Why Remove Colon Polyps, and What Do Colorectal Cancer Tests Look For? — A Floor Relaid Every Day, Bumps That Mostly Never Become Cancer

"Please Send Us a Small Sample of Your Stool"

Your health screening packet includes a small container. It is an instruction to collect and send a small stool sample. A few weeks later, someone who receives a result saying "further testing is needed" is handed bowel-cleansing medicine, and someone who undergoes the procedure is told, "We removed a polyp."

Being told a polyp was removed sounds frightening. Was that polyp cancer? Why did we look at stool first when nothing hurt at all? Why did we have to clean out the bowel so thoroughly the day before the test?

This article follows the path of how small polyps sprout on the inner wall of the large intestine, and how some of them turn into cancer over a long time. We will also look at what two tests that try to catch the middle of that path find and miss, and how major trials measured those benefits. The major trials come from Europe, the screening guidelines mainly from the United States, and South Korea's national screening system and Korean data are also covered together.

A Floor That Is Always Being Replaced

The large intestine is the thick tube that follows the small intestine, reclaiming water and salt from what is left over and shaping stool to be sent out. What the large intestine does and the bacteria living inside it are followed in "What Do Gut Microbes Do? — Guests Who Eat the Leftovers, Guests Who Hold the Room", and this article looks into the wall of that tube.

The mucosa is the thin layer covering the inner wall of a hollow organ like the large intestine, and in the large intestine, the innermost cells are constantly being replaced. The colonic mucosa is densely lined with glands dug out like small wells. New cells are made at the bottom of the glands and pushed upward, and old cells fall away. This innermost single layer in the mammalian intestine is replaced every 4 to 5 days (PMID 18808327). The inner wall of the large intestine is a floor that is always being replaced.

Lumps can also sprout on this floor. A polyp is a lump that rises from the mucosa into the inside of the intestine. Tracing backward from colorectal cancer, changes in genes accumulate in mucosal cells over many years, turning normal mucosa into a benign lump, and the cells of that lump become increasingly disorganized until they turn into cancer (PMID 32419916). Starting from the lump tells a different story.

Colorectal cancer is cancer in which cells starting from the mucosa of the large intestine multiply wildly and become able to burrow into the wall and spread. Most colorectal cancers start from polyps, which are the precursor stage to cancer (PMID 35361331). A 1978 review noted that when comparing the number of lumps classified as precursor stages to cancer and colorectal cancer, the transition of a lump into cancer is uncommon (PMID 359943). Most colorectal cancers start from polyps, but most lumps classified as precursor stages to cancer also do not become cancer.

Most Polyps Stay Put, While Some Deepen into Cancer

Polyps are classified based on the appearance of their cells under a microscope. An adenoma is a polyp formed by an overgrowth of cells that make up the glands of the mucosa, and some can progress to cancer over a long period.

The adenoma-carcinoma sequence is the pathway in which an adenoma accumulates changes over many years to become cancer. The 1978 review we saw earlier estimated that the time this pathway takes varies by person, but averages roughly 10 to 15 years (PMID 359943). This is an estimate from over 40 years ago.

There are other pathways, too. Until around 2010, serrated polyps were mostly considered harmless bumps, but evidence accumulated that some of them can become colorectal cancer (PMID 35457279). Serrated lesions are a group of polyps where the edges of the glands look jagged like a saw under a microscope, and they can lead to cancer through a different pathway than adenomas. Among this group, the most common precancerous stages are the sessile serrated lesion and the traditional serrated adenoma (PMID 35457279).

Estimates for the share that the serrated pathway accounts for in colorectal cancer vary across reviews, from 10 to 20% (PMID 23431044) to as much as 30% (PMID 32419916). Sessile serrated lesions also appear to linger for a relatively long time, much like adenomas (PMID 30625264).

Because colorectal polyps take a long time and go through multiple stages to become cancer, screening to find precancerous polyps in average-risk individuals has become possible (PMID 27486317). Most polyps do not become cancer, and even the few that do take cancer's path usually require many years. This long middle period is what testing targets.

Common Cancers, Growing Young Patients

Colorectal cancer is the third most commonly diagnosed cancer worldwide and the second leading cause of cancer death (PMID 39938491).

Risk factors cited for adenomas and colorectal cancer include age, male sex, family history, obesity and physical inactivity, and red meat consumption. For serrated polyps, smoking and drinking are cited as important risk factors, which sets them apart from adenomas (PMID 35361331). Lifestyle factors are associations that moved together with risk in studies that observed people over time, so they do not tell us how much the risk changes if we alter a single one of those factors.

Long-standing inflammation of the intestine is also linked to risk. Inflammatory bowel disease is a group of conditions where inflammation in the intestine recurs over a long period, and ulcerative colitis is one of them. People with ulcerative colitis have a higher risk of colorectal cancer than the general population of the same age, and that risk increases the earlier the disease starts, the longer it lasts, and the more widely it is spread (PMID 37311872).

Early-onset colorectal cancer is colorectal cancer diagnosed before the age of 50. In the United States, the incidence of colorectal cancer under age 50 has nearly doubled since the early 1990s (PMID 35577711). In cancer registry data from 50 countries and regions, over the 10 years leading up to around 2017, incidence in the 25–49 age group rose in 27 places, and South Korea ranked among the higher places for incidence in this age group at 14.3 per 100,000 population (PMID 39674189). Another study looking at data up to around 2012 found that the speed at which incidence under age 50 was rising was steepest in South Korea (PMID 31488504).

