT) - This detailed radiological examination may reveal the presence of peritoneal cancer or ascites in some cases. Unfortunately, a CT scan is not very good for detecting nodules of peritoneal cancer, especially not in the early stages of the disease. In general, tumor nodules should be at least 1cm, which is larger than the size of typical tumor nodules in peritoneal cancer, to be detected by CT
• PET-SCAN - This is a study that combines CT with nuclear radiology (positron emission tomography), and is especially useful for detecting metastases outside the peritoneal cavity. Similar to what happens with CT, peritoneal cancer is typically not visualized by PET.
• BIOPSY - If peritoneal cancer is suspected, a biopsy can be obtained. In a biopsy, a small piece of tissue is removed from the body for microscopic research. The biopsy can be taken after the patient has received local anesthesia through the skin or during a laparoscopy when the patient is under general anesthesia. A biopsy can confirm the presence of cancer cells in the peritoneal cavity. If the primary tumor is located in the gastrointestinal tract, an endoscopy may be necessary to take a biopsy.
• LAPAROSCOPY - When peritoneal cancer is suspected, a laparoscopy may be performed. This procedure is especially useful for collecting biopsies of tumor nodules and determining the extent of peritoneal cancer. Usually, the extent of the cancer is determined by calculating the peritoneal carcinomatosis index (PCI).
• PATHOLOGY - Small parts of the tumor can be removed and examined under a microscope. Cancer can be confirmed and tumor aggressiveness can be predicted based on these observations. Using specific techniques, the origin of the tumor can be determined, which is especially useful in cases where the location of the primary tumor is still unknown.
• ENDOSCOPY - When the primary tumor is potentially located in the gastrointestinal tract, an endoscope can be inserted through the mouth to investigate the esophagus, stomach and duodenum ("esophagogastroduodenoscopy") or through the anus to investigate the large intestine ("colonoscopy"). The deepest layer of the gastrointestinal tract is visualized with the endoscope. Most gastrointestinal tumors originate from the deepest layer and can be detected by endoscopy. In addition to the visualization of the tumor, biopsies of the tumor can be obtained with the endoscope, which can help confirm the presence of cancer and direct subsequent treatment.