Colorectal cancer

Colorectal cancer is a malignant tumour of the colon or rectum, the two segments that form the large intestine and the final part of the digestive system. It begins when epithelial cells in the inner lining of these organs grow out of control. Most cases start as benign adenomatous polyps that gradually accumulate genetic and epigenetic changes until they become invasive cancer.

The disease matters for public health because it is one of the most common cancers and a major cause of cancer death. In the United States it ranks as the third most common cancer in both men and women and the second leading cause of cancer death, and worldwide it causes roughly 550,000 deaths a year. It is also unusually preventable: screening can find and remove precancerous polyps and detect tumours at an early stage, when treatment works best. Although incidence is falling in older adults, it is rising among people under 50, which has made the disease a current focus of epidemiology and public health research.

This article covers the anatomy and classification of the disease; its global burden and the rise of early-onset cases; risk factors and hereditary syndromes; molecular pathogenesis and biomarkers; diagnosis and staging; screening; multimodal treatment with surgery, chemotherapy, radiation therapy and immunotherapy; prognosis and survivorship; prevention; the history of research; and unresolved questions.

Definition, anatomy and classification

The large intestine consists of the colon and the rectum, and colorectal cancer is the umbrella term for malignant tumours arising anywhere along this tract. The point at which a tumour counts as rectal rather than colonic is set by convention, and it differs between guidelines. Staging and treatment texts generally place the cut-off at about 15 cm above the anal verge, with tumours higher than that managed as colon cancer. Above 15 cm, tumors are staged and treated as colon cancer. The distance thresholds within the rectum also depend on the measuring method. Measured by rigid endoscopy, the UICC classifies tumors beginning at the anal verge up to 6 cm as low rectal cancer, between 6 and 12 cm as mid rectal cancer, and between 12 and 16 cm as high rectal cancer, whereas for MRI measurements, ESMO sets the cut-off beginning at the anorectal junction up to 4 cm for low, from 4 to 8 cm for mid, and from 8 up to 12 cm for high rectal cancer. Even the upper boundary of the rectum is not defined uniformly in practice. In one survey of surgeons, about a third used the peritoneal reflection, while others used the rectosigmoid junction or a fixed distance from the verge.

Histologically, glandular tumours dominate. Case-series reviews commonly report that adenocarcinoma accounts for about 98% of colorectal tumour types. A tumor is labeled SRCC when >50% of tumor cells show a "signet ring," whereas ACs with <50% signet ring cells are still classified as ACs with a signet ring cell component. The signet-ring cell carcinoma variant is therefore a histological threshold rather than a separate tissue type. It is a rare histological colorectal cancer subtype defined by the WHO classification of tumors as a variant of colorectal adenocarcinoma with > 50% of tumour cells with prominent intracytoplasmic mucin. In the other direction, mucinous adenocarcinoma is defined by the extracellular mucin content of the tumour volume, and the two subtypes together are usually estimated to make up roughly 10–15% of cases. MAC accounting for 10% of cases, and SRCC accounting for 1% of cases is a commonly cited split. Signet-ring tumours have a poorer outlook than other subtypes. In a large SEER analysis, SRCC had the worst survival among the three histological subtypes of CRC. Rare neuroendocrine tumours arise from a different cell lineage and are classified apart from adenocarcinomas.

Pathologists also describe colorectal tumours by how deeply they invade the bowel wall, and this depth forms the core of the T category in TNM staging. T1 tumors are confined to the mucosa and submucosa, whereas T2 tumors extend into to the muscularis propria. Tumours that pass beyond the muscularis propria are subdivided by how far they extend into the surrounding fat, and T4 tumors are those that invade the anterior peritoneal reflection (stage T4a) or other pelvic organs or structures, including skeletal muscle (T4b). Depth is a deliberate choice here: the T stage for rectal cancer is determined by the depth of tumor penetration, whereas the T stage for anal cancer is determined by the tumor size. Extension beyond the muscular layer is also linked to outcome, since the extramural depth of invasion refers to extension of tumor beyond the muscularis propria and is a prognostic factor.

Naming conventions add a further layer of variation. In Britain, "bowel cancer" is the common public and health-service term, while clinical literature more often refers separately to colon or rectal cancer. This entry uses "colorectal cancer" as the umbrella term, applies "colon" and "rectal" only where the anatomical or treatment distinction matters, and otherwise treats the two sites together.

Global burden and epidemiology

Colorectal cancer is among the most frequently diagnosed cancers worldwide and is one of the leading causes of cancer death. In the GLOBOCAN 2022 estimates, it ranked third for incidence and second for mortality among all cancers. About 1.93 million new cases and roughly 904,000 deaths were recorded globally in 2022, accounting for about 9.6% of new cancer diagnoses and 9.3% of cancer deaths. Of the cases, about 59% were colon cancers and about 38% rectal cancers, with anal cancer making up the small remainder.

Burden differs sharply by place. In absolute terms, Eastern Asia had the largest number of cases, and China alone accounted for about 517,000 new cases and 240,000 deaths in 2022. Age-standardized rates, which remove the effect of differing population age structures, show a different picture. Within the 2020 GLOBOCAN data, mortality rates ranged from about 20 per 100,000 among males in Eastern Europe to about 2.5 per 100,000 among females in Southern Asia. Rates are consistently higher in men than in women across all world regions. Incidence and mortality also rise steeply after about age 40. Analyses of GLOBOCAN data link higher rates to higher Human Development Index levels, and projections suggest that most future cases will occur in countries with high or very high HDI, with the total rising to around 3.2 million new cases and 1.6 million deaths a year by 2040. These projections reflect population ageing and growth as much as changes in risk.

Most global figures come from the International Agency for Research on Cancer's GLOBOCAN database. It combines data from population-based cancer registries, modelled cause-of-death estimates and Bayesian statistical methods to produce estimates for 36 cancer types across 185 countries. Because the quality of these estimates depends on the national source data, figures for countries with limited registration are less certain. Age-standardized rates are calculated using the 1966 Segi–Doll world standard population so that countries can be compared. Within individual countries, the Surveillance, Epidemiology, and End Results (SEER) programme of the United States provides long-running registry data on incidence by age, sex and race, which are often used for trend analysis.

Incidence in adults aged 50 and older has fallen in several high-income countries. In the United States, incidence in this age group dropped by about 30% between 2000 and 2010, a decline attributed mainly to the spread of screening. The pace of decline then slowed, from roughly 3–4% per year in the 2000s to about 1% per year between 2011 and 2019. Analyses attribute the fall to a combination of screening uptake, particularly colonoscopy after Medicare expanded coverage in 2001, and changes in risk-factor exposure, such as lower smoking rates and greater use of non-steroidal anti-inflammatory drugs. Mortality has continued to fall more slowly. Over the same decades, incidence among adults under 50 has risen, a trend covered in the early-onset section.

Early-onset colorectal cancer

Early-onset colorectal cancer (EO-CRC) is usually defined as a diagnosis before age 50. Some registry analyses define early-onset disease as diagnosis before age 50 and late-onset as age 50 or older, while an epidemiological analysis of US data from the Global Burden of Disease (GBD) study set its cut-off at 45, so published figures are not always directly comparable. Reports of very early-onset cases in adults under 40, many without a known genetic or familial predisposition, have also appeared.