Other observations looked at different time periods. A 2019 review by Korean researchers reported that both under-50 and over-50 colorectal cancer had decreased in South Korea over the few years around that time, and attributed this likely to younger people also undergoing many tests that look directly inside the large intestine for screening purposes (PMID 31344768). Because the periods and age brackets of the data differ, the trend among young people in South Korea is not yet settled in a single direction.

Why it has increased in the young is not yet solved. Most young adults diagnosed with colorectal cancer do not have known risk factors like a family history or genetic disorders (PMID 35577711). Because colorectal cancer has increased in nearly all regions among generations born since the early 1950s, evidence is building that exposures received from the fetal period through young adulthood are important risk factors (PMID 37723270). In a meta-analysis pooling associations with metabolic disorders, obesity around the time of diagnosis was associated with a 1.45-fold higher risk, and in men aged 20–49, type 2 diabetes was also associated with a higher risk direction. Other metabolic disorders like hyperlipidemia or hypertension were also reported to have an association toward higher risk, but results were inconsistent across studies, and the authors wrote that more research is needed to clarify the causal pathway (PMID 41579288). Colorectal cancer in young people is increasing in many countries, but its causes are not yet settled.

A Disease That Arrives Late

Colorectal cancer usually has no symptoms in its early stages (PMID 32903904). That is why we run tests to catch it before symptoms appear. A screening test is a test that aims to find people who have a disease in advance among those who have no symptoms. Colorectal cancer screening works along two tracks: finding and removing precancerous polyps to reduce the occurrence of colorectal cancer, and catching cancer at an early stage to reduce deaths (PMID 29846947).

For people who already have symptoms, a separate diagnostic evaluation is in place. Citing the fact that colorectal cancer is increasing in people under 50, a 2017 recommendation by the U.S. Multi-Society Task Force on Colorectal Cancer (USMSTF) recommended a thorough diagnostic evaluation for young people suspected of bleeding from the colon (PMID 28555630).

These are the symptoms that studies classify as warning signs. In a meta-analysis of 81 studies that gathered people diagnosed with colorectal cancer before age 50, the most common initial symptoms were bleeding from the anus (45%), abdominal pain (40%), and changes in bowel habits (27%). Anal bleeding, abdominal pain, and anemia were linked to an increased likelihood of early-onset colorectal cancer, and it was common for 4 to 6 months to pass between the appearance of symptoms and diagnosis (PMID 38787555).

Evidence pointing in the opposite direction also exists. In a 2011 systematic review gathering the relationship between bowel symptoms and colorectal cancer and polyps, the symptoms associated with colorectal cancer were anal bleeding and weight loss, while changes in bowel habits, constipation, diarrhea, and abdominal pain showed no clear association with colorectal cancer or polyps. Most of the included studies had methodological flaws (PMID 21624112). Abdominal pain yields conflicting results between the two different syntheses. Even in studies gathering people who visited primary care for anal bleeding, an average of 7.0% actually had colorectal cancer (PMID 19935790). Most bowel symptoms do not mean colorectal cancer or polyps, and symptoms alone cannot distinguish whether the cause is cancer. Confirmation is done through tests that look directly inside the bowel and with excised tissue.

Colorectal cancer usually has no symptoms in its early stages, so the absence of symptoms does not mean the absence of disease.

Invisible Blood in the Stool

Cancers and large polyps in the colon can bleed a little, and that blood mixes into the stool. A fecal occult blood test is an examination that looks for blood mixed in the stool that is too small to see with the eye. A fecal immunochemical test is a fecal occult blood test that looks for blood in the stool using antibodies that react only to hemoglobin, a component of human blood. This test has gradually replaced older methods of stool blood testing, and it is now the most widely used test in colorectal cancer screening programs worldwide (PMID 32903904).

In a meta-analysis of 19 studies gathering asymptomatic, average-risk adults, this test was positive in 79% of people who had colorectal cancer and negative in 94% of people who did not have colorectal cancer. Lowering the threshold that defines a positive result reduces missed cancers, but instead increases positive results in people without cancer, and the variation across studies was large (PMID 24658694). In a synthesis of reviews for the U.S. Preventive Services Task Force (USPSTF), these two percentages were 74% and 94% (PMID 34003220).

Let us read these numbers from both sides. "Negative in 94% of people without cancer" means, conversely, that out of 100 people without cancer, about 6 will test positive. A positive result is a signal to move on to the next test. A colonoscopy is an examination that looks directly inside the colon by inserting a thin tube with a camera at the tip through the anus. American Cancer Society (ACS) guidelines state that when a non-colonoscopy test is positive, timely follow-up with a colonoscopy is required to complete the screening process (PMID 29846947), and the 2026 revised edition specifies within 6 months if possible (PMID 42200680).

In a small study from a medical center following 150 asymptomatic screening participants who tested positive and all underwent colonoscopy, cancer was found in 4 people (2.6%) and adenomas were found in 106 people (70.6%) (PMID 33965367). In a pragmatic trial in the U.S., the proportion of people with a positive result who were found to have advanced polyps or cancer ranged from 17% to 30 depending on the test product (PMID 30101513). Most people with a positive result do not have cancer, but a substantial number are found to have polyps that need to be removed.

