Colorectal Polyp Types: Adenomas vs. Serrated Lesions Explained

What Are Colorectal Polyps?

Colorectal polyps are small growths that stick out from the inner lining of your colon or rectum. They’re common-about 30 to 50% of adults will have at least one by age 60. Most don’t cause symptoms, and many are harmless. But some can turn into cancer over time. That’s why finding and removing them during a colonoscopy is one of the most effective ways to prevent colorectal cancer.

Not all polyps are the same. Two main types carry cancer risk: adenomas and serrated lesions. They look different under the microscope, grow in different ways, and need different follow-up plans. Knowing the difference helps doctors decide how often you need a repeat colonoscopy-and whether you’re at higher risk.

Adenomas: The Classic Precancerous Polyp

Adenomas make up about 70% of all precancerous polyps. They’re the type most people think of when they hear "polyp." These growths start as small, benign bumps but can slowly turn into cancer if left alone.

There are three subtypes, based on how they grow:

• Tubular adenomas (70% of adenomas): These are the most common and least risky. They’re usually small, round, and grow like tiny tubes. If they’re under half an inch, the chance of cancer is less than 1%.
• Tubulovillous adenomas (15%): These mix tube-like and finger-like growth patterns. They’re more likely to become cancerous than tubular ones, especially if they’re larger than 1 cm.
• Villous adenomas (15%): These are flat, spread out, and harder to remove completely. They carry the highest cancer risk-even a 1 cm villous adenoma has a 10-15% chance of already containing cancer cells.

Size matters. A polyp smaller than 0.5 cm has almost no chance of being cancerous. But once it hits 1 cm or larger, the risk jumps. And if it has villous features, that risk goes up another 25-30% compared to a tubular polyp of the same size.

Serrated Lesions: The Stealthy Path to Cancer

Serrated lesions account for 20-30% of all colorectal cancers, even though they’re less common than adenomas. They’re called "serrated" because under the microscope, their edges look like tiny saw teeth. These polyps are sneaky-they’re often flat, hard to spot during colonoscopy, and tend to hide in the upper part of the colon.

There are three types:

• Hyperplastic polyps: These are usually harmless, especially if they’re small and found in the lower colon. They don’t turn into cancer.
• Sessile serrated adenomas/polyps (SSA/Ps): These are the big concern. They’re flat, often larger than 1 cm, and located in the cecum or ascending colon. About 13% of SSA/Ps already show high-grade dysplasia or early cancer when removed. Their shape makes them easy to miss during colonoscopy-up to 6% of these polyps go undetected.
• Traditional serrated adenomas (TSAs): These are rarer but also precancerous. They often look like small, mushroom-shaped growths and carry a cancer risk similar to adenomas.

Unlike adenomas, which grow slowly over 10-15 years, SSA/Ps can turn into cancer faster-sometimes in just 5-7 years. That’s why finding them early is critical.

Why Detection Is So Hard

Not all polyps are easy to see. The shape and location make a huge difference.

Pedunculated polyps grow on a stalk, like a mushroom. They’re easy to spot and remove.

Sessile polyps sit flat on the colon wall. No stalk. Just a broad base. These are harder to see, especially if they’re small or hidden behind folds in the colon.

Flat polyps are even trickier. They’re flush with the lining-almost invisible unless you’re using high-definition scopes or AI-assisted tools.

SSA/Ps are especially problematic because they’re often in the right side of the colon, where the bowel is wider and more curved. Standard colonoscopy misses them more often than adenomas. Studies show a 2-6% miss rate for sessile polyps, and it’s even higher for flat ones. That’s why newer tools like AI-assisted colonoscopy systems (like GI Genius) are now being used in many clinics-they’ve been shown to boost adenoma detection by 14-18%.

How They Turn Into Cancer

Adenomas and serrated lesions follow different paths to cancer.

