Introduction: Who Should Undergo Diagnostic Assessment

The peritoneal cancer index, or PCI, is a measurement tool used when cancer has already spread to the peritoneum—the thin membrane that lines the inside of your abdomen and covers your organs. This condition is called peritoneal carcinomatosis, and it happens when cancer from another part of your body, such as your ovaries, colon, stomach, appendix, or pancreas, travels to this lining and forms tumors there.^[2][5]

Diagnostic assessment using the peritoneal cancer index is particularly important for patients who have been diagnosed with advanced cancers that are known to spread to the peritoneum. The most common cancers that lead to peritoneal spread include ovarian cancer, colorectal cancer, gastric cancer, appendiceal tumors, and a rare condition called pseudomyxoma peritonei.^[3][5] If you have one of these cancers and your doctor suspects it may have spread to your peritoneum, they will likely recommend tests to calculate your peritoneal cancer index.

People who should undergo diagnostic evaluation with PCI typically fall into several groups. First, those who have already been diagnosed with a cancer type known to spread to the peritoneum and are experiencing symptoms such as abdominal swelling, persistent abdominal pain, nausea, or unexplained weight changes.^[5][8] Second, patients who are being considered for specialized surgery called cytoreductive surgery, which aims to remove all visible tumors from the peritoneum. The PCI score helps surgeons decide whether complete removal is possible and whether the operation will benefit the patient.^[2][3]

It is advisable to seek diagnostic testing when symptoms suggest peritoneal involvement. Many patients do not experience symptoms until the cancer affects a large portion of the peritoneum. Common warning signs include a swollen abdomen due to fluid buildup (called ascites), persistent abdominal discomfort, changes in bowel habits like constipation, back pain, feeling full quickly when eating, and fatigue.^[5][8][14] These symptoms can be vague and similar to many other less serious conditions, which is why consulting a healthcare provider is essential to rule out cancer spread.

Diagnostic Methods: Measuring Tumor Spread in the Peritoneum

Diagnosing peritoneal cancer and calculating the peritoneal cancer index involves multiple steps and different types of tests. Doctors need to not only confirm that cancer has spread to the peritoneum, but also accurately measure the extent of that spread. This information is crucial for planning treatment and predicting how well a patient might respond to surgery or other therapies.

Imaging Tests

The first step in evaluating peritoneal cancer usually involves imaging tests. These non-invasive procedures allow doctors to see inside your abdomen and look for signs of tumor spread. The most commonly used imaging methods include computed tomography (CT) scans, magnetic resonance imaging (MRI), and sometimes positron emission tomography (PET) scans.^[5][8][11]

A CT scan is often the method of choice because it is widely available, relatively quick, and cost-effective. It can show the location and size of tumors, fluid buildup in the abdomen, and how organs are affected.^[12] However, CT scans have limitations. They may not detect very small tumor deposits, especially those smaller than a few millimeters. This means that even if a CT scan looks relatively clear, cancer may still be present in small amounts.^[11]

MRI scans can provide more detailed images of soft tissues and may be better at detecting certain types of peritoneal tumors, particularly when the deposits are larger than 10 millimeters.^[12] MRI is sometimes used when CT results are unclear or when doctors need more information before making treatment decisions.

PET scans, which use radioactive tracers to highlight areas of active cancer, are sometimes considered the most sensitive imaging method for detecting peritoneal spread. However, their role in routine practice remains somewhat controversial, and they are not always necessary for all patients.^[12]

Ultrasound can also be used, particularly pelvic ultrasound or transvaginal ultrasound in women, but it has lower sensitivity and specificity compared to CT or MRI. It plays a more limited role in diagnosing peritoneal carcinomatosis.^[12]

Blood Tests and Tumor Markers

Blood tests are another important diagnostic tool. Doctors may check for elevated levels of tumor markers—substances that tumors release into the bloodstream. Different cancers produce different markers. For example, patients with ovarian cancer may have elevated levels of a protein called CA-125, while those with colorectal cancer may show increased levels of CEA (carcinoembryonic antigen).^[5][8][16]

High levels of these markers can suggest that cancer has spread, but they are not definitive on their own. Tumor markers can be elevated for reasons other than cancer, and some people with cancer have normal marker levels. Therefore, these tests are most useful when combined with imaging and other diagnostic procedures.