The same numbers speak for the other side as well. If it is "positive in 79% of people with cancer," about one in every four or five people with colorectal cancer will test negative on a single test. Hemoglobin degrades in the stool, and bleeding comes and goes. One review notes that because of this limitation, one in four colorectal cancers is still diagnosed at a late stage (PMID 32903904). The power to find advanced adenomas before they become cancer is even weaker (PMID 41132013).

A positive fecal occult blood test does not mean you have cancer; it is a signal to look further with a colonoscopy, and even with a negative result, there are cancers and polyps that are missed.

Seeing Directly and Removing on the Spot

Colonoscopy not only finds precancerous polyps and cancer, but also removes them right where they are found (PMID 27217671). Polypectomy is the act of cutting and removing a polyp right there while looking through a colonoscopy.

A 2019 review considers that polyp removal changes the path that a precancerous growth was walking, reducing the occurrence of and death from colorectal cancer. As evidence, it points to the incidence of colorectal cancer in the U.S. that fell alongside screening, a trial comparing fecal blood tests and a short examination looking only at the end of the colon (flexible sigmoidoscopy) by lottery, and studies following people who received colonoscopies. The same review also notes that the degree of protection after removing polyps varied across studies (PMID 31445683). What trials comparing colonoscopy screening itself by lottery showed will be seen later.

When to get the next colonoscopy is determined by the size, number, and type of the removed polyps (PMID 40576881). In its 2020 revision, the U.S. Multi-Society Task Force on Colorectal Cancer lengthened the interval from 5-10 years to 7-10 years for people who had 1-2 low-risk adenomas under 10mm removed (PMID 35361341), and there is also a 2025 review that writes 5 years for 1-2 polyps under 1cm (PMID 40576881). South Korea's 2022 guidelines list findings such as adenomas 10mm or larger or 3-5 or more as high-risk findings, while noting that more research is needed to clarify who benefits most from follow-up colonoscopies and what the best interval is (PMID 36225130).

Colonoscopy aims to find and remove growths right where they are found, cutting off in the middle the path that a growth could take toward cancer.

Clearing the Floor to See the Lumps

If stool remains inside the colon, lumps rising from the floor are hidden. Bowel preparation is the process of drinking medication and restricting meals before a colonoscopy to empty the inside of the colon. One review considers the quality of bowel preparation to be strongly linked to how many adenomas are found, and notes that this directly affects the incidence of and mortality from colorectal cancer that develops after a colonoscopy. The same review notes the recommendation to have people with inadequate preparation undergo repeat examination within 1 year (PMID 39229776).

The US Multi-Society Task Force on Colorectal Cancer recommends a split-dose regimen where the bowel-cleansing medication is taken in two parts, the day before and the day of the test (PMID 40035345).

The eyes of the examining doctor also shape the results. The adenoma detection rate is the proportion of screening colonoscopies performed by a single doctor in which at least one adenoma is found. A 2015 review noted adenoma detection rate benchmarks of over 25% for men and over 15% for women (PMID 27217671), and a 2019 review considers that these benchmarks may be too low. Two studies found that people examined by doctors with a high adenoma detection rate had a lower likelihood of developing or dying from colorectal cancer after a colonoscopy (PMID 30815756). The fact that doctors differ in their lesion-detection rates and lesions that were missed are cited as core causes of colorectal cancer arising after a colonoscopy (PMID 40610186).

An interval cancer is a colorectal cancer diagnosed after a colonoscopy was reported to show no cancer, and before the time scheduled for the next examination arrives. The majority of interval cancers are attributed to polyps missed during the examination and polyps incompletely removed (PMID 25210470). Serrated lesions, particularly those located on the proximal side of the colon, are cited as a primary cause of interval cancers because they have frequently been missed (PMID 23431044), and they account for a larger share of post-colonoscopy colorectal cancers than of colorectal cancers overall (PMID 30625264).

Although colonoscopy is considered a low-risk procedure, harm occurs on rare occasions (PMID 31563271). In a meta-analysis pooling 21 population-based studies, per 1,000 colonoscopies, perforation—a hole in the bowel wall—occurred in 0.5 cases, bleeding occurred in 2.6 cases, and death was much rarer at 2.9 cases per 100,000 procedures. In examinations where polyps were removed, perforations were higher at 0.8 cases and bleeding at 9.8 cases per 1,000 procedures. There was wide variation across studies, and the authors concluded that complication rates remained stable or decreased over the past 15 years (PMID 27296945).

Clearing the bowel is preparation to sweep the floor and see the lumps, and even after a colonoscopy is finished, the burden of missed lumps and incompletely removed lumps remains.

What the Trial Said — Those Invited and Those Who Actually Received It

A randomized controlled trial is a study where participants are divided randomly like drawing lots, given a drug on only one side, and compared for outcomes with the other side. In screening trials, the invitation to test takes the place of the drug.

There is a large European trial that compared colonoscopy screening in this way. Healthy individuals aged 55 to 64 in Poland, Norway, and Sweden were divided in a 1 to 2 ratio, with one side receiving an invitation to have a single colonoscopy and the other side not receiving one. Among the 28,220 people who were invited, the proportion who actually underwent the test was 42.0% (PMID 36214590).

