Colorectal Polyp Types: Understanding Adenomas vs. Serrated Lesions

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 1 in every 2 adults over 60 will have at least one. Most don’t cause symptoms, which is why screening is so important. Left alone, some types can turn into colorectal cancer over time. But here’s the good news: if caught early and removed, they almost never become cancer.

Not all polyps are the same. The two main types that matter for cancer risk are adenomas and serrated lesions. These aren’t just different shapes-they follow different paths to cancer, show up in different parts of the colon, and require different approaches during colonoscopy.

Adenomas: The Classic Precancerous Polyp

Adenomas make up about 70% of all precancerous polyps. They’re the kind doctors have been watching for decades. Under the microscope, they look like disorganized glandular tissue. There are three subtypes, and each carries a different level of risk.

• Tubular adenomas are the most common-about 70% of all adenomas. They’re usually small, round, and grow like little mushrooms on a stalk. If they’re under half an inch, the chance of cancer is less than 1%.
• Tubulovillous adenomas are a mix of tubular and villous features. They’re bigger and flatter, making them harder to remove completely. About 15% of adenomas fall into this category, and they carry a higher risk.
• Villous adenomas are the rarest (about 15%) but the most dangerous. They’re flat, spread out like a carpet, and often grow in the rectum or lower colon. Even at the same size, they’re 25-30% more likely to contain cancer than tubular ones.

Size matters a lot. An adenoma larger than 1 centimeter has a 10-15% chance of already having cancer cells inside. That’s why doctors don’t just remove them-they send them to the lab to check for dysplasia, which means abnormal cell growth that’s a step closer to cancer.

Serrated Lesions: The Hidden Threat

Serrated lesions are trickier. They account for 20-30% of colorectal cancers, yet many doctors missed them for years because they don’t look like typical polyps. Under the microscope, they have a saw-tooth edge-that’s where the name comes from. There are three kinds:

• Hyperplastic polyps are usually harmless, especially if they’re small and in the lower colon. They rarely turn into cancer.
• Sessile serrated adenomas/polyps (SSA/Ps) are the real concern. They’re flat, often hidden in the right side of the colon (the cecum or ascending colon), and easy to overlook during colonoscopy. About 13% already show signs of high-grade dysplasia or early cancer when removed.
• Traditional serrated adenomas (TSAs) are less common but aggressive. They often have a more raised shape and are more likely to develop cancer than hyperplastic polyps.

SSA/Ps are especially dangerous because they grow slowly and don’t bleed much. You won’t feel them. They’re often found only during screening. Studies show they’re missed in 2-6% of colonoscopies because they blend in with the normal colon lining.

Why Detection Is So Hard

Not all polyps are created equal when it comes to visibility. Pedunculated polyps-those with a stalk-are easy to spot and remove. But flat and sessile polyps? They’re the silent problem.

SSA/Ps are often flush with the colon wall, making them look like normal tissue. Magnifying colonoscopy can reveal their telltale signs: enlarged, irregular crypt openings and twisted blood vessels. But not every endoscopist has the training or equipment to see them.

That’s why AI-assisted colonoscopy systems like GI Genius are changing the game. In recent trials, they boosted adenoma detection by 14-18%. For SSA/Ps, the improvement is even more critical. One study showed detection rates jumped from 45% to 78% when AI flagged subtle surface changes doctors might have missed.

Location matters too. SSA/Ps are most often found in the proximal colon-the part farthest from the rectum. That’s harder to clean and inspect thoroughly. If your bowel prep wasn’t perfect, these polyps can slip through.

What Happens After Removal?

Once a polyp is removed, it’s sent to a pathologist. The report tells you two things: what type it is, and whether it was fully removed.

For adenomas, if the entire growth was taken and there’s no cancer, your next colonoscopy is usually in 5-10 years-unless you had multiple or large ones. But for SSA/Ps, the rules are stricter.

The American Cancer Society recommends a 3-year follow-up if the SSA/P is 10 mm or larger, has dysplasia, or wasn’t completely removed. Some European guidelines suggest 5 years, based on lower progression rates in their populations. This is still debated, and your doctor will tailor the plan based on your specific case.

Here’s what you need to know: if your polyp was removed completely and shows no cancer, your risk of colon cancer drops dramatically. But having any precancerous polyp means you’re at higher risk than someone who’s never had one. That’s why ongoing screening is non-negotiable.

Who’s at Risk?

Age is the biggest factor. Most polyps appear after 50. But here’s something worrying: colorectal cancer is rising in people under 50. While we don’t fully understand why, it’s likely tied to diet, gut bacteria, and lifestyle.

Other risk factors include:

• Family history of colorectal cancer or polyps
• Smoking and heavy alcohol use
• Obesity
• Type 2 diabetes
• Inflammatory bowel disease (like ulcerative colitis)

Even if you feel fine, screening is your best defense. Most polyps cause no symptoms. When they do, it’s usually rectal bleeding, anemia from slow blood loss, or changes in bowel habits-like diarrhea or constipation that lasts more than a few days.

Molecular Pathways: Why It Matters

Not all colon cancers are the same. Adenomas usually follow the chromosomal instability pathway, driven by mutations in the APC gene. Serrated lesions follow the serrated pathway, linked to BRAF mutations and DNA methylation changes.

This isn’t just academic. These differences explain why some polyps progress faster, why some respond differently to treatments, and why future screening may be personalized. Within the next five years, labs will likely test removed polyps for specific gene markers to predict cancer risk. That could mean fewer unnecessary colonoscopies for low-risk people-and tighter monitoring for those who need it.

What You Can Do Now

Screening saves lives. If you’re 45 or older, get a colonoscopy. If you’re younger but have a family history, talk to your doctor about starting earlier.

Don’t wait for symptoms. By the time you feel something, it might already be too late. The goal isn’t to find cancer-it’s to find and remove the polyps before they get there.

After your colonoscopy, ask your doctor:

• What type of polyp was found?
• Was it completely removed?
• What’s my follow-up schedule?
• Should I consider genetic testing if I have multiple polyps?

Knowing the difference between adenomas and serrated lesions isn’t just for doctors. It’s your key to understanding your own risk-and taking control of your health.