Understanding Neoplastic Colon Polyps: Sessile vs Pedunculated vs Villous

What Are Colon Polyps?

Colon polyps are abnormal mucosal growths projecting into the lumen of the colon. They vary widely in:

• Shape (gross appearance)
• Histological features
• Risk of malignancy

Colon polyps are abnormal mucosal growths that project into the lumen of the colon. They are common findings during colonoscopies and gross dissection of bowel specimens. Not all polyps are created equal – what are the gross differences and the malignant potential of each of them?

The most common neoplastic polyps are colonic adenomas, which are the precursors to the majority of colorectal adenocarcinomas (aka colon cancer). Colorectal adenomas are characterized by the presence of epithelial dysplasia. Typical adenomas range from 0.3 to 10 cm in diameter and can be pedunculated (think mushroom) or sessile (think flattened). 

Gross Morphology: Sessile vs Pedunculated

Identifying differences in what polyps look like can tell you (and the pathologist) how likely they are to become cancerous. Recognizing these differences helps guide your sampling and can ultimately affect patient outcomes.

Sessile Polyps

Pedunculated Polyps

Gross Features:

• Have a stalk that connects them to the mucosal surface (like a mushroom)
• Project into the lumen of the colon

Examples:

• Tubular adenomas
• Some tubulovillous adenomas

Malignancy Risk:

• Generally lower than sessile polyps of the same size
• Risk increases with size and degree of dysplasia

Architectural Subtypes

Adenomas are further categorized histologically based on their glandular architecture:

Tubular Adenomas

• Typically small and pedunculated
• Comprised mostly of tubular glands
• Malignancy risk: Relatively low

Villous Adenomas

• Often larger and sessile
• Covered by finger-like (villous) projections
• Malignancy risk: Highest among adenomas
• Appearance: May appear shaggy or cauliflower-like

Tubulovillous Adenomas

• Features of both tubular and villous types
• Malignancy risk: Intermediate (increases with the amount of villous architecture present)

Invasion and Malignancy Potential

When dysplastic epithelial cells invade the lamina propria, the lesion becomes an intramucosal carcinoma. However, true malignancy is only diagnosed when the cells breach the muscularis mucosae and enter the submucosa:

Invasive adenocarcinoma: Invades beyond muscularis mucosae; potential for metastasis

Intramucosal carcinoma: Limited to mucosa; no lymphatic access; polypectomy is usually curative

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Let’s pause for a second because this is really important—and I missed it the first (and second) time I learned it.

There are no working lymphatic vessels in the lamina propria (the layer just under the colon epithelial lining). That means if a cancer starts in the colon and breaks through the basement membrane into the lamina propria, it still can’t spread anywhere else yet. There’s no way out—no lymphatics means no escape route for the cancer cells.

For the cancer to keep spreading, it has to push deeper into the next layer, called the muscularis mucosa. Once it gets through that layer, then it can reach the lymphatic system and might spread. At that point, doctors may need to do a bigger surgery and check nearby lymph nodes.

But if the cancer hasn’t made it past the lamina propria, removing it can actually cure the patient!

Key Risk Factors for Malignancy

• Size: Larger polyps are more likely to be malignant
  • <1 cm: Rarely malignant
  • 4 cm: ~40% contain invasive cancer
• Histology: Villous > Tubulovillous > Tubular
• Growth pattern: Sessile > Pedunculated (in terms of risk)
• Number of polyps: Multiple or recurrent polyps increase overall risk

Study Tips

• Flat = sneakier but riskier
• Villous architecture is most associated with malignant potential

Use the mnemonic: “Villous = Villainous” to recall high malignancy risk.

• Bigger = badder: Size correlates with malignant potential (esp > 1cm)