The results of such a trial are counted using two denominators. The approach that counts the entire group of invited people (intention-to-screen) includes even the 58% who did not take the test, looking at what the recommendation of screening does to the population as a whole. The approach that counts only those who actually received it (per-protocol) is closer to what changes for the people who take the test. However, because people who respond to an invitation and those who do not may differ in health or lifestyle from the start, this approach disrupts the balance that the random draw set up.

Over a median of 10 years and based on all invited individuals, the rate at which colorectal cancer developed was 18% lower in the invited group. This is a magnitude where 455 people need to be invited to prevent one case of colorectal cancer. There was no clear difference in colorectal cancer death and all-cause death combined. Among those who received the test, 15 experienced major bleeding after polyp removal, and there were no perforations within 30 days or deaths due to the test (PMID 36214590).

At 13 years of follow-up, the direction was the same. Based on all invited individuals, the incidence of colorectal cancer was 1.46% versus 1.80%, representing a factor of 0.81. This is a difference of roughly 3 fewer cases per 1,000 people. Based on those who actually received the test, it was a factor of 0.55. Colorectal cancer death was 0.88 times on an invitation basis and 0.70 times on a received basis, but neither was a statistically clear difference. Cancers in the distal colon decreased, but cancers in the proximal colon showed no clear difference. The researchers concluded that a single colonoscopy clearly reduced the incidence of colorectal cancer over 13 years but did not clearly reduce mortality, and added that colorectal cancer mortality in both groups was lower than when the trial was designed. Actual colorectal cancer mortality in the uninvited group was 0.47%, which is less than 60% of the 0.82% anticipated at the design stage. This trial is still ongoing (PMID 42102826).

In a randomized trial inviting people to a single colonoscopy, based on all invited individuals, colorectal cancer incidence decreased over 13 years, but colorectal cancer mortality did not clearly decrease. Among those invited, 42% actually received it, and colorectal cancer mortality was lower in both groups than anticipated at design.

There are also screenings that underwent trials before colonoscopy. In a review for the US Preventive Services Task Force, 4 randomized trials that invited participants to sigmoidoscopy once or twice showed a colorectal cancer mortality of 0.74 times on an invitation basis. Five trials of the older guaiac test, which looks for blood in the stool every 1 to 2 years, showed a mortality of 0.78 times at 30 years of follow-up (PMID 34003220). Observational studies are studies that do not interfere with people's lives, leaving them to live as they normally do and observing what happens afterward. The findings that screening colonoscopy and fecal immunochemical tests lower the incidence or mortality of colorectal cancer are associations derived from such observational studies. The reviewers concluded that each screening method differs in the level of evidence reducing mortality, the power to detect, and harms (PMID 34003220).

Where Each Organization Draws the Age Line

In 2021, the U.S. Preventive Services Task Force gave its highest grade of recommendation for colorectal cancer screening to average-risk, asymptomatic adults aged 50 to 75. It gave a one-grade-lower rating to ages 45 to 49, judging the benefit to be moderate, and left screening for ages 76 to 85 to individual choice based on health status, prior screening history, and personal preference (PMID 34003218).

In 2018, the American Cancer Society lowered the recommended starting age for screening to 45, keeping the recommendation for age 45 as a weaker conditional recommendation than the one for age 50 and older (PMID 29846947), and reaffirmed the 45 to 75 age range in its 2026 update (PMID 42200680). The U.S. Multi-Society Task Force on Colorectal Cancer stated that no studies show screening under age 50 reduces colorectal cancer incidence or mortality. Even so, citing the increased disease burden and modeling studies, it suggested starting at age 45 (PMID 34794816). A 2024 review notes that starting at age 45 remains under debate (PMID 38588547).

An expert review by the American Gastroenterological Association (AGA) without graded evidence classifies individuals with a parent, sibling, or child who has had colorectal cancer as higher risk, and recommends starting screening 10 years before the age at which that family member was diagnosed or at age 40, whichever comes first (PMID 37737817).

There is also more than one test available. U.S. guidelines list several stool tests, colonoscopy, and CT colonography as options at varying intervals (PMID 35501176). The American Cancer Society considers offering multiple options helpful for supporting personal choice and increasing participation, noting that the most effective test is the one the person completes (PMID 42200680).

When screening should start and which test to use are lines drawn by organizations after reading the evidence, and those lines differ from one organization to another.

National Colorectal Cancer Screening in South Korea

In South Korea, colorectal cancer is the third leading cause of cancer death, ranking fourth in men and third in women. According to a 2023 review, the South Korean government has recommended an annual fecal immunochemical test as the first step of colorectal cancer screening for people aged 50 and older since 2004. A positive result leads to a follow-up test such as a colonoscopy, whereas a negative result does not warrant one (PMID 38076601).

Whether this screening reduces colorectal cancer mortality has been evaluated using observational data. A study compared screening records of approximately 5.94 million individuals aged 50 and older in 2004 with matched control groups of individuals who died from colorectal cancer. Having undergone screening at least once was associated with a lower colorectal cancer mortality of 0.74 times, and the reduction was greater with a higher frequency of screenings. The difference was not clear among those aged 75 to 79, and the direction was reversed for those aged 80 and older. The researchers concluded that it is effective for ages 50 to 74 (PMID 39766178).