In the United States, the trend has been upward in younger adults while older adults have moved in the opposite direction. In a population-based analysis of cancer registry data from 2000 to 2019, age-adjusted incidence under 50 rose steadily from 6.0 per 100,000 in the early 2000s, while the rates for people 50 and older declined over the same period (rates among older adults have declined). New York State data show the same divergence over a longer span: the under-50 rate rose from 5.9 to 10.0 per 100,000 between 1976–1980 and 2021–2022, whereas the rate at age 50 and above fell from 229.5 to 106.3 (the overall incidence rate of all CRC combined decreased for ages 50 and above from 229.5 in 1976–1980 to 106.3 in 2021–2022). In 2022, 20,422 of the 147,931 new colorectal cancers diagnosed in the United States, or 13.8%, occurred in patients younger than 50. Projections suggest colorectal cancer could become the leading cause of cancer death among Americans under 50 by 2030.

Globally, the GBD 2019 estimates show that the number of cases rose sharply between 1990 and 2019, from 95,737 to 226,782 (Global age-standardized incidence per 100,000 population rose from 2.95 in 1990 to 4.04 in 2019). The gap between the rise in case counts and the much smaller rise in age-standardized rates indicates that population growth and ageing account for much of the increase in absolute numbers. Deaths also increased: the global numbers of early-onset CRC for incidence, deaths, and DALYs in 2019 were 225,736, 86,545, and 4,259,922, respectively, and despite large variations at the regional and national levels, the global incidence rate, death rate, and DALY rate increased from 1990 to 2019.

The strongest analytical signal is the birth-cohort pattern. CRC rates have increased in the U.S. among individuals born since the early 1960s, and incidence rates are 1.89-fold and 2.98-fold higher among individuals born in 1980 to 1984 and 1990 to 1994, respectively, compared with earlier cohorts. An age-period-cohort model of US data from 1990 to 2021 found that being born after 1983 negatively impacted EO-CRC incidence rates, with a sharp rise after 2000 and a reduction among females from 2017 to 2021. This pattern points to exposures acting early in life, although age-period-cohort models cannot cleanly separate cohort effects from changes in diagnosis. Within the under-50 group, incidence is also rising faster among people aged 20–44 than among those aged 45–54 (the incidence was increasing in patients aged 20–44 years at a significantly faster pace compared to patients aged 45–54 years).

Proposed drivers fall into several groups. Established epidemiological associations include diabetes, obesity, diet, sedentary time, alcohol consumption and smoking, whereas data regarding potential risk factors of prior antibiotic exposure and microbiome alterations or direct carcinogen exposure are still emerging. The GBD analysis of attributable risk found that diets low in milk, diets low in calcium, and alcohol use were the leading risk factors in 2019, and that a high body mass index and high fasting plasma glucose ranked remarkably higher among males and females over the study period. Some studies also link the rise to diagnostic delay, meaning that symptomatic younger patients are diagnosed later than their symptoms would suggest. The relative weight of these explanations remains unresolved, and the GBD estimates carry wide uncertainty intervals, which limits how precisely individual contributions can be assigned.

Risk factors and modifiable exposures

Age is the strongest non-modifiable risk factor. Most colorectal cancers are diagnosed after 50, and the risk rises steadily with each decade of life. Family history also matters: a first-degree relative with colorectal cancer or advanced polyps raises risk, and the effect is larger when several relatives are affected or diagnosed young. Sex affects risk as well. Men have a higher incidence overall, and the risk linked to alcohol and to excess body weight appears to be stronger in men than in women. Hereditary syndromes, discussed in a separate section, account for a minority of cases, so most risk arises from the combination of age, genetics and exposures described here.

Dietary pattern is among the best-studied modifiable factors. Diets high in red meat (beef, pork, lamb and goat) and in processed meat, meaning meat preserved by smoking, curing, salting or the addition of nitrites, are consistently associated with higher risk. Meta-analytic estimates from AICR's review put the increase at about 12% for each 100 grams per day of combined red and processed meat, and the risk rises with intake. Several mechanisms have been proposed. Meat cooked at high temperatures forms heterocyclic amines, polycyclic aromatic hydrocarbons and N-nitroso compounds, which can damage DNA, and heme iron from red meat has been linked to epithelial injury and inflammatory signalling in the intestinal lining. Low intake of dietary fibre, whole grains, fruit and vegetables is associated with higher risk. Fibre is fermented by colonic bacteria into short-chain fatty acids, which are thought to have protective effects on the colonic epithelium.

Alcohol is an established risk factor, and the association is dose-dependent. Estimates from the same AICR review suggest roughly a 7% increase in risk for each 10 grams of ethanol per day. Proposed mechanisms include the conversion of ethanol to acetaldehyde, a reactive carcinogenic metabolite, and impairment of DNA repair. Alcohol may also increase the penetration of other carcinogens into the colonic lining. Current guidance on drinking limits varies between national bodies, but most recommend that intake be kept low and that people who do not drink avoid starting for cancer prevention.

Obesity, physical inactivity and smoking are consistently linked to colorectal cancer. Excess body fat is associated with raised circulating insulin and insulin-like growth factor signalling, which can stimulate proliferation of colonic epithelial cells, and with a pro-inflammatory state. Obesity is also associated with genomic instability and with altered gut microbial communities. Regular physical activity is associated with lower risk, and the benefit appears to be partly independent of body weight. Tobacco smoking increases risk for cancers of the colon and rectum, and the association is strongest for rectal cancer in some analyses. Sedentary behaviour, as distinct from low exercise, is also treated as a separate exposure in some risk tables.

Chronic inflammatory conditions raise risk through a more direct route. Patients with long-standing ulcerative colitis or Crohn's disease, collectively termed inflammatory bowel disease, have a substantially higher risk of colorectal cancer, and the risk increases with the extent and duration of colonic involvement. Type 2 diabetes is associated with a 40–60% higher risk of colorectal cancer in several meta-analyses. Hyperinsulinaemia, hyperglycaemia, oxidative stress and visceral adiposity are proposed links, and diabetes and obesity share some of these pathways. Diabetes and colorectal cancer also share patterns of gut dysbiosis.

The gut microbiota is an active area of study in this context. The colon holds more than 10^13 bacteria, and the composition of this community is shaped by diet, antibiotic exposure and early-life factors. Certain bacteria, including Fusobacterium nucleatum, have been repeatedly detected in colorectal tumours across different populations, and dysbiosis has been linked to chronic inflammation, the production of genotoxic metabolites, and immune dysregulation. Whether these microbial changes cause cancer, reflect the tumour environment, or both, is not yet settled, and most evidence remains observational. The microbiota is therefore considered a mediator between diet and disease rather than an independent cause.

Hereditary syndromes and familial risk

Hereditary syndromes account for a small share of colorectal cancers, but they carry some of the highest individual risks and determine who needs closer monitoring. Estimates of how common these conditions are differ by source. Lynch syndrome accounts for only 3%–5% of colorectal cancer cases, while another clinical reference gives a figure of 3–8%. Published carrier frequencies range from approximately 1 in 279 individuals to a prevalence of 1 in 440 in the general population.

Lynch syndrome is the most common inherited form. It arises when a person inherits a faulty copy of a mismatch-repair gene, most often MLH1, MSH2, MSH6 or PMS2, and the variant is usually paired with a later acquired event that disables the surviving copy in tissue. The defect in mismatch repair produces microsatellite instability, and its risk depends on the gene involved. Carriers of MLH1 or MSH2 variants face a lifetime colorectal cancer risk of 58–82%, whereas MSH6 and PMS2 carriers face a considerably lower 10–22% risk, diagnosed at a later mean age. Lesions also progress faster than in sporadic disease, with adenomas becoming cancers within roughly 1–3 years rather than 8–17. Because of this, carriers are advised to undergo colonoscopy every one to two years starting at age 25. Lynch syndrome also raises the risk of endometrial, ovarian, gastric and other extracolonic cancers.

