A villous adenoma is a type of polyp that grows in the lining of the colon or rectum, characterized by long, finger-like projections of tissue. Among the three types of adenomatous polyps, villous adenomas carry the highest risk of becoming cancerous: 35% to 40% of them contain malignant cells at the time of discovery. They account for roughly 5% to 10% of all adenomatous polyps found during colonoscopy.
How Villous Adenomas Differ From Other Polyps
Adenomatous polyps are classified by their internal architecture. Tubular adenomas, the most common type (75% to 85% of all adenomatous polyps), are built from branching tube-shaped glands and have less than a 5% chance of harboring cancer. Tubulovillous adenomas are a middle category with a mix of both patterns, and 20% to 25% of them contain malignancy. A polyp officially qualifies as a villous adenoma when more than 80% of its structure is made up of the villous (finger-like) growth pattern.
Villous adenomas tend to be sessile, meaning they grow flat against the intestinal wall rather than dangling from a stalk. They’re often broader and flatter than tubular polyps, which can make them harder to spot and more challenging to remove completely.
Why They’re Considered High Risk
Size is the single strongest predictor of whether any adenoma contains cancer, and villous adenomas tend to be larger at the time of detection. The numbers are striking: polyps smaller than 5 mm contain high-grade abnormal cells or invasive cancer about 3.4% of the time. That jumps to 13.5% for polyps between 5 and 10 mm, and to 38.5% for those larger than 10 mm. For villous adenomas specifically, the risk reaches roughly 50% once they exceed 2 cm in diameter.
Patients with a villous adenoma in the rectum or sigmoid colon larger than 1 cm face about 3.6 times the risk of developing colorectal cancer compared to the general population. The National Cancer Institute identifies three features of adenomas that most strongly predict cancer transformation: larger size, villous architecture, and the degree of abnormal cell growth (dysplasia) within the polyp. Villous adenomas frequently check all three boxes.
The Genetic Path From Polyp to Cancer
Villous adenomas don’t appear out of nowhere. They develop through what’s known as the adenoma-carcinoma sequence, a step-by-step accumulation of genetic damage in colon cells. One of the earliest and most common changes is the loss of a gene called APC, which normally acts as a brake on cell growth. When APC stops working, cells begin dividing more freely, forming the initial polyp.
As additional mutations pile up in genes that control cell growth and DNA repair, the polyp can progress from mildly abnormal tissue to severely disordered cells and, eventually, to invasive cancer. This progression typically takes years, which is why regular screening colonoscopies are effective at catching and removing polyps before they become dangerous. Villous adenomas also occur outside the colon in rarer locations, including the duodenum and the ampulla of Vater (where the bile duct meets the small intestine), where cancer rates of 20% to 50% have been reported.
Symptoms Most People Notice
Most villous adenomas cause no symptoms at all and are found incidentally during routine colonoscopy. When they do cause symptoms, the most common are rectal bleeding, mucus in the stool, or a change in bowel habits.
Large villous adenomas, particularly those in the rectum, can produce an unusual and dramatic complication. These growths sometimes secrete large volumes of mucus and fluid directly into the bowel, causing watery, mucus-filled diarrhea multiple times per day. Over weeks or months, this fluid loss can deplete the body’s sodium, potassium, and protein levels severely enough to cause muscle cramps, fatigue, weakness, dangerously low blood pressure, and even kidney failure. This constellation of problems is called McKittrick-Wheelock syndrome. It’s rare, with only about 50 cases reported in the medical literature, but it’s a recognizable pattern: chronic mucus diarrhea plus kidney problems plus a large rectal tumor should raise immediate suspicion.
How They’re Found and Diagnosed
Colonoscopy is the primary tool for finding villous adenomas. During the procedure, the gastroenterologist can often recognize the characteristic flat, velvety, or shaggy surface of a villous polyp, but visual appearance alone isn’t enough for a definitive diagnosis. The polyp must be removed and examined under a microscope by a pathologist, who determines the percentage of villous architecture and checks for dysplasia or invasive cancer.
Distinguishing a villous adenoma from other growths like serrated lesions (sometimes called serrated polyps) matters because the two follow different genetic pathways and carry different surveillance schedules. The pathologist’s report will specify the polyp type, size, and whether cancer cells have invaded beyond the surface layer.
Removal Techniques
Small villous adenomas can be removed during a standard colonoscopy using a wire loop called a snare. For polyps under about 10 mm, cold snare removal (without electrical current) has the fewest complications and near-zero perforation rates. Larger polyps often require endoscopic mucosal resection (EMR), where fluid is injected beneath the polyp to lift it away from the underlying tissue before snaring it off. When a polyp is too large to remove in one piece, it’s taken out in sections, though this approach requires close follow-up because of higher recurrence rates.
A newer variation called underwater EMR skips the injection step entirely. The bowel is filled with water, which causes the polyp to float upward and makes it easier to snare. For villous adenomas in the duodenum or at the ampulla, removal is more complex and carries additional risks including pancreatitis, so specialized techniques and stenting are used.
Very large villous adenomas, particularly those in the rectum that have grown beyond what endoscopy can handle, or those where pathology confirms invasive cancer, may require surgical removal of the affected segment of bowel.
Follow-Up After Removal
Because villous adenomas carry a high recurrence and cancer risk, surveillance colonoscopy is scheduled sooner than for lower-risk polyps. Current guidelines from the U.S. Multi-Society Task Force on Colorectal Cancer recommend a repeat colonoscopy in 3 years after complete removal of an adenoma with villous features during a high-quality exam. This is more aggressive than the 5- to 10-year intervals recommended after removing small, low-risk tubular adenomas.
If the polyp was removed in pieces rather than intact, or if the pathologist finds cancer cells, follow-up is even more frequent. Your gastroenterologist will set a specific schedule based on the polyp’s size, the completeness of removal, and the pathology results. Sticking to that schedule is one of the most effective things you can do to prevent colorectal cancer from developing.