In the 2022 national cancer statistics, the proportion of colorectal cancer diagnosed at a localized stage increased by 18.5 percentage points compared to 2005, and the 5-year relative survival rate for colorectal cancer rose by 7.6 percentage points since that time. The authors of the report concluded that early detection has improved since the national cancer screening program began in 2002 (PMID 40083085). This is a conclusion about the entire program, which included stomach and breast cancer together, and colorectal cancer screening was introduced in 2004 (PMID 38076601). This conclusion comes from observational data comparing periods before and after screening.

There is also the follow-up story of individuals who did not have cancer found on a colonoscopy after a positive result. Following 150,660 cases where no cancer was found on a colonoscopy after a positive fecal immunochemical test in the 2005–2010 national screening, 280 interval cancers subsequently occurred. This amounts to 0.49 cases per 1,000 person-years of follow-up. The researchers concluded that the burden of interval cancer following a positive result and subsequent colonoscopy is substantial (PMID 28231692).

A Person Who Received a Notice

A fifty-two-year-old person receives a notice of a positive result on a fecal immunochemical test from a national screening. A positive result means that human blood came out in the stool, crossing the threshold. It is a result that appears in about 6 out of 100 people even without colorectal cancer (PMID 24658694), and in one study, among those who tested positive, about 3% had cancer and about 70% had an adenoma (PMID 33965367).

This person restricts their diet the day before the test, takes divided doses of medicine, and empties the bowel. The floor must be cleared so that polyps can be seen.

During the colonoscopy, a small polyp is seen, and the doctor removes it on the spot (PMID 27217671). The removed polyp is classified by type under a microscope. Most early-stage polyps do not become cancer (PMID 359943), and this polyp is now outside the body. However, along with incompletely removed or missed polyps, it is considered the cause of most interval cancers (PMID 25210470). The interval for the next colonoscopy is determined by guidelines based on the size, number, and type of the removed polyp, and even for the same findings, the recommended interval differs between guidelines and revised editions (PMID 35361341).

What if not a single polyp had been seen? Even then, the share of polyps missed by the test remains. Missed polyps are considered one of the causes of most interval cancers (PMID 25210470).

A remaining share also applies to the person next to them whose notice was negative from the start. A single test can miss about one in four or five colorectal cancers (PMID 24658694, PMID 34003220). Korean screening recommends this test every year (PMID 38076601). What we followed in this scene is a single polyp rising from a floor that is constantly being relaid. Most are polyps that will not become cancer, and the test tries to find and remove the ones among them that could deepen, amidst a long middle.

Where to Look Next

There are still open questions. Why colorectal cancer in younger adults has increased remains unsolved (PMID 35577711), and just how much screening colonoscopy reduces colorectal cancer deaths awaits longer follow-up from ongoing trials (PMID 42102826).

Why a positive result does not immediately mean disease, and why the same test is read differently in a group where the disease is common versus rare, are covered in "What a Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability". The story of how what is measured as an outcome shapes judgments continues in "Better Numbers, Better Patients? — An Introduction to Surrogate Endpoints". Type 2 diabetes, which is linked to early-onset colorectal cancer, can be explored in "What Is Diabetes? — The Letter That Never Comes, the Letter That Isn't Read".

Upper endoscopy and gastric cancer screening in Korea are followed by "What Do the Stomach and Esophagus Do? — A Pouch That Holds Acid, and Its Two Doors". Troubles that arise at the exit where digestive juices flow into the intestine are taken up by the installment covering the gallbladder and pancreas in other parts of this series.