Fluid Sampling (Paracentesis)

If you have fluid buildup in your abdomen (ascites), your doctor may perform a procedure called paracentesis. This involves inserting a thin needle through the abdominal wall to remove a sample of the fluid. The sample is then examined under a microscope in a laboratory to check for cancer cells.^[5][8]

Paracentesis is a relatively simple and safe procedure that can provide direct evidence of cancer spread. It is particularly useful when imaging tests are inconclusive or when doctors need to confirm the type of primary cancer that has spread to the peritoneum.

Biopsy

A biopsy involves removing a small sample of tissue from a suspected tumor so it can be examined under a microscope. This is the most definitive way to confirm that cancer is present and to identify what type of cancer it is. Biopsies of peritoneal tumors can be done in different ways, depending on the location and size of the tumor.^[5][8]

In some cases, a biopsy is performed during a minimally invasive procedure called staging laparoscopy (discussed below). In other cases, a needle biopsy may be done using imaging guidance to ensure the needle reaches the right spot.

Staging Laparoscopy and Calculating the PCI

The peritoneal cancer index is most accurately calculated during a surgical procedure called staging laparoscopy or at the time of open surgery. Staging laparoscopy is a minimally invasive procedure in which a surgeon makes small incisions in the abdomen and inserts a thin camera (laparoscope) to look directly at the peritoneum and organs inside.^[11][17]

This direct visualization allows the surgeon to see tumors that are too small to be detected by imaging tests. During the procedure, the surgeon can also take tissue samples (biopsies) and collect fluid for testing. Most importantly, the surgeon can calculate the peritoneal cancer index by evaluating the size and location of tumor deposits throughout the abdomen.^[2][17]

The peritoneal cancer index divides the abdomen into 13 regions: the upper, middle, and lower sections of the abdomen, as well as specific areas around the small intestine. In each region, the surgeon assigns a score from 0 to 3 based on the size of the largest tumor deposit found. A score of 0 means no tumor is present; 1 means tumor deposits are smaller than 0.5 centimeters; 2 means they are between 0.5 and 5 centimeters; and 3 means they are larger than 5 centimeters or involve a large portion of that region.^[2][4]

The scores from all 13 regions are added together to produce the total peritoneal cancer index, which can range from 0 (no tumor) to 39 (extensive tumor involvement throughout the abdomen).^[2][4] A higher PCI score indicates a greater tumor burden and generally suggests a more advanced disease state.

Selected PCI Regions

In some cases, doctors focus on specific regions of the abdomen that are particularly important for surgical outcomes. These are called selected PCI regions and typically include the small intestine (regions 9 through 12) and the area near the liver (region 2), known as the hepatoduodenal ligament or hepatic hilum.^[3][10]

Tumors in these areas are technically more difficult to remove completely during surgery. Research has shown that the selected PCI score (which only counts these specific regions) can be just as useful as the total PCI in predicting whether complete surgical removal is possible, and it may save time during the surgical assessment.^[3][10]

Peritoneal Washing Cytology

Another diagnostic test that may be performed during laparoscopy or open surgery is peritoneal washing cytology. In this procedure, the surgeon rinses the abdominal cavity with a sterile fluid solution and then collects the fluid to examine it under a microscope for the presence of cancer cells.^[11][17]

Even when no visible tumors are seen during surgery, a positive cytology result—meaning cancer cells are found in the fluid—strongly suggests that peritoneal spread has occurred. This test is especially valuable for detecting early or microscopic disease that might not be visible to the naked eye.

