Peritoneal cancer is primarily classified into two types: primary and secondary. Secondary peritoneal cancer accounts for more than 90% of clinical cases and is most often caused by metastasis from tumors such as gastric cancer, colorectal cancer, and ovarian cancer. Primary peritoneal cancer is rare and predominantly consists of high-grade serous carcinoma; its clinical presentation is insidious, with early symptoms typically limited to abdominal distension, abdominal pain, or mild ascites. Imaging diagnosis centers on the combined use of contrast-enhanced CT and MRI with diffusion-weighted imaging (DWI). The Peritoneal Cancer Index (PCI) serves as a key tool for assessing tumor burden, and patients with a PCI ≤ 15 may be candidates for cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC). Differential diagnosis should emphasize conditions such as peritoneal tuberculosis and peritoneal lymphoma.

I. Anatomical Characteristics of the Peritoneum

1. Basic Structure

- Parietal peritoneum and visceral peritoneum: The peritoneum is a single layer of mesothelial cells that lines the inner wall of the abdominal cavity (parietal layer, red) and covers the surfaces of the viscera (visceral layer, blue); the two layers transition to form the potential peritoneal cavity. In males, the peritoneal cavity is a closed space, whereas in females it communicates with the external environment via the fallopian tubes.

- Key anatomical regions:

- Omentum: Omentum majus (The omentum, a four-layered fold of visceral peritoneum that forms an apron-like structure covering the intestinal tract, and the lesser omentum, which connects the stomach to the liver) are common sites for peritoneal metastases; involvement of the greater omentum may result in the formation of an “omental cake.”

- Mesentery: Mesentery When the fan-shaped structure that anchors the small intestine is involved, it manifests as the “foggy mesentery” sign, which is an important contraindication to surgery.

- Special regions: The right paracolic gutter is most susceptible to involvement due to its hydrodynamic characteristics, and the Douglas pouch (the lowest point of the pelvic cavity) is a typical site for ovarian cancer metastasis.

2. Functional Features

The peritoneum performs lubrication, absorption, and immune defense functions; however, its fragile barrier properties facilitate the dissemination of tumor cells throughout the abdominal cavity via peritoneal fluid circulation.

II. Common Causes of Peritoneal Cancer

1. Primary Peritoneal Cancer

- Definition: A malignant tumor originating from the peritoneal mesothelium (e.g., high-grade serous carcinoma), with histological features similar to ovarian cancer, but with the ovary itself remaining normal or showing only superficial involvement.

- Etiology: The precise mechanism remains unclear; it may be associated with ectopic implantation of fallopian tube epithelium or mutations in the TP53 gene. It is most commonly observed in middle-aged and older women, with an average age of 51 years.

2. Secondary Peritoneal Cancer

- Primary source:

- Gastrointestinal tumors: gastric cancer (approximately 50% develop peritoneal metastases over the course of the disease), and colorectal cancer (5%–15% at initial diagnosis, with a postoperative recurrence rate of 20%–50%).

- Gynecologic malignancies: At the time of diagnosis, 70% of ovarian cancers have already metastasized to the peritoneum, making it the most common cause of secondary peritoneal cancer.

- Others: appendiceal mucinous neoplasms (which can readily lead to pseudomyxoma peritonei), pancreatic cancer, and others.

- High-risk factors: T4 stage of the primary tumor, perforation, positive lymph nodes, and mucinous adenocarcinoma histology.

III. Clinical Manifestations

1. Early symptoms

- Subtle onset: In the early stages, specific symptoms are often absent, with common nonspecific manifestations such as:

- Mild abdominal distension and intermittent abdominal pain (often characterized as a dull ache or a feeling of fullness).

- Minimal ascites (often missed on routine imaging).

- Elevated tumor markers (e.g., CA125, which is highly sensitive in peritoneal metastases associated with ovarian cancer).

2. Late-stage symptoms

- Typical triad: progressive abdominal distension, abdominal pain, and increasing abdominal circumference.

- Severe complications:

- Massive ascites causes abdominal distension and dyspnea.

- Intestinal obstruction (due to tumor involvement of the mesentery or intestinal tract).

