A colon polyp is a growth on the inner lining of the large intestine. These growths are extremely common, and while most are benign, certain types can develop into colorectal cancer over time. Serrated polyps are a specific class distinguished by their unique appearance under a microscope, where the tissue folds into a characteristic saw-tooth or corrugated pattern. This particular morphology has placed them at the center of attention within gastroenterology, as they represent a pathway to cancer development that is distinct from the traditional model.
Defining Serrated Polyps and Their Subtypes
Serrated polyps are classified by the World Health Organization (WHO) into distinct subtypes, and the risk of progression varies significantly between them. The most frequent type is the Hyperplastic Polyp (HP), accounting for approximately 70% to 90% of all serrated lesions. HPs are generally considered low-risk and are not thought to develop into cancer, especially when small and located in the lower parts of the colon, such as the rectum or sigmoid colon.
The subtype with the greatest clinical significance is the Sessile Serrated Lesion (SSL). These lesions are considered precancerous. SSLs are typically flat or slightly elevated (sessile) and often occur in the right side of the colon. Though difficult to distinguish from HPs under a microscope, SSLs contain architectural distortions that signal their malignant potential.
The third classification is the Traditional Serrated Adenoma (TSA), the rarest of the three subtypes. TSAs are also considered precancerous and tend to present with a more classic polypoid shape, often with villous features. The distinction between these subtypes is paramount because the treatment and follow-up plan depend entirely on whether the lesion is categorized as a low-risk HP or a high-risk SSL or TSA.
The Serrated Pathway to Cancer
The serrated pathway is the distinct method by which certain serrated polyps transform into cancer. This pathway is responsible for an estimated 15% to 30% of all colorectal cancer cases, operating differently from the more common adenoma-carcinoma sequence. Molecularly, the pathway is frequently initiated by an activating mutation in the BRAF gene, which regulates cell growth.
The BRAF mutation often cooperates with the CpG island methylator phenotype (CIMP), where widespread hypermethylation occurs. This epigenetic change essentially silences the normal function of several tumor suppressor genes. For high-risk SSLs, this molecular progression often leads to the inactivation of the MLH1 gene, a DNA mismatch repair gene, resulting in a high level of microsatellite instability (MSI).
This unique molecular profile means that certain serrated polyps, particularly SSLs, can progress to cancer more rapidly than conventional adenomas. Understanding the molecular drivers of the serrated pathway reinforces the need for accurate pathological diagnosis after a polyp is removed.
Detection and Removal
Detecting Sessile Serrated Lesions during a colonoscopy presents a challenge due to their subtle physical characteristics. Unlike conventional polyps, which are often raised and easily visible, SSLs are characteristically flat, pale, and blend into the surrounding colon lining. They frequently possess a mucus cap that obscures their borders, making identification difficult.
Endoscopists employ careful inspection techniques to overcome detection difficulties. Techniques such as spraying water to wash away the mucus cap or using advanced imaging, like chromoendoscopy, help highlight subtle differences in tissue structure. Recognizing tell-tale signs, such as an irregular, dilated crypt pattern known as an O-pit, is a specialized skill that improves the detection rate.
Once a serrated polyp is found, the goal is complete removal through a procedure called polypectomy. For smaller lesions, cold snare polypectomy (CSP) is typically used. Larger or flatter SSLs, particularly those 10 millimeters or greater, often require Endoscopic Mucosal Resection (EMR). EMR involves injecting fluid beneath the lesion to lift it away from the muscular layer before removal. This lifting technique helps ensure the entire base of the lesion is excised, which is crucial because SSLs are known to have a higher rate of incomplete resection compared to other polyp types.
Post-Removal Surveillance
Following the removal and pathological diagnosis of a serrated polyp, the patient enters a surveillance protocol designed to reduce the risk of future cancer development. The timing of the next colonoscopy is determined by the size, number, and specific type of serrated polyp that was removed. Isolated, small Hyperplastic Polyps located in the rectosigmoid are generally considered low-risk and may not require surveillance beyond the standard screening schedule.
However, the discovery of a Sessile Serrated Lesion (SSL) requires a shortened surveillance interval. For a small SSL (less than 10 millimeters without cellular dysplasia), a follow-up colonoscopy is typically recommended in five years. If the finding is considered high-risk—such as an SSL 10 millimeters or larger, an SSL exhibiting dysplasia, or a Traditional Serrated Adenoma—the surveillance colonoscopy is scheduled within three years.
Patients who have large serrated polyps removed in fragments, known as piecemeal resection, may require a very short follow-up interval, sometimes as early as six months. This is done to confirm that the entire resection site has healed without recurrence. Adherence to these personalized surveillance schedules is paramount for cancer prevention, as the goal is to detect and remove any new or recurring lesions before they have a chance to progress.