Familial adenomatous polyposis (FAP) results from inactivating germline variants in the APC gene, which lead to hundreds to thousands of adenomas in the colon and rectum. Without intervention, colorectal cancer becomes nearly certain over a person's lifetime. The standard response is prophylactic colectomy to prevent the progression to CRC. The timing of surgery is shaped by age, family history, polyp count and the presence of pre-malignant change. Surgery does not eliminate the need for monitoring, since adenomas will arise and grow in the retained rectum or ileal pouch, so lifelong endoscopic surveillance is required. Chemopreventive drugs remain largely investigational; major guideline bodies still restrict them to research settings.

Several rarer syndromes follow different patterns. MUTYH-associated polyposis is an autosomal recessive mutation of both alleles of the MUTYH gene. Compared with FAP, it is characterized by a low polyp burden, right-sided disease, later onset of CRC, and colectomy is not always necessary. Peutz–Jeghers syndrome is caused by germline STK11 changes and produces hamartomatous polyps, most often in the jejunum, together with pigmented spots on the lips and digits. It is the second highest incidence among hereditary gastrointestinal polyposis syndromes, following familial adenomatous polyposis, and in some series up to 50% of PJS cases have no detectable LKB1 mutation. Juvenile polyposis is linked mainly to SMAD4 and BMPR1A variants, and it is associated with a colorectal cancer risk of 38.7% in one reported figure. SMAD4 carriers typically develop polyps in both the upper and lower digestive tract, whereas BMPR1A-related polyps are exclusively located in the colorectum, and SMAD4 mutations have been linked to a more aggressive phenotype.

Identifying people who need closer surveillance depends on family history, genetic counselling and tumour testing. Universal screening of colorectal tumours for mismatch-repair deficiency is one route to finding at-risk individuals. Lynch syndrome nonetheless remains underdiagnosed, even where national guidelines recommend testing all new colorectal and endometrial cancers. Once a germline variant is confirmed, relatives can be offered targeted surveillance programmes.

Pathogenesis: the adenoma–carcinoma sequence

Most colorectal cancers are thought to arise from benign epithelial growths called adenomas. The classical route from normal mucosa to invasive carcinoma was set out in the 1990 genetic model proposed by Fearon and Vogelstein. In that model, a normal epithelial cell becomes malignant through a series of driver-gene changes, with APC, KRAS, TP53 and SMAD4 as the central examples. The sequence was inferred statistically from many tumours rather than observed in any single patient, so it describes a general pattern. Individual tumours do not have to show every step, and the key idea is that multiple mutations must accumulate, not that they occur in one fixed order.

Early and late genetic events

The sequence is usually initiated by mutation or loss of APC, located on chromosome 5q21–22. KRAS activation follows and is generally an early event, occurring mainly as small adenomas grow into intermediate-sized ones. Changes in TP53 and the loss of material on chromosome 17p are considered late events, and they are thought to let a growing tumour escape cell-cycle arrest and programmed cell death. Losses in SMAD4 and DCC appear in the same general phase of progression. The germline form of SMAD4 causes juvenile polyposis, which underlines its role in the disease.

Wnt signalling and the destruction complex

The APC protein normally works as a scaffold in a cytoplasmic complex with axin, GSK-3β and casein kinase 1 that tags β-catenin for degradation. When APC is lost or truncated, this complex fails, and β-catenin accumulates in the nucleus. There it binds TCF/LEF transcription factors and switches on genes that drive cell proliferation and suppress differentiation, a process described under Wnt signalling. Estimates of APC inactivation vary between about 35% and 88% of colorectal tumours, but APC or Wnt-activating mutations are found in the great majority of cases. APC and β-catenin mutations appear largely mutually exclusive, which supports the view that they act on the same pathway. Epithelium with this activation proliferates to form the adenoma, and the expansion of this population allows further mutations to accumulate.

Progression from adenoma through dysplasia to invasive carcinoma typically takes years rather than months. This slow timescale is the rationale for screening intervals, because removing adenomas before they progress can prevent cancer.

The serrated pathway

An alternative serrated pathway accounts for an estimated 20–30% of cases, with one review giving a figure of roughly 30%. Its precursors are serrated polyps, including hyperplastic polyps and sessile serrated lesions, named for the saw-tooth crypt architecture. These lesions most often involve a BRAF V600E mutation, or sometimes a KRAS mutation, together with a CpG island methylator phenotype in which widespread promoter methylation silences tumour-suppressor genes such as MLH1 and p16. Methylation of MLH1 can cause mismatch-repair deficiency, and the resulting microsatellite instability is a hallmark of this route. Serrated tumours are more often located in the proximal colon, and CIMP-high lesions show a strong association with BRAF mutation. Because these precursors are anatomically and histologically subtle, they can be overlooked at endoscopy and in pathological assessment, which makes their recognition a continuing challenge for surveillance.

Taken together, the two pathways reflect different routes to the same end point. The conventional route is driven mainly by APC loss and chromosomal instability, while the serrated route is driven by epigenetic silencing and mismatch-repair defects. These distinctions carry through to the molecular classification of tumours discussed below.

Molecular classification and biomarkers

Molecular profiling now sits alongside histology and stage in deciding treatment. Tumours are sorted by mismatch-repair status, RAS and BRAF alterations, rarer amplifications and fusions, and gene-expression subtypes that are still being evaluated. Biomarkers such as microsatellite instability, RAS and BRAF mutations, HER2 amplification or NTRK fusions are major tools for personalised therapeutic strategies.

Mismatch-repair deficiency and microsatellite instability

Microsatellite instability (MSI) is a hypermutable phenotype caused by mismatch repair deficiency, which accounts for 10–15% of all CRCs. The main defects involve the proteins that normally correct base-pair mismatches, chiefly MLH1, MSH2, MSH6 and PMS2 (see mismatch repair). In sporadic tumours, loss of MLH1 is often caused by promoter methylation rather than an inherited mutation, whereas Lynch syndrome carries a germline defect. The resulting accumulation of errors in short repeated DNA sequences produces many neoantigens, which underlies the durable responses seen with PD-1 blockade in mismatch-repair-deficient disease (see immune checkpoint and cancer immunotherapy). MSI-high tumours are significantly associated with BRAF V600E mutations but tend to correlate inversely with KRAS mutation.

RAS and BRAF

KRAS and NRAS mutations determine whether anti-EGFR antibodies such as cetuximab or panitumumab can be used, because tumours carrying them do not respond to EGFR blockade; RAS testing is therefore required before these drugs are prescribed. KRAS-mutant tumours are concentrated in one gene-expression group, as CMS3 (metabolic) has a distinctive global genomic and epigenomic profile. BRAF alterations are not uniform: BRAF mutations are classified into three groups: class 1 (V600E), class 2 (codons 597/601), and class 3 (codons 594/596). V600E tumours are generally associated with poorer prognosis and limited benefit from anti-EGFR therapy alone, which is why they are treated with BRAF inhibitors combined with EGFR blockade.

