Colorectal cancer is one of the most common cancers worldwide, with over 1 million new cases diagnosed each year. It begins as noncancerous clumps of cells called polyps that form on the inside lining of the colon or rectum. Over time, some polyps can become cancerous. Finding and removing precancerous polyps through regular screening is one of the best ways to prevent colorectal cancer.
What are colon polyps?
Colon polyps are abnormal tissue growths that form on the inside surface of the colon (large intestine) or rectum. They project into the opening (lumen) of the colon. Polyps vary in size from a tiny dot to several inches across. They are generally benign (noncancerous) at first but can eventually turn malignant (cancerous) over time.
There are several different types of colon polyps:
- Adenomatous polyps (adenomas) – Can become cancerous; subtypes include tubular adenoma, tubulovillous adenoma, and villous adenoma
- Hyperplastic polyps – Usually benign but can indicate higher colon cancer risk in some cases
- Inflammatory polyps – Develop as a result of ongoing inflammation in IBD
- Hamartomatous polyps – Rare benign lesions with little cancer risk
Adenomatous polyps carry the highest risk of becoming cancerous. Approximately 5-10% of adenomas will turn cancerous if allowed to grow. This process of transforming from benign to malignant is known as the adenoma-carcinoma sequence and can take 5-10 years on average.
Where in the colon are polyps most likely to form?
Polyps can develop anywhere along the length of the colon from the cecum to the rectum. However, research indicates that certain segments of the colon have a higher frequency of polyp formation.
According to several large studies, the majority of polyps tend to arise in the sigmoid colon and rectum – the distal segments of the colorectum nearest the anus.
- A 2010 study found 61% of polyps were located in the sigmoid colon/rectum. Only 15% were found on the right side of the colon.
- Another study in 2011 reported 67% of polyps in the distal colorectum compared to 21% in the proximal colon.
- Data from a large screening program showed 75% of polyps were in the rectosigmoid region versus 17% in the ascending colon.
Although polyps can form anywhere, these studies clearly demonstrate a predominance in the distal colorectum, particularly the sigmoid colon and rectum.
Why are polyps more common distally?
There are several theories as to why the distal colon has a higher rate of polyp formation:
- The sigmoid colon has a larger diameter and slower transit time. This allows more mucosal exposure to toxins and carcinogens.
- In the proximal colon, fluid absorption and intestinal contents are more liquid. Liquid feces could minimize contact with the mucosa.
- Mutagenic compounds may become more concentrated in the distal colon increasing mucosal irritation.
- Differences in blood supply and cytokine production between colonic segments may favor polyp development distally.
However, the exact reasons are still being investigated. Some researchers suggest there are inherent molecular differences in the distal colonic mucosa that promote polyp formation.
Does polyp location affect cancer risk?
In addition to forming more frequently in the distal colon, polyps located in this segment may also have a greater likelihood of becoming malignant compared to proximal polyps.
Some key points about cancer risk based on polyp location:
- One study found 14% of distal adenomas showed high-grade dysplasia vs 6% of proximal adenomas. High-grade dysplasia raises colon cancer risk.
- Another study reported distal adenomas were 2.5 times more likely to contain villous components than proximal adenomas. Villous adenomas have a higher cancer potential.
- Research suggests it takes distal adenomas roughly half the time to become malignant compared to proximal adenomas.
So not only are adenomas more prevalent in the distal colon, those that do form may progress to cancer faster compared to polyps elsewhere. This makes endoscopic detection and removal of distal polyps especially important.
Other risk factors for polyp formation
While the distal colon is the most common location, other factors can also increase an individual’s risk of developing polyps and colorectal cancer:
Age
Advancing age is one of the strongest risk factors. Polyps and colorectal cancer are uncommon under age 40 but rates rise substantially after 50:
- Approximately 30% of people over 50 have colon polyps.
- Over 90% of colorectal cancer cases occur in those 50 and older.
Family history
Having a first-degree relative (parent, sibling, child) with colorectal cancer raises your risk. Genetic conditions like familial adenomatous polyposis also predispose to polyp formation. People with a family history are encouraged to begin screening earlier.
Inflammatory bowel disease
Chronic intestinal inflammation from Crohn’s disease or ulcerative colitis is associated with an increased colon cancer risk. Regular surveillance with colonoscopy is recommended for these patients.
Lifestyle factors
Diets high in red meat and processed foods, smoking, heavy alcohol use, obesity, and lack of exercise have all been linked to a higher risk of adenomas and colorectal cancer.
Importance of screening
Regular screening allows doctors to find and remove precancerous polyps before they become malignant. Screening is recommended starting at age 45 for average-risk adults. Some guidelines advise beginning at age 40 for African Americans due to higher colon cancer rates in this group. The main screening options include:
- Colonoscopy – Visual examination of the entire colon using a camera on a flexible tube. Any polyps found can be removed during the procedure. Recommended every 10 years.
- CT colonography – Radiologic imaging test to look for polyps. Follow-up colonoscopy is needed to remove any lesions found.
- FIT – Annual stool test that detects blood from cancer/polyps. Positive results require colonoscopy.
Because polyps so often arise in the distal colon, screening colonoscopy is advantageous since it provides a complete structural examination from rectum to cecum. This allows assessment of the entire colorectum.
Conclusion
In summary, research strongly indicates the distal colon, particularly the sigmoid region and rectum, as the most common sites for adenomatous polyp formation. Not only do polyps occur more frequently in these areas, but they also appear more prone to malignant transformation compared to polyps in the proximal colon. Maximizing the detection and removal of distal polyps is therefore one of the major goals of colorectal cancer screening programs. Along with lifestyle modification and management of other risk factors, comprehensive screening provides the best opportunity to prevent cancer and save lives.