Villous adenoma of the colon and rectum

by Jason Wasserman MD PhD FRCPC
July 18, 2025

A villous adenoma is a type of polyp that forms on the inner lining of the large intestine, which includes the colon and rectum. It is made up of abnormal glandular cells that grow in long, finger-like projections called villi. These polyps are considered precancerous, meaning they are not cancer, but the abnormal cells have the potential to turn into colorectal cancer if left untreated. Villous adenomas are less common than other types of adenomas, but they carry a higher risk of cancer.

Is a villous adenoma a type of cancer?

No, a villous adenoma is not cancer. It is a benign (non-cancerous) growth, but it contains dysplastic cells, which are abnormal and have the potential to become cancer over time. Because of this risk, villous adenomas are removed when found.

What are the symptoms of a villous adenoma?

Most villous adenomas do not cause symptoms and are found during routine colonoscopy. However, larger villous adenomas are more likely to cause symptoms, such as:

  • Rectal bleeding or blood in the stool.

  • Mucus in the stool.

  • Changes in bowel habits such as diarrhea or constipation.

  • Abdominal discomfort or cramping.

In rare cases, large villous adenomas can cause a condition called McKittrick–Wheelock syndrome, which leads to watery diarrhea and loss of electrolytes.

What causes a villous adenoma?

Villous adenomas develop due to genetic changes called mutations in the cells lining the colon that allow them to grow abnormally. These changes accumulate over time and are influenced by both inherited and lifestyle factors.

Risk factors include:

  • Age over 50.

  • A personal or family history of polyps or colorectal cancer.

  • Genetic conditions such as familial adenomatous polyposis (FAP).

  • Diets high in fat and low in fiber.

  • Smoking, obesity, and low physical activity.

How is this diagnosis made?

A villous adenoma is diagnosed after the polyp is removed during a colonoscopy and examined under a microscope by a pathologist. The pathologist looks at the structure of the adenoma and the appearance of the cells to confirm the diagnosis and determine if there is any dysplasia.

What does a villous adenoma look like under the microscope?

Under the microscope, a villous adenoma is made up of long, finger-like projections lined by abnormal glandular cells. These projections resemble the villi seen in the small intestine. To be classified as a villous adenoma, more than 75% of the polyp must show this villous architecture. The abnormal cells show changes that pathologists call dysplasia, and all villous adenomas contain at least low grade dysplasia.

Villous adenoma with low grade dysplasia

In a villous adenoma with low grade dysplasia, the cells show early signs of abnormal growth. The nuclei (the central part of the cell) may appear darker and slightly enlarged, and the cells may be arranged less evenly than normal. However, these changes are considered mild. Most villous adenomas show this pattern, and if completely removed, the risk of cancer developing is low.

Villous adenoma with high grade dysplasia

A villous adenoma with high grade dysplasia shows more advanced changes. The cells are much more abnormal in size and shape, and the architecture of the glands is often distorted. These changes suggest a higher risk that the adenoma could develop into cancer. If high grade dysplasia is found, complete removal of the adenoma is strongly recommended.

What does it mean if the margin is mentioned in the pathology report?

The margin is the edge of the tissue that was removed during the colonoscopy. If the margin is free of abnormal cells, it means the adenoma was likely removed completely. However, if dysplastic cells are found at the margin, there is a chance that some of the adenoma remains. Sometimes, especially when a polyp is removed in several fragments, the pathologist may not be able to evaluate the margin.

What is the risk of developing cancer?

Villous adenomas have a higher risk of progressing to cancer than other types of adenomas, especially if they are large or show high grade dysplasia. Because of this, they are always removed and closely monitored. Removing a villous adenoma helps prevent the development of colorectal cancer.

Can a villous adenoma come back after being removed?

If the entire villous adenoma is removed, it is unlikely to grow back in the same place. However, new polyps may develop elsewhere in the colon over time. This is why doctors recommend follow-up colonoscopies based on the number, size, and type of adenomas found.

Questions to ask your doctor

  • Was the villous adenoma completely removed?

  • Did the pathology report describe low grade or high grade dysplasia?

  • How large was the adenoma?

  • Were the margins clear of dysplasia?

  • When should I have my next colonoscopy?

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