Diagnostics for Clinical Trial Qualification

Clinical trials are research studies that test new treatments or combinations of treatments to see if they are safe and effective. For patients with peritoneal cancer, participating in a clinical trial may offer access to cutting-edge therapies that are not yet widely available. However, to enroll in a clinical trial, patients must meet specific criteria, and diagnostic testing plays a key role in determining eligibility.

Confirming Peritoneal Cancer and PCI Score

Most clinical trials for peritoneal cancer require confirmation that the cancer has indeed spread to the peritoneum. This confirmation typically comes from imaging tests, biopsy results, or direct surgical observation during laparoscopy.^[1][4] Trials may also require a specific peritoneal cancer index score range for enrollment. For example, some studies may only accept patients with a PCI score below a certain threshold because higher scores may indicate that the treatment being tested is unlikely to work.

The PCI score is also used as a standard criterion to predict which patients are likely to benefit from specialized surgical procedures like cytoreductive surgery combined with heated intraperitoneal chemotherapy (HIPEC). Clinical trials testing these approaches often use PCI cutoff points to select patients who have the best chance of achieving complete tumor removal.^[3][10]

PCI Cutoff Points for Different Cancer Types

Different types of cancer that spread to the peritoneum have different PCI cutoff values for predicting surgical outcomes. These cutoff points are based on research studies that have analyzed large groups of patients.

For example, in patients with pseudomyxoma peritonei (a rare cancer that originates in the appendix and produces thick, jelly-like mucus), research has identified optimal PCI cutoff points for predicting whether complete surgical removal is possible. For low-grade pseudomyxoma peritonei, the optimal total PCI cutoff is 21, while for high-grade disease, it is 25.^[3][10][15] When the selected PCI (regions 2 plus 9 to 12) is used, the cutoffs are 5 for low-grade and 8 for high-grade tumors.

In patients with advanced ovarian cancer, a PCI score of 13 has been identified as a cutoff value above which survival outcomes are worse.^[4] Another study found that a PCI score higher than 24 was associated with incomplete cytoreductive surgery in a significant portion of patients, although some patients with scores above this threshold were still able to achieve complete tumor removal.^[9]

Other Diagnostic Criteria for Clinical Trials

In addition to PCI scores, clinical trials often require other diagnostic tests to ensure patients are suitable candidates. These may include:

• Blood tests to assess overall health, organ function (such as kidney and liver function), and tumor marker levels.
• Imaging studies to evaluate the extent of disease and ensure no distant metastases (spread to other parts of the body outside the peritoneum) are present.
• Performance status assessment, which measures how well a patient can carry out daily activities. Patients who are too weak or unwell may not be eligible for certain trials.
• Histological confirmation of cancer type through biopsy, since some trials are specific to certain cancer types or subtypes.

Some trials may also use advanced diagnostic tools, such as circulating tumor DNA (ctDNA) tests. These are newer blood tests that look for tiny fragments of DNA released by cancer cells into the bloodstream. ctDNA testing can help detect cancer that does not show up on imaging scans and may be used to monitor how well treatment is working during a clinical trial.^[11][17]

Role of PCI in Treatment Planning

The peritoneal cancer index is not only important for diagnosing and staging peritoneal cancer, but it also plays a central role in treatment planning. Doctors use the PCI score to help decide whether a patient is a good candidate for cytoreductive surgery, which involves removing all visible tumors from the peritoneum.^[2][5]

The goal of cytoreductive surgery is to achieve what is called a completeness of cytoreduction (CC) score of 0 or 1, meaning that no tumor or only very small tumor deposits remain after surgery. Patients with lower PCI scores are more likely to achieve complete cytoreduction, while those with very high scores may not benefit from surgery because complete removal is not possible.^[3][9]

Clinical trials often use PCI scores to stratify patients—that is, to divide them into groups based on their tumor burden—so that researchers can better understand how different treatments work in patients with varying degrees of disease severity.