A polyp is an abnormal, often benign, growth that develops on the inner lining, or mucosa, of the colon or rectum. The Sessile Serrated Polyp or Adenoma (SSP/SSA) represents a distinct and particularly significant type of precancerous lesion. These lesions are a major concern in colorectal cancer prevention because they follow a unique and often accelerated path toward malignancy. Due to their subtle appearance, SSP/SSAs are often difficult to detect during standard screening, making their identification and complete removal a high priority for gastroenterologists.
Distinguishing Characteristics of Sessile Serrated Polyps
The term “sessile” means the growth is flat or broad-based, lacking the stalk-like structure seen in conventional polyps. This flat morphology causes the lesion to blend into the surrounding colonic mucosa, contributing to the difficulty in visual detection during colonoscopy.
The “serrated” part of the name refers to the saw-toothed appearance of the glandular structures when viewed under a microscope. This distinct histological architecture results from abnormal cell growth within the crypts of the colon lining. Pathologists look for specific markers, such as architectural distortion where the crypts branch or extend horizontally, to differentiate SSP/SSAs from benign hyperplastic polyps.
SSP/SSAs are classified as high-risk precancerous lesions, unlike most common hyperplastic polyps. They are frequently found in the right (proximal) side of the colon, a location where detection can be more challenging.
The Unique Cancer Progression Pathway
SSP/SSAs progress toward cancer via a unique molecular mechanism known as the serrated pathway. This mechanism is responsible for an estimated 20% to 30% of all colorectal cancer cases, operating independently of the more common adenoma-carcinoma sequence.
A key molecular feature of the serrated pathway is the strong association with the BRAF gene mutation, particularly the V600E variant, which is found in a high percentage of SSP/SSAs. This mutation activates a cell signaling pathway that promotes uncontrolled cell survival and proliferation. The genetic change is often accompanied by the CpG island methylator phenotype (CIMP), which involves the widespread silencing of tumor-suppressor genes through a process called hypermethylation.
The accumulation of these epigenetic changes, including the hypermethylation of the MLH1 gene promoter, eventually leads to high-level microsatellite instability (MSI-H). This instability speeds up the mutation rate and drives the rapid progression from a precancerous polyp to an aggressive carcinoma. The SSP/SSA is recognized as a significant source of “interval cancers,” which are cancers detected between scheduled colonoscopies.
Challenges in Identification and Removal
The appearance of Sessile Serrated Polyps creates significant challenges for endoscopists during screening colonoscopy. Their flat structure means they lack the prominent, raised profile of conventional polyps and can easily be overlooked. They often have a color similar to the surrounding healthy mucosa, or they may be covered by a subtle, yellowish cap of mucus.
To improve detection rates, endoscopists employ quality measures, such as ensuring an adequate withdrawal time—the time spent inspecting the colon lining while removing the scope. Specialized techniques, including high-definition endoscopy and chromoendoscopy, which uses dye spray to highlight subtle surface changes, are often necessary to define the lesion’s vague borders.
Once detected, the removal of SSP/SSAs also presents difficulties because of their broad base and indistinct margins. There is an increased risk of incomplete resection compared to traditional polyps. For larger SSP/SSAs, particularly those over 10 millimeters, specialized techniques like Endoscopic Mucosal Resection (EMR) are used to ensure the entire lesion is removed, often requiring the injection of fluid beneath the polyp to lift it away from the muscular wall before cutting.
Post-Removal Surveillance and Follow-Up
Due to the biological aggressiveness and high recurrence risk associated with SSP/SSAs, long-term surveillance protocols are mandatory following their removal. The surveillance schedule is individualized based on the size, number, and presence of dysplasia within the removed lesions. This proactive management aims to clear the colon of precancerous tissue and monitor for the development of new lesions.
For patients with a high-risk finding, defined as an SSP/SSA 10 millimeters or greater in size, any SSP/SSA that shows dysplasia, or multiple SSP/SSAs, a repeat colonoscopy is typically recommended in three years. If the removed SSP/SSA was smaller than 10 millimeters and did not contain dysplasia, the interval for the next surveillance colonoscopy may be extended to five years. These intervals are designed to intercept any new or recurrent serrated polyps before they progress to cancer.
If a very large sessile polyp was removed in pieces (piecemeal resection), a short-interval follow-up colonoscopy is often scheduled within six months to confirm the removal site is clear of residual tissue. Adherence to these strict surveillance guidelines is crucial for a patient’s long-term health management after an SSP/SSA diagnosis.