A polyp is an abnormal tissue growth that forms on a mucous membrane, most commonly found in the colon and rectum. Understanding the classification of a polyp is the first step toward determining its significance. The term “sessile” describes a specific physical shape: a growth attached directly to the tissue wall without a connecting stalk. This morphology has implications for both detection and removal.
Defining Sessile Polyps
The word “sessile” in a medical context refers to a growth that is fixed in place, lacking a stem or stalk. A sessile polyp is characterized by a broad, flat base that anchors it firmly to the underlying mucosal layer. This dome-shaped or flattened appearance makes the growth difficult to see during standard screening procedures, as it blends in with the surrounding tissue. This structure contrasts with a pedunculated polyp, which grows on a thin stem, giving it a distinct mushroom-like appearance. The lack of a separating stalk in a sessile polyp means abnormal cells are immediately adjacent to the deeper layers of the organ wall.
Common Locations and Histological Varieties
Sessile polyps are most frequently encountered within the large intestine, encompassing the colon and rectum. Classification depends heavily on the cellular structure observed under a microscope, known as histology. Polyps are broadly categorized into non-neoplastic (generally benign) and neoplastic (having the potential for malignant transformation) types.
A common non-neoplastic variety is the hyperplastic polyp, which is typically small and rarely develops into cancer. The neoplastic forms, known as adenomas, carry a greater risk and account for the majority of colon polyps. Adenomas are subclassified based on glandular patterns, including tubular, villous, and tubulovillous types. Villous adenomas often present with a sessile morphology and have a higher potential for advanced cellular changes compared to purely tubular adenomas.
The Significance of Sessile Serrated Lesions
The most concerning type of sessile growth is the Sessile Serrated Lesion (SSL), historically called a Sessile Serrated Adenoma. These lesions are challenging because they are often flat and subtle, making them difficult to spot during a colonoscopy, especially in the proximal (right) side of the colon. Under a microscope, SSLs feature a distinctive saw-toothed or serrated pattern within the lining of the crypts.
SSLs progress to colorectal cancer through the serrated pathway, which accounts for up to 30% of all colorectal cancers. This pathway is often driven by the CpG island methylator phenotype (CIMP), an epigenetic process that can silence tumor-suppressor genes. SSLs frequently harbor the BRAF mutation, and this combination is a molecular hallmark of this aggressive pathway. Progression to cancer can sometimes occur more rapidly than the traditional adenoma-to-carcinoma sequence, highlighting the need for prompt and complete removal.
Detection and Removal Procedures
Sessile polyps are typically discovered during routine screening, most commonly through a colonoscopy. Because of their flat nature, these polyps can be easily overlooked, which emphasizes the need for skilled endoscopists and high-quality bowel preparation. Once identified, the removal procedure often requires specialized techniques due to the polyp’s broad attachment to the wall.
For smaller sessile polyps (generally those less than 10 millimeters), Cold Snare Polypectomy (CSP) is a common and safe method. This technique uses a wire loop, or snare, to remove the tissue without using electrical current, which minimizes the risk of thermal injury to the colon wall.
Larger sessile polyps, especially those 10 millimeters or more, frequently require Endoscopic Mucosal Resection (EMR). EMR involves injecting a solution beneath the lesion to lift it away from the deeper muscle layer, creating a protective fluid cushion. This lifting helps define the boundaries of the flat lesion and allows for safer, more complete removal, often in multiple pieces. Following the removal of any sessile polyp, a surveillance plan is established, involving subsequent colonoscopies at specific intervals determined by the size and histological type of the removed tissue.