Colon polyps are growths on the inner lining of the large intestine (colon) or rectum. While many are harmless, some types can develop into colorectal cancer over time if left untreated. These polyps are often without symptoms, making regular screening important for early detection. Medical imaging, such as a computed tomography (CT) scan, visualizes internal structures like the colon.
Understanding CT Scans and Contrast
A CT scan, also known as a CAT scan, uses X-rays to create detailed cross-sectional images of the body. Unlike a standard X-ray, a CT scanner rotates around the patient, taking multiple X-ray pictures from different angles. A computer processes these images to construct detailed two-dimensional or three-dimensional views of bones, soft tissues, and blood vessels.
Contrast material is sometimes used during a CT scan to enhance visibility. This material, often iodine-based, appears bright on images, highlighting specific organs, blood vessels, or tissues that might otherwise be difficult to distinguish. It can be administered orally, rectally, or intravenously, depending on the body part examined.
CT Scans for Polyp Detection: Capabilities and Considerations
CT scans can detect colon polyps, particularly through CT colonography, also known as virtual colonoscopy. This method uses CT technology to create detailed images of the colon, allowing providers to look for polyps without inserting a traditional scope. CT colonography effectively identifies larger polyps (10 millimeters or more) with over 90% sensitivity, and polyps in the 6-9 mm range with around 84% sensitivity.
Despite its capabilities, CT colonography has limitations. It may not reliably detect very small polyps (under 5 millimeters), which have a lower cancer risk. Flat or sessile polyps, which lie flat against the colon lining, are also more challenging to identify than mushroom-shaped (pedunculated) ones. If a polyp is found during CT colonography, a separate traditional colonoscopy is still required for removal, as it is a diagnostic tool, not therapeutic. Patients undergoing CT colonography are also exposed to a small amount of ionizing radiation, with an effective dose ranging from 2 to 8 mSv.
Standard Approaches to Colon Polyp Screening
Beyond CT colonography, other established methods are commonly used for colon polyp screening. Traditional colonoscopy is a primary and highly effective screening tool, allowing direct visualization of the entire colon and immediate removal of detected polyps. During a colonoscopy, a flexible tube with a camera is inserted into the rectum and advanced through the colon, enabling the physician to identify and remove polyps during the same procedure.
Stool-based tests offer less invasive screening options. The Fecal Immunochemical Test (FIT) looks for hidden blood in stool, an early sign of polyps or cancer. Another option is the guaiac-based Fecal Occult Blood Test (gFOBT), which detects hidden blood but may require dietary restrictions. Stool DNA tests, such as Cologuard, examine stool samples for altered DNA and blood, indicating precancerous growths or cancer cells. While convenient, positive results from stool-based tests necessitate a follow-up colonoscopy to confirm findings and remove polyps.
Next Steps After Polyp Discovery
If a colon polyp is detected by any screening method, including a CT scan, next steps involve further evaluation and removal. Most polyps are noncancerous when found, but because some can become cancerous over time, removal is usually recommended. The standard procedure for removing colon polyps is a polypectomy, often performed during a traditional colonoscopy.
During a polypectomy, the physician uses specialized instruments passed through the colonoscope to snip or snare the polyp. The removed polyp tissue is sent to a laboratory for examination by a pathologist to determine its type and whether cancerous cells are present. The type, size, and number of polyps found, along with microscopic features, guide recommendations for future screening intervals. If cancer is identified within the polyp, additional imaging or surgical consultations may be recommended to assess spread and plan further treatment.