Source Papers

• "Colorectal Cancer: Epidemiology, Risk Factors, and Public Health Strategies." (2025) — PMID 39938491 — Colorectal cancer is the third most diagnosed cancer globally and the second leading cause of cancer death, with the highest incidence in high-income countries; authors' conclusion: screening, early detection, and public health interventions can significantly reduce mortality.
• "Risk Factors for Colorectal Polyps and Cancer." (2022) — PMID 35361331 — Most colorectal cancers arise from precursor adenomas or serrated polyps, and established risk factors for adenomas and colorectal cancer include age, male sex, family history, obesity, lack of physical activity, and red meat, whereas serrated polyps differ in risk profile, associated with smoking and alcohol.
• "The adenoma-carcinoma sequence." (1978) — PMID 359943 — Most colorectal cancers arise from adenomas, a process that varies widely but averages roughly 10 to 15 years, though transition is rare when comparing the absolute numbers of adenomas and cancers, with malignant potential related to size, growth pattern, and degree of epithelial dysplasia.
• "Curriculum review: serrated lesions of the colorectum." (2020) — PMID 32419916 — Sporadic colorectal cancer develops through the accumulation of genetic and epigenetic alterations, typically over many years in a sequence of normal epithelium to benign adenoma to dysplasia to cancer, with the serrated pathway accounting for up to 30% of colorectal cancers.
• "Stem cells, self-renewal, and differentiation in the intestinal epithelium." (2009) — PMID 18808327 — The mammalian intestine is covered by a single layer of epithelial cells that is entirely replaced every 4 to 5 days, consisting of proliferative crypts containing stem cells and villi (a physiological review based on animal studies).
• "Serrated Colorectal Lesions: An Up-to-Date Review from Histological Pattern to Molecular Pathogenesis." (2022) — PMID 35457279 — Until 2010, serrated lesions were mostly regarded as harmless hyperplastic polyps; WHO classification lists sessile serrated lesions with dysplasia and traditional serrated adenomas as the most common precursors.
• "Screening Relevance of Sessile Serrated Polyps." (2019) — PMID 30625264 — The traditional pathway accounts for about two-thirds of colorectal cancers and most of the remainder follows the serrated pathway, contributing disproportionately to post-colonoscopy colorectal cancers, while sessile serrated polyps linger for a relatively long time, similar to adenomas.
• "Serrated pathway: alternative route to colorectal cancer." (2013) — PMID 23431044 — The serrated pathway accounts for 10% to 20% of colorectal cancers, with proximal colon lesions in particular being less recognized and missed, serving as a major cause of interval cancers.
• "Colorectal cancer development and advances in screening." (2016) — PMID 27486317 — Most colorectal tumors develop through multistep changes over time, making it possible to screen average-risk individuals to find polyps before they become cancer.
• "[Ulcerative colitis-associated carcinogenesis : An update]." (2023) — PMID 37311872 — Patients with ulcerative colitis have a higher risk of colorectal cancer than the age-matched population, increasing with earlier onset, duration, and extent, following an inflammation-to-intraepithelial neoplasia-to-cancer sequence that differs from sporadic adenoma-to-carcinoma.
• "Colorectal Cancer in Younger Adults." (2022) — PMID 35577711 — Incidence in adults under 50 in the US has nearly doubled since the early 1990s, with about 15% projected to be young adults by 2030; most young patients have no known risk factors, and the mechanism remains a puzzle.
• "Red Flag Signs and Symptoms for Patients With Early-Onset Colorectal Cancer: A Systematic Review and Meta-Analysis." (2024) — PMID 38787555 — Across 81 studies, common first symptoms of early-onset colorectal cancer were rectal bleeding (45%), abdominal pain (40%), and bowel habit changes (27%), with rectal bleeding, abdominal pain, and anemia linked to increased odds of early-onset colorectal cancer, and diagnostic delays of 4 to 6 months common; conclusion: need to recognize concerning symptoms and complete timely diagnostic workups.
• "Most bowel cancer symptoms do not indicate colorectal cancer and polyps: a systematic review." (2011) — PMID 21624112 — Symptoms associated with colorectal cancer are limited to rectal bleeding and weight loss, whereas bowel habit changes, constipation, diarrhea, and abdominal pain show no significant association with colorectal cancer or polyps, and most studies suffer from methodological flaws; conclusion: guidelines should account for the limited value of symptoms.
• "Diagnostic accuracy systematic review of rectal bleeding in combination with other symptoms, signs and tests in relation to colorectal cancer." (2010) — PMID 19935790 — In 8 studies of primary care patients with rectal bleeding, the pretest probability of colorectal cancer averaged 7.0%, and even when overlapped with severe anemia, the posttest probability reached 21.6%; conclusion: the diagnostic value of additional warning symptoms is limited.
• "Colorectal cancer incidence trends in younger versus older adults: an analysis of population-based cancer registry data." (2025) — PMID 39674189 — Early-onset colorectal cancer increased in 27 out of 50 countries, with the age-standardized incidence rate for 25-49 year-olds in South Korea at 14.3 per 100,000 during 2013-17, placing it near the top, highlighting the need to identify causes and raise awareness for early detection.
• "Global patterns and trends in colorectal cancer incidence in young adults." (2019) — PMID 31488504 — During 2008-12, South Korea had the highest incidence in those under 50 at 12.9 per 100,000, and South Korea also showed the steepest rate of increase (AAPC 4.2), potentially signaling changes in early-life exposures.
• "[Early-onset Colorectal Cancer]." (2019) — PMID 31344768 — Early-onset colorectal cancer tends to be diagnosed at later stages and with symptoms, whereas in South Korea both early- and late-onset cases have decreased in recent years (likely thanks to screening colonoscopy in younger age groups).