HER2 amplification, NTRK fusions and rare alterations

The frequency of HER2 overexpression/amplification ranges from 1.3 to 6.3% of unselected colorectal cancers. In one molecular analysis, HER2-amplified tumours were mutually exclusive of MSI and harbor less frequent KRAS/NRAS/BRAF mutations but frequent TP53 mutations. Detection is therefore needed to select patients for HER2-directed therapy, and the US Food and Drug Administration has approved tucatinib + trastuzumab combination therapy for patients with HER2 amplified CRC. NTRK fusions are rarer still; they are targetable with NTRK inhibitors regardless of tumour site, and their detection depends on sequencing methods that go beyond routine hotspot panels.

Consensus molecular subtypes

Gene-expression classification has attempted to reconcile competing schemes. An international consortium merged six independent classification systems into four consensus molecular subtypes (CMS). CMS1 includes ∼14% of CRCs, and is associated with microsatellite instability (MSI), BRAF mutation, promoter hypermethylation, and immune infiltration. CMS2 is the most prevalent CRC subtype (37%) and shows the hallmarks of canonical CRC carcinogenesis, including activation of the Wnt and Myc pathways. CMS4 (23%) is described as a mesenchymal, stroma-rich group, associated with poor prognosis, with TGF-β activation and angiogenesis. Reported proportions differ between cohorts; one analysis gives CMS4 as 27.5%, so the figures should be read as approximate. Because whole-genome sequencing is costly for routine use, various studies have focused on the use of other methods, such as immunohistochemistry to determine CMS. Whether CMS assignment should guide treatment choice remains under evaluation.

Circulating tumour DNA

The routine implementation of molecular predictive tests provides new perspectives and challenges for the therapeutic management of CRC patients, such as liquid biopsies. Circulating tumour DNA (ctDNA) detected in a blood sample can be sequenced with next-generation methods to identify actionable biomarkers without tissue, and to track residual disease after treatment. Its use to guide adjuvant chemotherapy is still being studied, and it has not yet replaced tissue-based testing for most decisions.

Clinical presentation, diagnosis and staging

Symptoms

Presenting features range from rectal bleeding, abdominal pain, and change in bowel habits to fatigue, weight loss, and anemia in advanced stages. Bowel-habit changes that persist beyond two weeks, a sense of incomplete emptying, and rectal pressure are among the most frequently reported complaints. Iron-deficiency anaemia can be the first clue, because slow occult bleeding gradually depletes iron stores, so blood counts form part of the initial work-up. Tumours frequently produce no warning signs in their early stages, which is why screening matters.

Diagnostic evaluation

A positive stool-based screening test or suggestive symptoms usually leads to a diagnostic colonoscopy. If suspicious areas are found, the doctor can take biopsies during the procedure for further analysis. Histological confirmation remains necessary, since a biopsy, where a small sample of tissue is removed and examined under a microscope, is the only way to confirm the presence of cancer cells. The pathology report then records the histological type and grade, which together with invasive features feed into staging and prognosis. Blood tests such as carcinoembryonic antigen (CEA) may help monitor treatment response. They are therefore used mainly to follow patients after diagnosis rather than to establish it.

Staging imaging

Once malignancy is confirmed, imaging determines how far the tumour has spread. For colon cancer, contrast-enhanced multidetector CT of the abdomen and chest is preferred over chest X-ray for assessing nodal disease and distant metastases, and CT of the abdomen and pelvis primarily seeks to exclude T4 disease with infiltration into other organs. Node size alone is an imperfect guide, because metastases can be found even in normal-sized lymph nodes. PET-CT is used selectively: FDG-PET is not recommended for staging, although it may be considered for patients with CT-detected synchronous liver metastases scheduled for liver surgery.

For rectal tumours, local staging relies on MRI and endorectal ultrasound, which answer different questions. MRI is currently the only imaging modality that enables highly accurate evaluation of the topographic relationship between lateral tumor extent and the mesorectal fascia, and it is the only reliable tool for determining the status of the circumferential resection margin. Endorectal ultrasound is accurate mainly for discrimination between tumours that are limited to the submucosa (T1) versus tumours showing ingrowth in the muscularis externa (T2), but its limited field of view makes it less suitable for the mesorectal fascia and high nodes. CT, by contrast, significantly lacks resolution of soft tissue contrast so that its utility in T staging of rectal cancer is limited.

TNM classification and the role of stage

Findings are expressed using the TNM system, which describes depth of invasion (T), regional node involvement (N) and distant metastasis (M), and assigns stages I to IV. Stage I disease is confined to the inner layers of the colon and has not spread to nearby lymph nodes or distant organs. Stage is determined by tumor size, lymph node involvement and spread to other organs, guiding treatment decisions. In rectal cancer, accurate preoperative staging is especially consequential, since it has a critical role in determining whether patients should undergo neoadjuvant therapy. Neoadjuvant treatment has been associated with decreased local recurrence and increased survival, which makes the accuracy of the initial imaging a determinant of downstream management.

Screening and early detection

Screening serves two purposes: finding and removing precancerous growths before they become invasive, and detecting cancers early enough for treatment to work. Detection and removal of precancerous growths is the route by which regular screening prevents the disease. Earlier detection can also lower deaths, because treatment is more likely to succeed when the cancer is found sooner. Early-stage detection is associated with five-year survival rates of more than 90% in the United States.

The American Cancer Society's 2026 update, published in May 2026, keeps age 45 as the starting point for average-risk adults. Screening should continue through age 75 for those whose life expectancy is greater than 10 years. For adults aged 76 to 85, the decision to continue is discussed with a health care provider, and screening is not continued after 85 or when life expectancy is under 10 years. The update was the first revision of the guideline since 2018, and it formally adds a blood-based test and two newer stool-based tests to the recommended menu.

The tests fall into tiers. Visual examinations, most commonly colonoscopy, and stool-based tests are recommended and preferred for screening. Among stool tests, highly sensitive fecal immunochemical tests are preferred annually, while the next-generation multi-target stool DNA test and the multi-target stool RNA test are preferred at three-year intervals. Both multi-target tests combine markers for DNA or RNA with a hemoglobin measurement. The blood-based test, Shield, is an FDA-approved blood test that detects tumor DNA, but it is not a preferred option. Compared with established stool-based tests, blood-based tests show lower sensitivity for both advanced precancerous lesions and stage I cancers, so the guideline states that blood-based tests should be recommended only to individuals who decline or do not complete preferred screening tests. Other recognised strategies include CT colonography and flexible sigmoidoscopy.

Follow-up is a required step in the screening process. All positive results on non-colonoscopy screening tests should be followed up with timely colonoscopy. For stool or blood tests, a positive result requires timely follow-up with a colonoscopy, preferably within 6 months, to complete the screening process. Without that step, a positive test does not translate into a diagnosis or a removed polyp, and the expected benefit is lost.

Participation remains the main limit on population impact. More than 20 million Americans who are eligible for screening have not been tested as recommended, about 1 in 3 adults. The guideline authors stress that the most effective screening test is the one that the patient completes, which is the rationale for offering several options. Organised population programmes in European countries, which send invitations and track follow-up, are generally regarded as a way of raising participation and reducing the gap between eligible and screened populations.

Surgical treatment

Surgery is the principal curative option for localised colorectal cancer. The operation depends on the segment of bowel involved, the depth of invasion, and whether the disease is confined to the bowel wall and nearby nodes.

