Peritoneal cancer (PC) is an uncommon malignancy that usually arises as a secondary spread from other cancers, most often originating in the ovaries, colon, or stomach. Diagnosing this condition is challenging because the cancer spreads diffusely across the peritoneum, the thin membrane lining the abdominal cavity. The computed tomography (CT) scan is the initial imaging tool utilized when a healthcare provider suspects this cancer. While the CT scan is not the final diagnostic step, it serves a foundational purpose in mapping the disease and guiding subsequent investigations.
Visual Markers of Peritoneal Cancer on CT
CT scans provide cross-sectional images of the entire abdomen, allowing radiologists to look for both direct and indirect signs of disease. Direct signs are the tumor deposits themselves, which appear as solid plaques, nodules, or masses. These implants show variable contrast enhancement after the injection of an intravenous dye and can be scattered across the peritoneal surface or clustered in specific areas.
A common indirect sign is ascites, the abnormal accumulation of fluid within the abdominal cavity, often containing malignant cells. While ascites is not exclusive to cancer, its presence alongside other findings increases suspicion for peritoneal malignancy. Another characteristic sign is omental caking, which describes extensive tumor deposits that infiltrate and thicken the greater omentum, a large apron of fat covering the intestines. This appears as a dense, heterogeneous mass on the CT image.
Radiologists also look for evidence of peritoneal thickening or nodular enhancement. This thickening often occurs in the parietal peritoneum lining the walls of the pelvis or in the mesentery, the tissue that anchors the intestines. The ability of the CT scan to visualize these abnormalities helps localize the extent of the cancer, but reliance on these visible markers highlights the inherent limitations of the technique.
Diagnostic Limitations of Computed Tomography
Despite its utility as a primary screening tool, the CT scan has limitations in detecting the full scope of peritoneal cancer, particularly the smallest tumors. The resolution of the scan is insufficient to reliably identify microscopic disease or small implants, often referred to as micrometastases. For tumors smaller than one centimeter, the sensitivity of the CT scan to detect the disease can drop to between 25% and 50%.
CT scans are very poor at detecting lesions under five millimeters, with sensitivity reported to be as low as 11% in some studies. Furthermore, the scan struggles to differentiate active, dense tumor tissue from non-cancerous conditions like scar tissue or inflammation, which can mimic the appearance of malignancy. This difficulty can lead to false-negative results or an underestimation of the true volume of the disease.
The diffuse, thin-layer spread of cancer, known as peritoneal carcinomatosis, can be easily missed if there are no large, coalesced masses or significant amounts of fluid. Because of these resolution limits, a “negative” CT scan cannot definitively guarantee the absence of peritoneal cancer. Imaging alone frequently underestimates the extent of the disease compared to what surgeons find during an operation, a difference that can significantly impact treatment planning.
Confirmation and Staging Tools
Due to the CT scan’s limitations, it is rarely the final diagnostic step for peritoneal cancer. The gold standard for confirmation is a biopsy, which involves collecting a tissue sample for microscopic analysis. This procedure is often guided by imaging, such as a CT scan or ultrasound, to ensure the needle accurately reaches the suspicious lesion.
For accurate staging, diagnostic laparoscopy is often necessary. This minimally invasive surgical procedure involves inserting a narrow scope through small incisions to directly visualize the entire abdominal cavity. Laparoscopy allows the surgeon to assess the full extent of peritoneal involvement and determine the Peritoneal Cancer Index (PCI), a scoring system used to quantify the disease burden. This surgical staging is performed prior to major treatment to assess feasibility.
Supportive imaging modalities provide additional information. Magnetic Resonance Imaging (MRI) is often superior to CT for visualizing soft tissues and detecting small lesions in specific areas, such as the pelvis or bowel walls. Specialized techniques like diffusion-weighted MRI (DWI-MRI) show high accuracy in identifying implants smaller than one centimeter. Positron Emission Tomography (PET) scans, often combined with CT, identify the metabolic activity of tumors and search for spread outside the abdominal cavity.