• "Changing epidemiology of colorectal cancer - birth cohort effects and emerging risk factors." (2024) — PMID 37723270 — Colorectal cancer has increased in almost all regions among those born since the early 1950s (birth cohort effect), supporting exposures during fetal life to young adulthood as critical risk factors.
• "Early-onset colorectal cancer is associated with metabolic disorders: a systematic review and meta-analysis." (2026) — PMID 41579288 — Across 38 studies, obesity at diagnosis was associated with a 1.45-fold risk of early-onset colorectal cancer, while associations with hyperlipidemia, hypertension, and metabolic syndrome were heterogeneous, calling for research to establish causal pathways.
• "Non-Invasive Colorectal Cancer Screening: An Overview." (2020) — PMID 32903904 — Colorectal cancer is asymptomatic in early stages, with the fecal immunochemical test being the most common tool in global screening, though due to hemoglobin degradation and intermittent bleeding, one in four colorectal cancers is still diagnosed at a late stage.
• "Colorectal cancer screening for average-risk adults: 2018 guideline update from the American Cancer Society." (2018) — PMID 29846947 — Screening reduces incidence by removing adenomas and decreases mortality through reduced incidence and early detection, with a conditional recommendation to start at age 45 and a strong recommendation for age 50 and older; positive non-colonoscopy tests require timely colonoscopy, and fecal immunochemical testing should be done annually.
• "Colorectal cancer screening: An update to the American Cancer Society guideline, 2026." (2026) — PMID 42200680 — Reaffirms ages 45 to 75, noting that the most effective test is the one completed through to the end, blood tests have low sensitivity for precursor lesions and stage I cancer and are meant only for refusers, and positives should ideally receive a colonoscopy within 6 months.
• "Screening for Colorectal Cancer: US Preventive Services Task Force Recommendation Statement." (2021) — PMID 34003218 — Recommends screening for asymptomatic average-risk adults, graded A for ages 50 to 75, B for ages 45 to 49, and C for ages 76 to 85.
• "Screening for Colorectal Cancer: Updated Evidence Report and Systematic Review for the US Preventive Services Task Force." (2021) — PMID 34003220 — Sigmoidoscopy RCT invitation-based mortality ratio 0.74, guaiac test 19.5-year ratio 0.91 and 30-year ratio 0.78, colonoscopy and fecal immunochemical test linked via observational studies, fecal immunochemical test sensitivity 0.74 and specificity 0.94, screening colonoscopy perforation 3.1 and major bleeding 14.6 per 10,000 procedures, with evidence levels varying by method.
• "Accuracy of fecal immunochemical tests for colorectal cancer: systematic review and meta-analysis." (2014) — PMID 24658694 — Across 19 studies, sensitivity for colorectal cancer was 0.79 and specificity was 0.94, with lowering the threshold increasing sensitivity and decreasing specificity; conclusion: moderate sensitivity and high specificity, with performance depending on the cutoff.
• "Positive predictive value of fecal immunochemical test for high-risk colonic adenomas and carcinoma: A health maintenance organization cohort screening study in Lebanon." (2021) — PMID 33965367 — Among 3,000 asymptomatic screening participants at one medical center, 150 tested positive and all underwent colonoscopy, finding 4 cancers (2.6%), 106 adenomas (70.6%), and 40 advanced adenomas; conclusion: helpful when discussing expectations before colonoscopy following a positive result.
• "Positive predictive values of fecal immunochemical tests used in the STOP CRC pragmatic trial." (2018) — PMID 30101513 — Among 1,130 individuals testing positive in a mailed fecal immunochemical test pragmatic trial, positivity ranged from 8% to 21%, and the positive predictive value for advanced neoplasia ranged from 0.30 to 0.17 (depending on test brand), with false positives increasing the burden of colonoscopy.
• "[National Colorectal Cancer Screening Program: Fecal Immunochemical Testing vs. Colonoscopy]." (2025) — PMID 41132013 — South Korea's national screening relies primarily on the fecal immunochemical test, facing limitations in advanced adenoma accuracy and sustained participation, whereas colonoscopy is invasive and resource-intensive but allows direct viewing and removal, with the pros, cons, and effectiveness of both methods depending on participation rate, quality, and infrastructure.
• "Colonoscopy in Colorectal Cancer Screening: Current Aspects." (2015) — PMID 27217671 — Colonoscopy finds and removes precursor and malignant lesions, with interval polyps and cancers remaining a challenge, bowel preparation being crucial, and adenoma detection rate benchmarks set at 25% for men and 15% for women.
• "Reviewing the Evidence that Polypectomy Prevents Cancer." (2019) — PMID 31445683 — Polypectomy alters the natural history of precursor polyps to reduce incidence and mortality, supported by US incidence trends, stool test and sigmoidoscopy RCTs, and colonoscopy observational studies, though the degree of protection following resection varies across studies.
• "Colorectal polyps: pathophysiology, malignant potential, and advancements in therapeutic strategies." (2025) — PMID 40576881 — Colonoscopy surveillance intervals are determined by the size, number, and type of polyps discovered.
• "Surveillance Recommendation for Colonoscopy after Polypectomy." (2022) — PMID 35361341 — 2020 US multi-society update: interval for 1 to 2 low-risk adenomas under 10 mm changed from 5-10 years to 7-10 years, and for 3 to 4 adenomas from 3 years to 3-5 years.
• "Korean guidelines for postpolypectomy colonoscopic surveillance: 2022 revised edition." (2022) — PMID 36225130 — High-risk findings after resection include adenomas 10 mm or larger, 3 to 5 or more adenomas, villous components, high-grade dysplasia, traditional serrated adenomas, and sessile serrated lesions with dysplasia, while target groups for surveillance benefit and optimal intervals require further research.
• "Optimal bowel preparation for colonoscopy." (2025) — PMID 39229776 — Bowel preparation quality is strongly correlated with adenoma detection rate and directly impacts post-colonoscopy colorectal cancer incidence and mortality, with inadequate preparation warranting a repeat exam within 1 year.