Endoscopic resection of early lesions

Endoscopic resection is used for lesions confined to the mucosa or only slightly into the submucosa. Guidelines indicate it for high-grade dysplasia or early cancer with minimal submucosal invasion. Whether a removed T1 tumour is considered cured depends on the pathologist's review of the specimen. The commonly used criteria require complete removal, clear margins, no lymphatic or vascular invasion, low to moderate grade, low tumour budding, and submucosal invasion of less than 1,000 µm (including a complete resection with horizontal and vertical margins free of tumour, the absence of lymphatic or blood vessel infiltration, low to moderate histological grading, and limited submucosal infiltration). Nodal spread remains a concern because lymph node metastasis occurs at a rate of approximately 10% among patients with surgically resected submucosally invasive colorectal cancer. Some series suggest that depth alone, without other high-risk features, carries a lower risk than once assumed. In one Japanese series, cases that met all criteria except the depth limit had a nodal metastasis rate of 1.2%, and no metastasis was found in the absence of deep invasion (≥1,800 µm) (when cases that met three of the JSCCR 2010 criteria were considered together, the incidence of lymph node metastasis was only 1.2%). Technically, the aim of the procedure is to obtain a specimen with sufficient submucosal tissue for accurate staging (endoscopic resection techniques should focus on acquiring qualitatively and quantitatively sufficient submucosal tissue).

Colectomy for colon cancer

Colon cancer is treated by colectomy with lymph node dissection, performed open, laparoscopically or with robotic assistance. Large comparative studies of minimally invasive techniques have generally found similar safety outcomes. In a national US database of elective resections, after adjustment, there were no differences between robotic and laparoscopic colectomy for 30-day mortality or overall complications. Anastomotic leak rates varied by side of the colon, with the highest rate in left-sided robotic resections. Results across individual studies are not uniform. A randomised interim analysis from the ESSIMIC trial found that grade III complications were significantly more frequent in the laparoscopic group, and a propensity-matched study reported reduced blood loss, fewer conversions to open surgery, lower overall complication rates, and shorter hospital stays, despite longer operative times for robot-assisted surgery. An earlier multicentre randomised trial, ROLARR, found no short-term clinical advantage of robotic over conventional laparoscopic surgery (robotic surgery did not gain a clinical advantage over conventional laparoscopic surgery in the short-term clinical outcome).

Rectal cancer surgery

Rectal cancer surgery centres on total mesorectal excision (TME), which removes the rectum together with its surrounding fat and draining lymphatics (complete removal of the rectum together with its draining lymphatics, resulting in low rates of recurrence). TME is regarded as the reference standard for this disease (TME represents the "gold standard" of rectal cancer surgery). Where it is technically feasible and oncologically safe, surgeons aim for sphincter-preserving resection, which has progressively reduced the use of abdominoperineal resection (APR), in which the anus and sphincter are removed with a permanent colostomy. Tumours requiring APR tend to be more advanced; one series found that low rectal cancer requiring APR seems to be a disease with more locally advanced disease and adverse pathologic features than are seen with mid-rectal cancers treatable by low anterior resection. Sexual function outcomes also differed in that series, with preservation reported in about 57% after APR compared with 85% after sphincter-preserving surgery.

Transanal approaches (TaTME) have been developed to improve visualisation of the lower pelvis. Their impact on function is still being evaluated. In a matched comparison, the median low anterior resection syndrome score at 3 months was higher after transanal TME than after the conventional approach, but the severity grading and Wexner scores were comparable at 6 and 12 months.

Functional consequences: low anterior resection syndrome and stoma care

Low anterior resection syndrome (LARS) is a common functional disorder after sphincter-preserving rectal surgery, characterised by altered bowel frequency, urgency and evacuation difficulty. Up to 80% of rectal cancer patients experience a range of evacuation difficulties following surgery, although reported rates differ by series. In one laparoscopic cohort, the prevalence of overall LARS was 35.9%. Risk is associated with tumour position and treatment: neoadjuvant radiotherapy and tumour localisation were independent risk factors for LARS after laparoscopic sphincter-saving TME, and tumours in the middle or upper rectum were less often affected than low tumours (tumour localisation in the middle and high rectum was a protective factor compared to low rectal localisation). Stoma-related complications, such as retraction, prolapse, parastomal hernia and skin problems, also affect quality of life after both colectomy and APR, and their management is part of long-term surgical care.

Chemotherapy and systemic adjuvant therapy

Systemic chemotherapy treats micrometastatic disease that surgery cannot see. Its core is a fluoropyrimidine, either intravenous fluorouracil given with leucovorin (folinic acid) or oral capecitabine. These are usually combined with oxaliplatin, and irinotecan is used in some regimens. The two standard oxaliplatin-based doublets for colon cancer are FOLFOX (infusional fluorouracil with leucovorin and oxaliplatin) and CAPOX (oxaliplatin with oral capecitabine). CAPOX consists of oxaliplatin given every 3 weeks along with oral capecitabine, in contrast to mFOLFOX6, which requires infusions every 2 weeks. In practice, the two are considered broadly comparable in efficacy, which is why the choice often turns on the infusion schedule, the need for a central line, and toxicity profile.

Adjuvant treatment after resection

Adjuvant chemotherapy is given after surgery to patients whose tumours have a meaningful risk of relapse. It is standard for stage III disease (node-positive) and is considered for selected high-risk stage II tumours, where the decision rests on features such as T4 stage, poorly differentiated histology, lymphovascular invasion or obstruction, rather than stage alone. For many years the standard duration was six months. On the basis of positive findings from three phase-3 trials, 6 months of FOLFOX or CAPOX became the standard adjuvant therapy for patients with stage III colon cancer.

The shorter-course question was tested in the International Duration Evaluation of Adjuvant Chemotherapy (IDEA) collaboration, which pooled six trials. Among patients receiving FOLFOX or CAPOX, noninferiority of 3 months of therapy compared with 6 months was not confirmed in the overall population. The benefit of the longer course, however, was not uniform. In patients treated with CAPOX, 3 months of therapy was as effective as 6 months, particularly in the lower-risk subgroup. The reduction in toxicity was clear. Grade 2 or higher neurotoxicity during therapy and in the month after it stopped was 16.6% with FOLFOX and 14.2% with CAPOX at 3 months, versus 47.7% and 44.9% at 6 months. The consensus that followed is risk-based. For low-risk stage III colon cancer (T1–3, N1), 3 months of CAPOX is considered appropriate, while high-risk disease (T4 and/or N2 or other high-risk factors) warrants 6 months of CAPOX or FOLFOX. Some commentators have argued that regimen choice and patient preference should weigh more heavily than these anatomical criteria, so the consensus is not uniform.

Neoadjuvant chemotherapy in rectal cancer

Rectal cancer follows a different sequence. Surgery alone has a relatively high risk of local recurrence, so locally advanced tumours are usually treated before resection. Patients with localised rectal cancer frequently benefit from neoadjuvant treatment with radiotherapy or chemoradiotherapy before surgery. The standard approach has been preoperative chemoradiotherapy followed by total mesorectal excision, but this model left a substantial share of patients with distant relapse. Total neoadjuvant therapy (TNT) addresses this by moving systemic chemotherapy into the preoperative period. TNT consists of neoadjuvant radiochemotherapy and consolidative or inductive chemotherapy, followed by curative resection. Several trial designs exist. In one common format, chemoradiotherapy with capecitabine is followed by consolidation chemotherapy with XELOX before surgery. The rationale is that the improvement in local control with chemoradiotherapy was not matched by a similar gain in long-term overall survival, which points to distant metastasis as the main obstacle. Duration is also debated. One review concludes that 3–4 months of neoadjuvant combined chemotherapy are recommended as the optimal length of TNT. Adjuvant chemotherapy after TNT is therefore not always given, and the completion of postoperative treatment is itself a concern, since poor completion has been linked to missed systemic control.

