Pancreatic cancer involves cells in the pancreas, located behind the stomach, growing out of control and forming a mass that can spread. The pancreas produces digestive enzymes and hormones like insulin. When a patient shows symptoms such as unexplained weight loss or jaundice, a computed tomography (CT) scan is often the initial and most informative imaging test. A multi-detector or helical CT scan uses a specialized protocol to provide cross-sectional images for diagnosis, characterization, and determining the extent of the disease. This technique visualizes the size, location, and potential spread of the mass.
How Contrast Imaging Highlights the Pancreas
Visualizing pancreatic cancer depends heavily on using an iodinated contrast dye administered intravenously. This material flows through the bloodstream, temporarily enhancing well-vascularized tissues and making them appear bright on the scan. The healthy pancreas has a rich blood supply, causing it to enhance brightly during the arterial or pancreatic parenchymal phase, approximately 40 seconds after injection.
Pancreatic ductal adenocarcinoma, the most common type, is hypovascular, meaning it has poor blood supply. Because the tumor receives significantly less blood than the surrounding healthy tissue, it takes up much less contrast dye. This difference creates a stark visual contrast: the brightly enhanced normal tissue sharply outlines the darker, less-enhanced tumor mass. This technique is necessary for distinguishing the cancer from the rest of the organ, especially for smaller lesions.
The Characteristics of the Primary Tumor Mass
The most direct visual sign of pancreatic cancer is a focal mass with low attenuation, meaning the tumor appears darker or less dense (hypodense) than the surrounding enhanced tissue. This appearance results from the tumor’s poor vascularity and its composition of dense, non-enhancing fibrous tissue. This difference in density is maximized during the specialized pancreatic phase of the contrast-enhanced CT.
Approximately 60 to 70 percent of these masses are found in the head of the pancreas, the widest part of the organ near the small intestine. The tumor typically presents with an irregular, ill-defined margin, indicating an infiltrative growth pattern. While the mass usually appears as an obvious focal enlargement, smaller tumors can be subtle or isoattenuating, meaning they have a density similar to the normal pancreas. When a tumor is isoattenuating, the mass may be nearly invisible, making diagnosis challenging. Radiologists must then rely on secondary signs. The identification of this low-attenuation mass remains the primary indicator of the disease.
Secondary Effects on Surrounding Structures
Beyond the primary mass, tumor growth often causes indirect signs by obstructing adjacent structures. The most common secondary finding is the dilation of the main pancreatic duct, known as upstream pancreatic duct dilation. As the tumor blocks the duct, the portion behind the obstruction swells.
If the tumor is in the head of the pancreas, it frequently compresses the common bile duct, which runs through that area. This compression leads to obstruction and subsequent dilation of the biliary system. When seen alongside the dilated pancreatic duct, this is called the “double duct sign.” Biliary dilation often causes the clinical symptom of jaundice. A further consequence of prolonged obstruction is pancreatic atrophy, where the tissue distal to the tumor shrinks because it is no longer functioning effectively. These indirect findings—duct dilation and parenchymal atrophy—are highly suggestive of pancreatic cancer, even when the primary mass is too small or subtle to be visible.
Determining Tumor Stage and Spread
The CT scan is the primary tool used to determine the stage of pancreatic cancer by assessing the extent of the disease’s spread. A major focus is evaluating the tumor’s relationship with major blood vessels supplying the intestines and liver, such as the superior mesenteric artery and vein, and the portal vein. This assessment is the most important factor in determining if the tumor is surgically removable, or resectable.
A tumor is considered less likely to be surgically removed if it is encasing or significantly invading these vessels, typically defined as having contact with more than 180 degrees of the vessel’s circumference. The scan allows the radiologist to measure the degree of tumor contact with the vessel walls. Determining this vascular involvement is a core component of the staging process.
The CT scan also checks for signs of distant metastasis, which is the spread of cancer to other organs. The most common site for distant spread is the liver, where metastatic lesions appear as multiple, small, low-attenuation masses distinct from the primary tumor. The scan also looks for enlarged lymph nodes outside the pancreas and for evidence of spread to the lining of the abdominal cavity, known as the peritoneum. The presence of distant metastasis automatically classifies the cancer as advanced stage disease.