• "Optimizing Bowel Preparation Quality for Colonoscopy: Consensus Recommendations by the US Multi-Society Task Force on Colorectal Cancer." (2025) — PMID 40035345 — Adequate preparation means a state that permits setting a standard interval, split-dose intake is recommended, dietary restrictions are limited to the day prior, and adequate preparation rates should target over 90%.
• "Quality in screening colonoscopy." (2025) — PMID 40610186 — Differences in detection rates among physicians and missed lesions are core drivers of post-colonoscopy colorectal cancer, standardizing quality indicators.
• "Does Increased Adenoma Detection Reduce the Risk of Colorectal Cancer, and How Good Do We Need to Be?" (2019) — PMID 30815756 — Adenoma detection rate is the most validated indicator of physician performance, benchmark thresholds may be too low, and two studies showed that individuals treated by physicians with high adenoma detection rates experienced lower post-colonoscopy colorectal cancer incidence and mortality.
• "Advances, problems, and complications of polypectomy." (2014) — PMID 25210470 — High incomplete resection rates, along with missed polyps, are considered the primary causes of most interval cancers, and serrated polyps are frequently missed or incompletely removed.
• "ASGE review of adverse events in colonoscopy." (2019) — PMID 31563271 — Colonoscopy is overall a low-risk procedure, meta-analyzing bleeding, perforation, and mortality.
• "Post-Colonoscopy Complications: A Systematic Review, Time Trends, and Meta-Analysis of Population-Based Studies." (2016) — PMID 27296945 — Across 21 population-based studies, perforation was 0.5 and bleeding was 2.6 per 1,000, mortality was 2.9 per 100,000, and during polypectomy perforation was 0.8 and bleeding was 9.8 per 1,000, with screening and surveillance lower than diagnostic indications; conclusion: complication rates have been stable or decreasing over 15 years.
• "Effect of Colonoscopy Screening on Risks of Colorectal Cancer and Related Death." (2022) — PMID 36214590 — NordICC randomized trial, 42.0% of 28,220 invitees underwent screening, 10-year intention-to-screen colorectal cancer 0.98% vs 1.20% (RR 0.82, 1 case per 455 invitees), colorectal cancer death 0.28% vs 0.31% (not significant), overall death equal; conclusion: lower 10-year colorectal cancer risk in the invited group.
• "Long-term effects of colonoscopy screening on colorectal cancer incidence and mortality: a multicountry, population-based randomised controlled trial." (2026) — PMID 42102826 — NordICC at 13 years, incidence intention-to-screen RR 0.81 and per-protocol RR 0.55, mortality intention-to-screen 0.88 and per-protocol 0.70, neither significant, reduction limited to the distal colon; conclusion: a single colonoscopy significantly reduced incidence over 13 years but not mortality, with mortality lower than trial design assumptions in both groups, and follow-up ongoing.
• "Updates on Age to Start and Stop Colorectal Cancer Screening: Recommendations From the U.S. Multi-Society Task Force on Colorectal Cancer." (2022) — PMID 34794816 — Although literature does not show that screening under 50 improves outcomes, starting at age 45 is proposed based on disease burden, advanced lesion prevalence in ages 45-49, and modeling, with discontinuation recommended after age 85.
• "Screening for Colorectal Cancer." (2024) — PMID 38588547 — Screening reduces incidence and mortality, suggested for ages 50 to 75, starting at age 45 remains controversial, and offering choices increases participation.
• "AGA Clinical Practice Update on Risk Stratification for Colorectal Cancer Screening and Post-Polypectomy Surveillance: Expert Review." (2023) — PMID 37737817 — Having a first-degree relative with colorectal cancer is regarded as increased risk, suggesting initiation 10 years before the youngest patient's diagnosis age or at age 40, whichever is earlier, in an expert review without formal evidence grading.
• "Screening for Colorectal Cancer." (2022) — PMID 35501176 — Regular screening for ages 45 to 75 based on 2021 USPSTF evidence, multiple test options and intervals, noting "the best test is the one that gets done."
• "Colorectal Cancer Screening: Recommendations for Physicians and Patients from the U.S. Multi-Society Task Force on Colorectal Cancer." (2017) — PMID 28555630 — Rising incidence under 50, recommending thorough diagnostic evaluation for young individuals with suspected colonic bleeding.
• "National cancer screening program for colorectal cancer in Korea." (2023) — PMID 38076601 — Colorectal cancer is the third leading cancer death in South Korea (4th for men, 3rd for women), with annual fecal immunochemical testing as the first step for those 50 and older since 2004, followed by colonoscopy or double-contrast barium enema if positive, and colonoscopy not recommended if negative.
• "Effectiveness of the Korean National Cancer Screening Program in Reducing Colorectal Cancer Mortality." (2024) — PMID 39766178 — Case-control study within a cohort of roughly 5.94 million people, colorectal cancer mortality odds ratio 0.74 for screening participants, decreasing with more rounds, not significant for ages 75-79, and reversed for age 80 and older; conclusion: effective for ages 50 to 74.
• "Cancer Statistics in Korea: Incidence, Mortality, Survival, and Prevalence in 2022." (2025) — PMID 40083085 — Localized stage diagnosis of colorectal cancer increased by 18.5 percentage points, and 5-year relative survival rose by 7.6 percentage points; authors' conclusion: early detection and survival improvements followed the 2002 National Cancer Screening Program.
• "Risk and Characteristics of Postcolonoscopy Interval Colorectal Cancer after a Positive Fecal Test: A Nationwide Population-Based Study in Korea." (2018) — PMID 28231692 — Among 150,660 negative colonoscopies following a positive fecal test between 2005 and 2010 in South Korea, there were 280 interval cancers (0.49 per 1,000 person-years) compared to 2,427 screen-detected colorectal cancers, more frequent in men and older adults, with quality improvement initiatives associated with lower short-term risk; conclusion: substantial burden of interval cancers.