Toxicity

The toxicities of these regimens shape both dose and duration. Oxaliplatin causes cumulative, sometimes persistent peripheral neuropathy, which is the main reason shorter courses have been tested. Fluoropyrimidines cause myelosuppression, diarrhoea, and hand–foot syndrome, the last being more pronounced with capecitabine. Dose modification and treatment delays are common, and they can affect how much of the planned therapy patients actually receive.

Radiation therapy and multimodal rectal cancer care

Because the rectum sits deep in the pelvis, where a tumour can invade surrounding tissue and be hard to clear with surgery alone, radiation is a central part of treatment for locally advanced disease. Giving radiotherapy and chemotherapy before the operation has largely displaced the postoperative approach. In the German CAO/ARO/AIO-94 comparison, the 5-year cumulative incidence of local cancer recurrence was 6% for patients randomly assigned to preoperative CT/RT, compared to 13% in the group of patients treated with postoperative CT/RT, and the 5-year overall survival rates did not differ significantly. The benefit therefore lies mainly in local control rather than in survival.

Two radiotherapy schedules are used before surgery. The long-course regimen usually delivers around 50 Gy over roughly 28 fractions, given alongside an oral fluoropyrimidine such as capecitabine, with the operation deferred by six to eight weeks to allow tumour shrinkage. The short-course schedule, more commonly used in Scandinavia and Poland, involves a 25 Gy dose in 5 fractions without chemotherapy, followed by surgery within one week of completing radiotherapy. A systematic review of randomised trials in lower rectal cancer found no evidence of a significant difference in either overall survival or rate of recurrence between the two approaches, although its authors described their conclusions as tentative because the included trials were variable and of limited quality. The choice therefore turns on trade-offs. Short-course treatment is shorter and cheaper, while long-course treatment is associated with lower surgical morbidity, increased sphincter preservation, and the ability to safely combine radiation with concurrent systemic chemotherapy. An economic evaluation concluded that the short-course approach was generally cost-effective, but that for the subset of patients with distal tumors, long-course chemoradiotherapy was the cost-effective approach. Concern also persists that a short course may limit the chance of sphincter-preserving surgery, because tumours shrink more slowly after it.

Organ-preservation strategies build on the response to neoadjuvant therapy. Pathological complete response is seen in roughly one fifth of patients after chemoradiation, and clinical assessment is used to identify those who may not need an operation. In one real-world cohort of 148 evaluable patients, clinical complete response was documented in 56 patients (37.8%), and a watch-and-wait strategy was implemented in 42 patients (28.4%). Tumour size and clinical stage were the main determinants of response, and small tumors (<4 cm) showed high response rates regardless of neoadjuvant regimen. Outcomes differ by response depth: in the OPRA trial analysis, the three-year probability of rectum preservation was 77% for patients with CCR and 40% for patients with NCR. Watch-and-wait carries a real risk of relapse, since nearly 30% of these patients will develop local regrowth usually within 3 years from initial decision to WW. Because clinical judgement is less precise than pathological assessment, strict surveillance is necessary to recognize tumor regrowth in a timely manner, when the tumor is still curable by resection. That surveillance is demanding, relying on repeated endoscopic examinations, digital rectal examination and MRI, and most of the supporting evidence remains retrospective.

Radiotherapy also has a palliative role in advanced pelvic disease, where it is used to control bleeding, relieve bowel obstruction and reduce pelvic pain, usually with shorter and less intensive schedules than curative treatment. In curative planning, the radiation field and dose are shaped to limit exposure of the small bowel, bladder and pelvic nerves, because radiation can cause long-term bowel, urinary and sexual dysfunction. These effects are weighed against the expected local control benefit when choosing a schedule for each patient.

Metastatic disease, targeted therapy and immunotherapy

The liver is the most frequent site of spread, and the lungs are the next most common. Liver involvement affects roughly four in five people with metastatic disease, yet resection at diagnosis is feasible for only 10–25% of them Although 80% of patients with metastatic colorectal cancer (CRC) experience liver metastases, only 10–25% undergo resection at the time of diagnosis. For limited hepatic disease, resection remains the central option, and ablation and stereotactic radiotherapy address lesions that are not suitable for surgery. Hepatic arterial infusion (HAI) is a regional approach in which a pump is implanted so that chemotherapy reaches the liver through the hepatic artery, the vessel that supplies most of the blood to colorectal liver metastases. HAI involves surgically implanting a pump during the same operation as the resection. The evidence is mixed: in randomized trials after complete resection, HAI improved hepatic recurrence-free survival without a measurable effect on overall survival.

Systemic therapy for metastatic disease depends heavily on the RAS status of the tumour. Anti-EGFR monoclonal antibodies such as cetuximab and panitumumab are given only when KRAS and NRAS are unmutated, because mutant RAS keeps downstream signalling active regardless of EGFR blockade. Bevacizumab, which targets VEGF-A, is added to chemotherapy as an anti-angiogenic agent. Mutations in BRAF are a separate high-risk group. They appear in as many as 15% of patients with metastatic disease and are linked to a poor outlook. Early trials tested the combination of the BRAF inhibitor encorafenib with cetuximab in patients who had already received therapy. The BEACON phase III study, which tested encorafenib with cetuximab in patients who had progressed after one or two prior regimens, supported the regimen's approval in that setting. The first-line picture has since shifted. In the BREAKWATER phase III trial, encorafenib with cetuximab and mFOLFOX6 produced an objective response rate of 61%, compared with 40% for standard chemotherapy. The FDA granted traditional approval for the combination with fluorouracil-based chemotherapy in February 2026, and the European Commission approved the cetuximab-plus-FOLFOX version for first-line use.

Immune checkpoint blockade works in only a small, molecularly defined subset of patients. About 4–5% of metastatic colorectal cancers are mismatch-repair deficient or microsatellite-instability-high, and these tumours carry a high mutational burden that makes them responsive to PD-1 inhibitors such as pembrolizumab and nivolumab. In the KEYNOTE-177 phase III trial, pembrolizumab roughly doubled median progression-free survival compared with chemotherapy, at 16.5 versus 8.2 months, and this led to its use as first-line standard therapy in this population. Overall survival showed a trend toward benefit in the same trial, though it was not conclusively significant. Combining CTLA-4 blockade with nivolumab raised response rates further: nivolumab alone produced an objective response rate of 31% in one study, while the combination reached 55%.

By contrast, tumours that are mismatch-repair proficient respond poorly. In one trial of pembrolizumab, the immune-related objective response rate was 40% in mismatch-repair-deficient colorectal cancers and 0% in proficient ones. Responses can also be deeper than imaging suggests, since most patients with a partial radiological response who underwent metastasectomy in one series had no viable tumour on pathology. Resistance remains a concern, including conversion of MSI-high tumours to microstable tumours under treatment pressure, and alterations in the PI3K and JAK pathways have been proposed as mechanisms of resistance.

Prognosis, recurrence and survivorship

Stage at diagnosis is the strongest determinant of survival. In SEER data for colon cancer diagnosed between 2014 and 2020, the five-year relative survival rate is 91% for localized, 74% for regional and 13% for distant disease. Rectal cancer follows a similar gradient, with a 90% rate for localized disease and 18% for distant disease. Figures from other cohorts differ. A single-centre series from Shanghai reported overall 5-year survival of 94.7% in stage I, 88.4% in stage II, 74.3% in stage III and 31.5% in stage IV, which shows how strongly estimates depend on the population and period studied.

