Polyps are abnormal tissue growths that can form on the inner lining of various organs, with the colon and rectum being common locations. While many polyps are benign, some have the potential to develop into cancer over time. Among the different types of polyps, sessile polyps represent a specific form of growth that is directly attached to the tissue with a broad base, rather than on a stalk. Understanding the characteristics of sessile polyps and their potential for malignancy is an important aspect of colon health.
Understanding Sessile Polyps
Sessile polyps are characterized by their flat or dome-shaped appearance, growing directly from the mucosal lining of the colon or rectum without a stalk. This broad-based attachment distinguishes them from pedunculated polyps, which are connected by a narrow stem. Sessile polyps can develop in various parts of the colon, though some types are more commonly found in specific regions. The flat or subtle nature of sessile polyps can sometimes make them challenging to detect during a colonoscopy, as they may blend in with the surrounding tissue. Complete removal of sessile polyps can also be more technically challenging compared to pedunculated polyps due to their broad attachment to the colon wall.
Sessile Polyp Types and Cancer Risk
The potential for a sessile polyp to become cancerous depends significantly on its histological type and size. Polyps are broadly classified as neoplastic, meaning they have the potential for malignancy, or non-neoplastic, which are generally benign. Most colon cancers originate from polyps, though only a small percentage of all polyps will ultimately become cancerous.
Hyperplastic Polyps
Hyperplastic polyps are a common type of serrated polyp, accounting for approximately 75% of all serrated polyps found. These are generally considered non-neoplastic and typically do not pose a risk of developing into cancer. Small hyperplastic polyps, particularly those in the rectum and sigmoid colon, are rarely malignant and usually do not require extensive follow-up.
Adenomatous Polyps
Adenomatous polyps are a different category and are considered pre-cancerous, meaning they have the potential to become cancerous if not removed. These are the most common type of colon polyp, making up about 70% of all polyps. Adenomas are further classified based on their microscopic growth patterns:
Tubular adenomas: The most frequent type, accounting for about two-thirds of adenomatous polyps, and carry a lower risk of malignancy, with less than 10% developing into cancer.
Tubulovillous adenomas: Exhibit a mixture of both tubular and villous growth patterns. Their risk of progression to cancer is intermediate, with studies indicating a malignancy risk around 20-25%.
Villous adenomas: Characterized by finger-like or leaf-like projections, these carry the highest risk among adenomas for progressing to cancer, with a malignant risk ranging from 15-25%. For villous adenomas larger than 2 cm, the chance of containing cancer can increase to 50%, and for those over 4 cm, the risk of adenocarcinoma can approach 40%.
Sessile Serrated Lesions (SSLs)
Sessile serrated lesions (SSLs), also known as sessile serrated adenomas, are a distinct type of polyp with malignant potential that can be difficult to distinguish from hyperplastic polyps during endoscopy. SSLs are recognized as precursors to colorectal cancer, developing through a different pathway than conventional adenomas. While estimates of their cancer risk vary, some studies suggest that up to 25% of people with SSLs may develop cancer. The prevalence of dysplasia in SSLs is estimated to be around 5%, though a larger cross-sectional study found that 14% had low- or high-grade dysplasia and 1% had adenocarcinoma.
Polyp Size and Risk
The size of a polyp is a significant factor influencing its cancer risk. Larger polyps are more likely to harbor cancerous cells. Polyps 10 mm or larger are considered to have a higher cancer risk. For example, while polyps 5 mm or less have a low risk of cancer, between 30-50% of polyps larger than 2 cm can be cancerous. Polyps with high-grade dysplasia, indicating more abnormal cells, also warrant increased attention.
Screening and Follow-Up
Regular screening is important for the detection and management of sessile polyps. Colonoscopy is the primary method for identifying polyps in the colon, as many polyps, including sessile ones, often do not cause noticeable symptoms. During a colonoscopy, if a polyp is discovered, it is typically removed in a procedure called a polypectomy.
The removal of sessile polyps can be performed using various endoscopic techniques. For smaller polyps, biopsy forceps or a snare may be used. Larger sessile polyps may require more specialized techniques such as endoscopic mucosal resection (EMR), where fluid is injected to lift the polyp from the underlying tissue, allowing for its removal. After removal, the polyp tissue is sent to a pathology laboratory for microscopic examination to determine its type and whether it contains cancerous cells.
Following polyp removal, surveillance colonoscopies are recommended to monitor for new growths. The frequency of these follow-up procedures depends on the characteristics of the removed polyp, including its type, size, and the presence of high-grade dysplasia. For instance, individuals with sessile serrated polyps or larger adenomas often require more frequent surveillance compared to those with small, low-risk polyps.