- Cachexia, wasting, and changes in bowel habits.

IV. Typical Sites of Peritoneal Metastasis

1. Distribution by anatomical region

- High-incidence areas:

- Right colic gutter: This is where tumor cells are most likely to accumulate due to the predominant direction of peritoneal fluid flow.

- Subdiaphragmatic region: Respiratory movements cause tumor cells to adhere to the anterior surface of the diaphragm.

- Douglas’ pouch: the lowest point of the pelvic cavity, a characteristic site for ovarian cancer metastasis.

- Mesenteric root involvement: Its presence indicates a poor prognosis and constitutes a critical contraindication to surgery.

2. Morphological Characteristics of the Lesion

Low-load findings: small nodules (<5 mm), cystic metastatic lesions (characteristic of mucinous tumors), and calcified foci.

High-load performance:

Omental cake: The greater omentum shows thickening with soft-tissue density, with loss of the fat planes.

“Armor‑like” changes: the entire thickness of the intestinal wall is involved, with fixed, rigid bowel segments.

V. Imaging Diagnosis and Scoring

1. Preferred examination method

Contrast-enhanced CT can demonstrate peritoneal thickening (>2 mm), nodules (>5 mm), omental plaques, and ascites, but it has low sensitivity for lesions smaller than 5 mm.

Mesenteric thickening (with a thickened mesentery attached to the distal small intestine) is clearly visualized against the background of intra-abdominal fluid.

Extensive mesenteric involvement (thickening with marked enhancement), with a small amount of intra-abdominal fluid.

“Omental cake”

Paracolic gutter (yellow arrow)

Diaphragm

MRI combined with DWI:

DWI significantly enhances diagnostic performance, with a sensitivity of 75%–80% for lesions smaller than 1 cm and superior specificity compared to CT.

Key areas of observation: the hepatic hilum, the pelvis, and the mesentery, to avoid missed diagnoses due to high background signal.

2. Peritoneal Cancer Index (PCI) score

Scoring criteria:

The abdominal cavity is divided into 13 anatomical regions (regions 0–12).

Each region is scored according to the maximum tumor diameter:

0 points: No lesions.

1 point: Lesion ≤ 0.5 cm.

2 points: 0.5 cm < lesion ≤ 5 cm.

3 points: Lesion > 5 cm or fused lesions.

The total score ranges from 0 to 39; the higher the score, the greater the tumor burden.

Clinical significance:

PCI ≤ 15 points: Suitable for CRS plus HIPEC, with a 5-year survival rate of 11% to 30%.

PCI > 20 points: Surgical benefits are limited, and palliative care is often preferred.

Intraoperative gold standard assessment: Imaging‑guided PCI and surgical PCI show a 15%–20% discrepancy, necessitating confirmation via laparoscopic exploration.

VI. Differential Diagnosis

1. Major diseases that require differential diagnosis

- Peritoneal tuberculosis:

- Smooth, uniform peritoneal thickening, accompanied by low-density necrotic lymph nodes; ascitic fluid ADA (adenosine deaminase) > 40 U/L.

- Unlike the miliary nodules or omental cake appearance seen in peritoneal carcinomatosis.

- Peritoneal lymphoma:

- A uniform, massive mass is formed, often accompanied by marked enlargement of the retroperitoneal lymph nodes.

- Peritoneal cancer typically presents with a multifocal nodular pattern, without predominant lymph node enlargement.

- Pseudomyxoma peritonei:

- Jelly-like ascites (mucinous), with septated hypoechoic findings on ultrasound/MRI, often originating from an appendiceal tumor.

- It differs markedly from the bloody or turbid fluid seen in malignant ascites.

2. Key Differentiation Points

- Primary tumor workup: It is essential to rule out metastases from primary tumors of the ovary, gastrointestinal tract, and other sites (for primary peritoneal cancer, the ovaries must be normal or only superficially involved).

- Pathological diagnosis: The positive rate of ascitic fluid cytology is only 40%–60%; laparoscopic biopsy remains the gold standard, particularly for determining the histologic subtype (e.g., serous carcinoma, mucinous adenocarcinoma).