Within each stage, further pathological features refine the estimate. Poor differentiation, lymphatic or vascular invasion, perineural invasion, bowel obstruction, perforation, a low number of examined lymph nodes and involved or close margins are all treated as high-risk markers in stage II disease for patients without these features, no routine CT imaging is recommended. Lymph node yield, tumour deposits and perineural invasion also remain central to prognostic models. In a cohort of patients who survived more than five years after surgery, only age, pathological stage and perineural invasion remained significant, which suggests that prognostic determinants shift over time.

Recurrence risk is concentrated early. Recurrence incidence rises at an exponential rate over the first two years and less frequently thereafter, which is the basis for the denser surveillance schedule in the early years. The liver is the most frequent site of early recurrence, while late recurrences are more often local or pulmonary (liver 40.5% of early recurrences; late recurrence locally 28.1% or in the lung 32.8%). Patients whose recurrence is found early may still be candidates for a second curative operation, with survival after such surgery of approximately 20%.

Surveillance protocols vary between guidelines. For stage II and III disease, the NCCN recommendations call for clinical examination and CEA monitoring every 3-6 months for 2 years, and then every 6 months up to 5 years, alongside chest, abdominal and pelvic CT at 6–12 month intervals and a colonoscopy one year after resection. The ESMO schedule is similar in the first three years, while in years four and five clinical examination and CEA are recommended at 6-12 months, and CT every 12 months. For stage I disease, only colonoscopy surveillance is generally indicated with the same frequency as above. Evidence on the value of tumour markers is mixed: one evidence-based guideline found that the use of CEA in combination with CT CAP does not lead to a survival advantage as compared with CT CAP alone. Adherence to recommended follow-up is also imperfect; in one study of three NCI-designated centres, overall adherence was 23%, although physician visits were almost universal.

Survivorship care extends beyond cancer detection. After rectal surgery, bowel, urinary and sexual dysfunction are common long-term effects, and chemotherapy-related neuropathy may persist after treatment ends. Stomas affect daily life and quality of life in ways that require ongoing adjustment. Psychosocial support is a standard component of follow-up, as is palliative care for advanced disease, which addresses symptoms and planning rather than only the end of life. Financial toxicity, meaning the economic strain of treatment, travel and lost work, is increasingly recognised as a factor that can affect adherence to surveillance and therapy.

Prevention and risk reduction

Population-level prevention targets the modifiable exposures described above, and the evidence is strongest for diet, body weight, alcohol and physical activity. A 2017 report from the World Cancer Research Fund panel concluded that large intakes of processed meat or alcoholic drinks, together with overweight and obesity, raise risk, whereas physical activity was judged protective against colon cancer specifically. The most significant of the earlier findings that were confirmed in this report are that people who consume large quantities of processed meat or alcoholic drinks, or who are overweight, obese or tall, are at higher risk of developing colorectal cancers. The dose-response estimates are concrete: each 50 g of processed meat eaten daily was linked to a 16% higher risk, and each 10 g of ethanol daily to about a 7% higher risk, with no clear increase detected below roughly 20 g of alcohol per day. Because these exposures cluster in the same populations, their combined effect is substantial. One estimate suggested that an estimated 47 percent of US colorectal cancers could be prevented each year with lifestyle changes. A cohort analysis of older adults found that meeting more of the report's recommendations was associated with lower incidence, with those meeting four to six of them showing 58% lower incidence (95% CI: 34%, 74%) in fully-adjusted analyses. Such findings are observational and show association rather than proof of causation.

Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) illustrate the tension between potential benefit and harm. Early epidemiological work suggested large reductions, with one review noting that aspirin use reduces the risk of colorectal cancer and adenoma by approximately 40 to 50%. Proposed mechanisms centre on cyclooxygenase inhibition, which lowers prostaglandin E2 and disrupts cancer-related signalling. Randomised evidence has been less favourable for people without elevated genetic risk. A 2026 Cochrane review of 10 randomised controlled trials including 124,837 participants found little to no short-term benefit and uncertain long-term effects, while the bleeding harms appeared quickly. Trials suggested that any protective effect would emerge only after a decade or more, and the certainty of that evidence was very low. Daily use increases the risk of serious bleeding, and low-dose ("baby") aspirin also raises bleeding risk. Users with a history of gastrointestinal ulcers, bleeding disorders or older age face higher risk. For this reason, the general-population evidence does not support aspirin as a routine chemopreventive for average-risk adults, although some high-risk groups, including people with Lynch syndrome, are considered separately.

Chronic inflammatory conditions and metabolic disease also form part of risk reduction. In people with inflammatory bowel disease, control of intestinal inflammation and surveillance colonoscopy for longstanding colitis are central elements of care. In people with type 2 diabetes and obesity, prevention focuses on glycaemic control, weight management and the broader metabolic risk profile that accompanies these conditions, which are linked to colorectal cancer risk discussed elsewhere in this entry.

Public health programmes combine several strands: population education about diet, alcohol and physical activity, the promotion of screening and follow-up of polyps, and access to primary care that allows these measures to reach people across income and geographic groups. Effectiveness depends on whether such programmes reach the populations with the highest exposure, and on whether individuals can act on the advice they receive.

Research history

The earliest systematic framework for colorectal cancer came from pathology and surgery. In 1932, the British pathologist Cuthbert Dukes described rectal cancers by how deeply they had penetrated the bowel wall and whether they had reached regional lymph nodes, sorting them into groups A, B and C. This scheme established that depth of spread and nodal involvement track with survival. Later modifications subdivided these groups, but the principle that local extent and nodal spread predict prognosis remained central to subsequent staging systems.

Detection changed with endoscopy. Early rigid instruments gave limited views of the colon, and the introduction of flexible fibre-optic colonoscopes in the 1960s and 1970s allowed clinicians to examine the whole colon and to remove polyps through the instrument. Endoscopic polypectomy, which had previously required open surgery, became practicable. This shifted attention from detecting symptomatic tumours to finding precursor lesions, and it gave the adenoma-to-carcinoma concept practical value, since removing adenomas was now a feasible way to prevent cancer. The approach also laid the groundwork for the surveillance intervals used in later screening programmes.

The molecular era began with the genetic model proposed by Eric Fearon and Bert Vogelstein in 1990. Their model described colorectal tumorigenesis as a series of sequential mutations that accumulate in a defined order, beginning with loss of the APC gene and extending to activation of KRAS and later losses of SMAD4 and TP53. During the 1990s, the APC gene was identified, and the mismatch-repair genes linked to hereditary nonpolyposis colorectal cancer, including MSH2 and MLH1, were characterised, which is now known as Lynch syndrome. These findings showed that inherited and sporadic disease act through overlapping pathways, and they connected cancer risk to defects in DNA mismatch repair rather than to tumour appearance alone.

Clinical research progressively replaced empirical treatment with evidence-based regimens. Randomised trials in the 1990s established fluorouracil-based adjuvant chemotherapy for stage III colon cancer, and the addition of oxaliplatin and later targeted agents such as bevacizumab extended treatment to metastatic disease. Subsequent trials showed that anti-EGFR antibodies such as cetuximab benefit only patients whose tumours carry wild-type RAS genes, which made molecular testing a routine part of treatment selection. In the 2010s, large-scale genomic characterisation of colon and rectal tumours, including the work of The Cancer Genome Atlas, identified hypermutated and microsatellite-unstable subgroups and recurrent alterations across many samples. Later analyses grouped tumours into consensus molecular subtypes based on gene expression. In parallel, early studies of immune checkpoint blockade in mismatch-repair-deficient metastatic tumours reported durable responses, which opened a distinct therapeutic route for a biologically defined subset of patients and linked immunotherapy to the mutational profile of the tumour.