Adenomas usually develop through the chromosomal instability pathway. That means they pick up mutations in genes like APC, which controls cell growth. Over time, extra mutations pile up, and the polyp becomes cancerous.

Serrated lesions follow the serrated pathway, also called the CpG island methylator phenotype (CIMP). These polyps have mutations in the BRAF gene and show heavy DNA methylation-essentially, their genes get switched off in the wrong places. This pathway leads to a different type of cancer, often with different features and worse outcomes if not caught early.

Knowing the molecular type isn’t routine yet, but experts predict that within five years, labs will routinely test removed polyps for these markers. That will help doctors tailor surveillance plans to your actual risk-not just the size or shape of the polyp.

What Happens After Removal?

Once a polyp is found, it’s removed during the colonoscopy. For most adenomas under 2 cm, success rates are 95-98%. But for large sessile serrated lesions, especially those over 2 cm, the success rate drops to 80-85% because they’re flat and harder to grab completely.

After removal, the pathologist checks the tissue. The key question: Was it removed fully? And does it show cancer?

If it’s a small tubular adenoma (under 1 cm) with no high-grade dysplasia, you’re usually asked to come back in 5-10 years. But if you had an SSA/P that’s 1 cm or larger, guidelines differ. The U.S. recommends a follow-up colonoscopy in 3 years. Some European guidelines say 5 years, based on lower observed progression rates in their populations.

And if you had any serrated polyp-even a small one-you’re at 1.5 to 2.5 times higher risk of developing colorectal cancer later. That doesn’t mean you will. Most people never do. But it does mean you need to stick to your screening schedule.

Who Needs Screening and When?

Everyone should start regular colorectal cancer screening at age 45, according to the American Cancer Society. But if you’ve had a polyp, your schedule changes.

Here’s a simple breakdown:

• 1-2 small tubular adenomas (<1 cm): Return in 7-10 years.
• 3-10 adenomas, or any adenoma ≥1 cm: Return in 3 years.
• Any SSA/P ≥10 mm: Return in 3 years (U.S. guidelines).
• More than 10 adenomas, or any adenoma with high-grade dysplasia: Return in 1 year.
• Complete removal of a large sessile serrated lesion with clear margins: Return in 5 years.

If you’re unsure, ask your doctor for a written summary of your polyp type, size, and location. That helps when you see a new gastroenterologist or need to explain your risk to family members.

What About Symptoms?

Most polyps cause no symptoms at all. That’s why screening is so important.

But if you notice any of these, get checked:

• Rectal bleeding (30-40% of symptomatic cases)
• Iron deficiency anemia (15-20% of cases)-often from slow, unnoticed bleeding
• Changes in bowel habits lasting more than a few days
• Unexplained abdominal pain or cramping

Don’t wait for symptoms. By the time they appear, the polyp may already be advanced.

What’s Next for Polyp Detection?

Research is moving fast. The 2023 SEER data shows colorectal cancer rates are dropping by 3% per year in people over 55-thanks to better screening and polyp removal. But in people under 50, rates are rising by 2% per year. We don’t fully understand why.

Future tools will include:

• AI-powered colonoscopies that flag missed polyps in real time
• Blood or stool tests that detect molecular signatures of high-risk polyps
• Personalized surveillance based on your polyp’s DNA profile, not just its size

Right now, we’re still using the same tools we’ve had for decades. But in the next five years, your colonoscopy could be followed by a quick molecular test that tells your doctor exactly how often you need to come back.

Bottom Line

Not all polyps are created equal. Adenomas are the classic precancerous growths. Serrated lesions, especially SSA/Ps, are the silent threat-flat, hidden, and fast-growing. Both need to be removed. Both need follow-up.

The good news? If you get screened regularly and your polyps are removed completely, your risk of colorectal cancer drops by 75-90%. You’re not just preventing cancer-you’re stopping it before it starts.

Don’t skip your colonoscopy. Don’t ignore your results. And don’t assume a polyp is harmless just because it’s small. The right follow-up saves lives.