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 over 50 will have at least one. Most don’t cause symptoms, and many are found during routine colonoscopies. But not all polyps are the same. Two major types - adenomas and serrated lesions - are known to turn into cancer over time. Knowing the difference matters because how they grow, how hard they are to spot, and how quickly they turn dangerous vary a lot.
Adenomas: The Classic Precancerous Polyp
Adenomas make up about 70% of all precancerous polyps. They’re the type doctors have been watching for decades. Under the microscope, they look like disorganized glandular tissue. There are three main subtypes, and size and shape tell you how risky they are.
- Tubular adenomas are the most common - about 70% of all adenomas. They’re usually small, round, and grow like little tubes. If they’re under half an inch (1.27 cm), the chance of cancer inside is less than 1%.
- Tubulovillous adenomas mix tube-like and finger-like growths. These make up about 15% of adenomas and carry a higher risk. When they’re bigger than 1 cm, about 10-15% may already have cancer cells.
- Villous adenomas are the rarest - only 15% of adenomas - but the most dangerous. They’re flat, spread out, and hard to remove completely. Their cancer risk jumps to 15-40% if they’re over 2 cm.
Size is critical. A polyp under 1 cm has a very low chance of cancer. One over 1 cm? That’s when you start worrying. Villous features - even if just a small part of the polyp - raise the risk by 25-30% compared to pure tubular ones. That’s why doctors don’t just remove them - they send them to the lab to check for these details.
Serrated Lesions: The Stealthy Pathway to Cancer
Serrated lesions are trickier. They account for 20-30% of all colorectal cancers, yet they’re less common than adenomas. The name comes from their "sawtooth" edge under the microscope - like tiny teeth along the crypts. There are three kinds, but only two really matter for cancer risk.
- Hyperplastic polyps are usually harmless, especially if they’re small and in the lower colon. They rarely turn cancerous.
- Sessile serrated adenomas/polyps (SSA/Ps) are the big concern. They’re flat, often hidden in the right side of the colon (cecum or ascending colon), and grow slowly but steadily. About 13% of SSA/Ps already show high-grade dysplasia or early cancer when removed. They’re hard to see during colonoscopy because they’re flat, pale, and blend into the colon wall.
- Traditional serrated adenomas (TSAs) are rarer but aggressive. They often have a mushroom-like shape and can turn cancerous faster than SSA/Ps.
SSA/Ps are especially dangerous because they hide. They don’t bulge out like a mushroom - they sit flat, sometimes with a slight bump. Under magnifying colonoscopy, they show round, open pits and tangled blood vessels. Because they’re often in the proximal colon, they’re missed more often than adenomas. Studies show a 2-6% miss rate during standard colonoscopy - meaning one in 20 of these could be overlooked.
How Detection Differs Between the Two
Not all polyps are easy to find. Pedunculated polyps - those with a stalk - are like little mushrooms on a stem. Easy to spot and remove. Sessile and flat polyps? Not so much.
Adenomas often stick out clearly. Even villous ones, though flat, tend to be redder and more textured. SSA/Ps are pale, sometimes covered in mucus, and blend in. That’s why AI-assisted colonoscopy systems - like GI Genius - are becoming standard. In trials, they boosted adenoma detection by 14-18%. For SSA/Ps, the improvement is even more critical. A missed SSA/P can mean cancer five to ten years later.
Location matters too. Adenomas are common in the lower colon and rectum. SSA/Ps? Up to 70% are found in the right side - the cecum and ascending colon. That’s harder to clean well before a colonoscopy, and the scope’s view is less clear there. Many patients with right-sided SSA/Ps have no symptoms at all. When symptoms do appear, they’re vague: blood in stool, unexplained anemia, or changes in bowel habits.
What Happens After Removal?
Removing a polyp during colonoscopy is the standard treatment. Success rates are high - 95-98% for adenomas under 2 cm. But for larger SSA/Ps (>2 cm), removal success drops to 80-85%. Why? Because they’re flat. If even a tiny bit is left behind, it can regrow.
After removal, the lab checks for cancer cells, depth of invasion, and margins. If cancer is found, you might need surgery. But if it’s just a precancerous polyp, your next step is surveillance. How often you get screened again depends on what you had.
For adenomas under 1 cm with low-grade dysplasia: repeat colonoscopy in 7-10 years.
For SSA/Ps ≥10 mm: most U.S. guidelines say 3 years. But in Europe, some recommend 5 years. Why the difference? European studies show slower progression. But since SSA/Ps can turn cancerous without warning, most U.S. doctors err on the side of caution.
And here’s the key: having any precancerous polyp - adenoma or serrated - raises your lifetime risk of colon cancer by 1.5 to 2.5 times. But that doesn’t mean you’ll get cancer. Most people never do. The goal isn’t fear - it’s vigilance.
Why Molecular Pathways Matter
It’s not just about what a polyp looks like. It’s about what’s happening inside.
Adenomas usually follow the "chromosomal instability" pathway. That means mutations in genes like APC kick off the process. Think of it like a broken brake pedal - cells keep dividing.
Serrated lesions follow the "CpG island methylator phenotype" (CIMP) pathway. This is about gene silencing - turning off tumor suppressor genes with chemical tags. BRAF mutations are common here. These polyps don’t grow fast, but they quietly turn cancerous through epigenetic changes.
This is why future screening is moving toward molecular testing. Soon, doctors may analyze a polyp’s DNA right after removal to predict if it’s likely to come back or turn cancerous. That could mean fewer unnecessary colonoscopies. Right now, over 6.5 million surveillance colonoscopies are done in the U.S. each year. Experts think personalized follow-up based on molecular markers could cut that by 20-30%.
What You Can Do
If you’ve had a polyp removed, stick to your follow-up schedule. Don’t skip it. Even if you feel fine. Most cancers develop silently.
If you haven’t been screened yet - get screened. Starting at age 45 (or earlier if you have family history) is now standard. Colonoscopy is still the gold standard because it finds and removes polyps in one go.
Don’t rely on stool tests alone if you’ve had a serrated polyp. They’re good for initial screening, but they can miss flat lesions. Colonoscopy is the only way to be sure.
And yes - lifestyle helps. Eat more fiber, limit red and processed meats, stay active, don’t smoke, and keep your weight in check. These won’t erase your risk if you’ve had a polyp, but they lower your overall chance of cancer.
Final Thought
Adenomas and serrated lesions are different, but both are preventable. The key is finding them early and removing them completely. It’s not about being scared of polyps. It’s about knowing they’re there, understanding what they mean, and taking action. Most people who have them never get cancer - because they got screened. That’s the real win.