Health equity and access

Screening participation and timely access to care vary by income, insurance coverage, geography and race, and these gaps are reflected in the stage at which tumours are found. In a cross-sectional analysis of average-risk US adults, screening uptake was 61.1% in 2008 and 67.6% in 2016, and it was highest among whites and lowest among Hispanics. Screening rates among Black adults did not change over the period and were 4.0% lower than the rate for white adults in 2016. Gains among Hispanic adults narrowed the gap somewhat, but the screening rate disparity between whites and Hispanics was still 17% at the end of the study period. A review of the literature found that African American men and women continue to have higher colorectal cancer incidence and mortality rates and are diagnosed at more advanced stages. Socioeconomic status and insurance status were identified as major determinants of this difference, although some studies found significant racial disparities even after controlling for these factors. A zip-code-level study concluded that the two most important factors are SDI and access to healthcare, where SDI refers to a social deprivation index. Insurance status also matters for survival: a SEER-based analysis reported that lacking insurance was associated with an increase in CRC related-deaths.

Racial patterns in early-onset disease have shifted over time. Between 2001 and 2018, compared to Whites, early-onset CRC rates are now 21% higher in American Indians/Alaskan Natives and 6% higher in Blacks. In a study of adults aged 40 to 49, colorectal adenocarcinoma incidence rates were increasing among White individuals but stabilized among Black individuals with absolute incidence rates becoming equivalent. The same study found that absolute rectal adenocarcinoma incidence rates were 39% lower in Black individuals, and its authors suggested that part of the divergence may reflect historical availability of screening options for those aged at least 45 years among Black individuals but not White individuals. This explanation remains a hypothesis rather than an established cause. Survival differences are more consistent. For patients diagnosed with early-onset disease in 2003–2013, survival for Black patients (59.3%) remained lower compared with survival for White (66.6%) and Asian (65.0%) patients, and the gap was widest for proximal colon tumours. A cohort study of 22,834 participants found higher EOCRC mortality for Hispanic, Native Hawaiian or Other Pacific Islander, non-Hispanic Black, and Southeast Asian adults compared with non-Hispanic White adults, and neighborhood socioeconomic status and insurance status attenuated the differences in EOCRC mortality when factored into the analysis.

The financial burden of treatment is a separate access barrier. Newer agents have raised costs, because the price of drugs has increased for many reasons, at least in part due to the new biologic and immunotherapy agents entering the market. These drugs can be more expensive to develop and produce in comparison to traditional chemotherapy. Monoclonal antibodies and immune checkpoint inhibitors are therefore a particular concern for coverage decisions. Financial toxicity extends beyond drug prices: it encompasses long-term indirect outlays, including lost income and travel expenses for cross-regional care. The evidence base is uneven geographically. A systematic review found that the majority (n = 12, 80%) were from high-income countries, and none from low-income countries. A 2026 meta-analysis reported a pooled prevalence of objective financial toxicity (catastrophic health expenditure) was 0.69. Its results also showed that younger patients (< 60 years) had a higher risk (OR = 2.13), and that insured patients had a higher risk compared with uninsured patients (OR = 1.88, 95% CI: 1.05–3.37). Its authors concluded that vulnerable groups, such as patients with low socioeconomic status, inadequate insurance coverage, or those living in low- and middle-income countries, appear to experience a disproportionate burden.

How screening and treatment reach patients also depends on the structure of each health system. Arrangements for universal coverage, the strength of primary care, and the way colonoscopy and follow-up are funded and scheduled shape whether people are invited to screening, whether abnormal results lead to timely investigation, and whether treatment starts without long delays. These structural differences mean that the same clinical pathway can produce very different stage distributions and outcomes across countries and within them.

Controversies and unresolved questions

Screening age, interval and blood-based tests

The American Cancer Society's May 2026 update kept age 45 as the starting point for average-risk adults, and it added a blood test for the first time. Compared with established stool-based tests, blood-based tests demonstrated lower sensitivity for both advanced precancerous lesions and stage I cancers, and the society recommends them only for people who decline or do not complete preferred tests. Stool tests remain the reference point for comparison. Modelling studies have therefore tried to predict whether a blood test would reduce incidence and mortality as well as stool tests do. Those models rely on assumed performance figures, and the recently-issued CMS guidance does not stipulate performance thresholds for the detection of pre-cancerous adenomas, a measure that strongly affects how well screening prevents cancer. Sources also differ on the regulatory status of the products. One expert update names Guardant Shield as the only FDA-approved blood test for CRC screening in average-risk individuals, whereas a later patient-facing summary lists two FDA-approved, blood-based tests for colorectal screening in people who are at average risk. The optimal interval for repeating each test type, particularly for people in their forties, is less well defined than the starting age.

Causes of early-onset disease

Explanations for the rise in cases under 50 remain speculative. One reason is that most epidemiological evidence was built on late-onset cases, so no causes specific to early-onset CRC have been identified. A proposed explanation is an unidentified carcinogen introduced in the last half-century and encountered mainly in childhood or early adulthood. A pooled analysis of three large European prospective cohorts found that adiposity, physical inactivity, smoking, and alcohol consumption are risk factors for EOCRC, and that risk factors were largely similar between EOCRC and LOCRC, except for adiposity, with stronger EOCRC association in men. These findings do not separate early-life exposures from adult lifestyle effects. Screening access is a further variable. Since screening was lowered to age 45 in the United States, some of the apparent increase may reflect earlier detection of cancers that would otherwise have surfaced later, and researchers continue to disagree on how large this effect is.

Adjuvant therapy for stage II disease

The DYNAMIC trial is the most cited test of circulating tumour DNA (ctDNA) to guide adjuvant decisions in stage II colon cancer. Among patients randomised, fewer patients overall in the ctDNA-guided arm received adjuvant CT compared to standard management (15.3% vs 27.9%), while ctDNA-guided management was non-inferior to standard management for 2-year RFS (93.5% vs 92.4%). At five years, 5-year RFS was 88% and 87% with ctDNA-guided and standard management. Critics note that the trial was a phase 2 study with a non-inferiority margin, and that for ctDNA-positive cases the benefit of adjuvant CT is unknown. Concerns about under- and overtreatment have been raised in published debate, and the assays used in different studies are not identical. Larger randomised trials, such as the MEDOCC-CrEATE study in the Netherlands, are testing whether treating ctDNA-positive patients actually prevents recurrence. The appropriate duration of chemotherapy for stage II disease is a separate question that is also still under study.

Rectal organ preservation and chemoprevention

Organ-preservation approaches for rectal cancer raise a comparison problem. Watch-and-wait strategies can spare patients a radical operation after a complete clinical response, but the long-term oncological outcomes relative to total mesorectal excision depend on how regrowth is detected and managed, and these outcomes are still being established in ongoing studies. Aspirin presents a different balance. Its potential to reduce colorectal cancer is clearer in high-risk groups such as carriers of Lynch syndrome than in average-risk adults, while gastrointestinal bleeding remains the principal harm. Whether the net benefit justifies routine use in the general population is still disputed, and the answer depends heavily on the baseline risk assumed for each group.

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