Polyps are abnormal growths that form in the colon and rectum. While most polyps are benign (non-cancerous), some polyps can eventually develop into colorectal cancer if not removed. Determining whether a polyp is benign or has the potential to become cancerous is an important part of colorectal cancer screening and prevention.
What are polyps?
Polyps are abnormal tissue growths that form on the inside lining of the colon (large intestine) and rectum. They are very common, especially as people get older. There are several different types of polyps:
- Adenomatous polyps (adenomas): These polyps are benign but have the potential to eventually become cancerous. Adenomas are the most common type of polyp and the kind most likely to turn into cancer.
- Hyperplastic polyps: These polyps are benign and are not likely to become cancerous.
- Inflammatory polyps: These polyps form as a result of inflammation in the colon and are not likely to become cancerous.
- Serrated polyps: These types of polyps have a saw-toothed appearance under the microscope. Some serrated polyps can progress to cancer.
How are polyps detected?
There are a few different screening tests that can detect polyps:
- Colonoscopy: This test uses a long, flexible tube with a camera on the end (colonoscope) to directly visualize the entire length of the colon and rectum. Any polyps can be removed during a colonoscopy. It is considered the gold standard for polyp detection.
- Flexible sigmoidoscopy: This test uses a shorter tube to view just the bottom third of the colon. It will miss polyps further up in the colon.
- CT colonography: This test uses CT scans to take cross-sectional images of the entire colon. It does not allow removal of polyps.
- Fecal tests: Stool-based tests check for cancer markers or blood in the stool. They can suggest presence of polyps but do not visualize polyps directly.
Colonoscopy is the best screening test because the doctor can identify, remove, and biopsy any polyps during the procedure.
How are polyps biopsied?
If a polyp is discovered during colonoscopy or sigmoidoscopy, the doctor will remove it using a wire loop or snare. The removed polyp is then sent to a lab where a pathologist will examine it under a microscope. This tissue examination is called a biopsy.
The pathologist looks at features of the polyp tissue to determine the type of cells and architecture present. This helps classify the polyp as benign (non-cancerous) or precancerous based on cell characteristics. Adenomatous polyps which have a higher chance of progressing to cancer display certain changes called dysplasia when examined under the microscope.
Features that indicate an adenoma is benign
An adenomatous polyp that is fully benign displays the following features under the microscope:
- Tubular architecture: The cells are arranged in tube or gland-like structures.
- Abundant mucus production: Mucus-secreting cells are abundant.
- Minimal abnormal changes: The cells look fairly normal in size and shape.
Features that indicate an adenoma is precancerous
An adenomatous polyp that has precancerous changes (dysplasia) displays these features under the microscope:
- Villous architecture: The cells have a shaggy or leaf-like appearance.
- Little mucus production: There is a decrease in mucus-producing cells.
- Significant abnormal changes: There are changes in the cell size, shape, and nucleus.
- Increased mitotic activity: There is an increase in dividing cells.
The more severe the dysplasia (abnormal changes), the higher the chance that the adenoma could progress to cancer. Adenomas with high-grade dysplasia are considered precancerous.
Categories of dysplasia
Pathologists categorize the degree of dysplasia into three grades:
- Mild dysplasia: There are early, minor changes to cells. Low risk of cancer.
- Moderate dysplasia: More significant changes are seen. Intermediate risk of cancer.
- Severe dysplasia: Abnormalities are widespread. High risk of progression to cancer.
Determining if a polyp is concerning
Based on the biopsy results, doctors can determine if an adenoma has concerning features. Polyps are considered advanced or potentially dangerous if they have any of these features:
- Size greater than 1 cm
- Villous or tubulovillous architecture (mixed tubular and villous features)
- Moderate or severe dysplasia
- Carcinoma in situ (very early stage cancer cells)
These high-risk polyps have an increased chance of eventually becoming cancerous. The doctor will recommend more frequent colonoscopies to monitor the colon in these cases.
Non-adenoma polyp results
If a polyp is identified as hyperplastic, inflammatory, or another benign type based on the biopsy, the risk of cancer is very low. No additional follow up beyond routine screening may be required.
Polyps that contain cancer cells
In some cases, the pathologist finds cancer cells within the polyp tissue. This indicates the adenoma has progressed to an adenocarcinoma – colorectal cancer. Additional treatment such as surgery and chemotherapy may be recommended if cancer cells have invaded deeper into the colon wall. Finding cancer at this localized polyp stage leads to better outcomes.
Key takeaways
- Adenomatous polyps have the highest risk of becoming cancerous.
- Microscopic features like dysplasia indicate precancerous changes.
- Size over 1 cm, certain architecture, and moderate/severe dysplasia increase cancer risk.
- Non-adenoma polyps are typically benign.
- Cancer cells within a polyp signify localized cancer requiring treatment.
Conclusion
Determining if a polyp is concerning for cancer involves microscopic analysis by a pathologist. The biopsy results look at the polyp’s cell characteristics and architecture to classify it as benign, precancerous, or malignant. This guides doctors in providing the appropriate follow up and surveillance to ensure early detection and prevention of colorectal cancer.