Quick Answer
Doctors cannot definitely determine if a polyp is cancerous just by looking at it with the naked eye. To tell if a polyp is cancerous, the polyp needs to be removed and sent to a lab for biopsy. Under a microscope, a pathologist can examine the cells of the polyp to see if they are cancerous or benign (non-cancerous).
What is a Polyp?
A polyp is an abnormal growth of tissue that develops on the lining of the colon (large intestine) or rectum. Polyps form when some cells in the lining multiply faster than normal cells. While most polyps are benign (non-cancerous), some polyps can become cancerous over time.
The two main types of polyps are:
Adenomatous Polyps
Also called adenomas, these polyps can develop into cancer. Adenomatous polyps are most likely to become cancerous.
Hyperplastic Polyps
These polyps are rarely pre-cancerous. But they can indicate increased risk of adenomas forming.
Polyps can vary greatly in size – from a tiny dot to over a centimeter wide. Small polyps may produce no symptoms. Larger polyps can sometimes cause rectal bleeding, intestinal blockages, or other problems.
Can Doctors Visually Diagnose if a Polyp is Cancerous?
Unfortunately, doctors cannot determine with certainty whether a polyp is cancerous or benign simply by looking at it during a colonoscopy or sigmoidoscopy examination.
Here are some key reasons why a visual inspection isn’t definitive:
Appearance is Not a Reliable Indicator
Cancerous and non-cancerous polyps often look alike. While there are some general traits that may point to cancer, a polyp’s appearance alone cannot provide a sure diagnosis. Non-cancerous polyps can have an irregular shape, variegated surface, large size, etc. – while cancer polyps can look harmless.
Microscopic Cellular Changes
What determines whether a polyp is cancerous or not is changes at the microscopic cellular level. These changes are impossible to detect with the naked eye. Examining the polyp’s cells under a microscope is needed to check for cancer.
Location Alone Doesn’t Indicate Cancer
Where a polyp is located does not necessarily mean it is more likely to be cancerous. While certain parts of the colon may have higher cancer risk, potentially cancerous polyps can form anywhere in the colon and rectum.
Most Are Non-Cancerous
The vast majority of colon polyps are benign growths rather than cancers. Estimates indicate around 90% of polyps will not become cancerous. So there is only a small chance of any particular polyp already being malignant.
How are Polyps Tested for Cancer?
If polyps are discovered during a colonoscopy, the doctor will remove them so they can be sent for biopsy analysis. Here are the steps for testing polyps:
Polyp Removal
Most polyps can be removed safely during a colonoscopy or sigmoidoscopy using a wire loop tool. Larger polyps may take longer to remove. This is a minimally invasive procedure.
Biopsy
The removed polyp is sent to a laboratory. A pathologist will prepare slides of the polyp tissue and examine the cells under a microscope. This allows them to determine if the cells show changes indicating cancer.
Genetic Analysis
Sometimes, doctors may perform additional genetic tests on polyp cells to look for DNA mutations associated with higher cancer risk.
Staging
If cancer is found in a polyp, the cancer will be staged. This means using scans and biopsies to see how far the cancer has progressed. Early stage detection improves outcomes.
What are the Typical Results of Polyp Biopsies?
Around 10-20% of polyps removed during colonoscopy will show concerning cellular changes when biopsied. Here are the possible results:
Benign (Non-Cancerous)
This is the most common finding. The cells appear normal under the microscope. The patient can be reassured the polyp was not malignant.
Pre-cancerous Changes
The cells display abnormalities like crowding, enlarged nuclei, and architectural distortions. This indicates precancerous changes. These polyps may turn cancerous if not removed.
Cancer Confirmed
The pathologist finds cancerous cells in the polyp tissue. Treatment and staging will begin immediately in cancer cases.
Uncertain Results
In some cases, biopsy results may be uncertain. Additional examination of the tissue may be needed to make a definitive diagnosis.
What Factors Increase Risk of Polyps Being Cancerous?
While there is no certain way to visually diagnose cancerous polyps, some features may raise the level of suspicion. Factors linked to higher cancer risk include:
Patient Age
Older patients are more likely to have cancerous polyps. After age 50, polyp cancer risk increases.
Polyp Size
Larger polyps have higher potential to contain cancerous cells. Polyps over 1 cm have a greater cancer risk.
Polyp Type
Adenomatous polyps are the most likely to become cancerous over time. Hyperplastic polyps rarely become malignant.
Polyp Number
Patients with multiple polyps have increased odds that one may contain cancer. But even solitary polyps can be cancerous.
Polyp Shape
Irregular, multilobular shaped polyps raise more concern than smooth, rounded polyps. But shape is not definitive.
Family History
Patients with a family history of colorectal cancer or polyps are more likely to have cancerous polyps themselves.
What Happens if a Polyp is Cancerous?
If a polyp contains cancerous cells, doctors will recommend prompt treatment and monitoring:
Cancer Staging
Scans and biopsies will be done to determine how far the cancer has progressed. This helps guide treatment.
Surgery
If the polyp was removed completely, surgery may not be needed immediately. If cancer remains, surgery to remove colon section(s) with cancer may be required.
Chemotherapy
Drugs to kill remaining cancer cells may be used after surgery. This helps lower the risk the cancer will return.
Follow-up Exams
Frequent colonoscopies will be scheduled to monitor for recurrence and to remove any new polyps before cancer develops.
Catching polyp cancers early improves prognosis significantly. If the polyp was fully removed, 5-year survival rates are typically 90% or better.
Summary
While concerning features may raise suspicion, determining if a colon polyp is cancerous cannot be done by eye alone. Biopsy is needed to examine polyp cells and make an accurate cancer diagnosis. If cancer is found, prompt treatment and follow-up can provide an excellent outlook when caught early. Most polyps are benign, but it is crucial to identify and remove precancerous adenomas before they become malignant.