What Are Colorectal Polyps?

“Size matters”. This statement might spark a debate in many contexts, but when it comes to colorectal cancer and colorectal polyps, there’s no room for argument. It’s not just about size, though. Hold on to your hats, because shape and number of polyps also play a crucial role.

After all, cancerous tumors are characterized by uncontrolled cell proliferation. Particularly for colorectal cancer where polyp formation and growth are typically slow, the size of a polyp is often testament to how long it has been growing in the large intestinal lumen.

That having been said, large polyps too were once small, but not all small polyps will develop into tumors. These small polyps are sometimes left till the next colonoscopy for removal, while larger polyps that have been detected are removed during the procedure. The question is, which of these polyps have cancerous potential, and which are unlikely to develop into cancer?

Many factors promote and influence polyp formation and growth, and these cause cells that should not grow to continue growing and proliferating. Polyps can form as a result of abnormal cell growth known as neoplasia, which originates from the Greek words neo (new) and plasis (formation).

Neoplasia is the formation of new tissue at the affected area. This growth is autonomous but can be reversed or stopped by removing the polyps or eliminating the risk factors. These cells usually have a similar genetic makeup that results from cellular transformation or loss of growth regulation, such as deactivation of tumor suppressor genes or overexpression of oncogenes.

Polyps may also arise as a result of hyperplasia, derived from Greek to mean ‘increased growth’. Tissue is made up of a group of cells arranged in a certain manner in order for these cells to perform specific functions. While both hyperplasia and neoplasia result in tissue growth due to increasing cell numbers, hyperplasia results in an enlargement of the affected area when cells that are normal components of the tissue multiply without changing the cellular arrangement.

While there are differences in how some polyps form, neoplastic and non-neoplastic polyps are both considered to have cancerous potential, though determining which lesions will progress to cancer requires a closer look at the molecular signatures. Hyperplastic polyps are usually small and benign, those determining if a polyp is hyperplastic is difficult during a colonoscopy. Other non-neoplastic polyps such as inflammatory pseudopolyps are at higher risk of developing into colorectal cancer. This is seen in individuals diagnosed with ulcerative colitis, which has been linked with an increased risk of colorectal cancer.

Of all polyps, adenomas, serrated polyps and hyperplastic polyps are the most common. The morphology — form and structure — of a lesion can be described as polypoid that protrude from the mucosal lining, or non-polypoid that have flat or depressed structures.

In general, polyps that form are either pedunculated or sessile, where they appear attached to the lining to a stalk (pedunculated, or resembling a cherry or mushroom) or have a dome shape that lies flat against the wall of the intestine (sessile).

On the other hand, non-polypoid lesions have flat or superficial forms. They can be flat and elevated, completely flat, superficially depressed or excavated/ulcerated. At times, some non-polypoid lesions may also appear flat and elevated with a central depression.

Adenomas (or adenomatous polyps) are made of tissue that resemble the colon’s inner lining, while serrated polyps (or serrated adenomas) resemble saw teeth. Adenomas can be further classified into two different growth patterns: tubular or villous patterns.

While many adenomas have a mixture of both tubular and villous growth patterns (called tubulovillous adenomas), smaller adenomas typically have tubular growth that resemble small tubes packed into the colon lining. Larger adenomas have a villous pattern that looks frond-like; the outward spreading pattern is indicative of rapid and more chaotic growth and a higher likelihood of cancer.

The size of a polyp remains a key determinant — the larger the polyp, the more likely it will develop into a cancerous tumor — but how large is large, exactly?

Large polyps that are more than 10 mm (1 cm) in diameter have the highest chance of becoming cancerous, as data has shown the larger a colon polyp becomes, the more rapidly it grows. Polyps that are below 5 mm in diameter are referred to as diminutive and carry very little cancerous risk. Polyps that are 6–9 mm in diameter are considered small and are easily removed during screening.

Visually speaking, neoplastic and non-neoplastic polyps can sometimes be differentiated with the help of narrow band imaging (NBI), which uses optical filters that allow only blue and green light to pass through. This causes blood vessels found on the mucosa to appear bluish-green, thereby differentiating and enhancing the visualization of the shape and structure of any polyps found. This visualization technique can be paired with colonoscopy to visualize and identify polyps in real time.

While NBI has been shown to enhance the detection of polyps as compared to standard colonoscopies, visualization alone does not provide conclusive results of a polyps benign or malignant nature. Doctors may need to biopsy any polyp tissue samples removed during a colonoscopy in order to accurately determine if the cells contain any abnormal DNA indicative of cancerous growth.

Nonetheless, removal of polyps is usually recommended and carried out during screening colonoscopies, or through surgery in the case of larger polyps.