Colorectal cancer

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Colorectal cancer is usually an adenocarcinoma arising through accumulated molecular change in colonic or rectal epithelium. Sporadic risk rises with age, smoking, excess adiposity and processed meat intake; important high-risk groups include people with longstanding colitis, previous colorectal neoplasia and inherited syndromes such as Lynch syndrome or familial adenomatous polyposis.

The central clinical decision is not simply whether bleeding is present, but whether the pattern needs urgent cancer investigation and whether an emergency complication is developing. Symptoms can be subtle, intermittent or attributed incorrectly to haemorrhoids, anticoagulation or irritable bowel syndrome.

Management depends on anatomical site, TNM stage, tumour biology, physiological fitness and patient priorities. Colon and rectal pathways differ: rectal MRI and decisions about neoadjuvant radiotherapy or systemic therapy are integral to rectal-cancer planning, whereas resectable colon cancer usually proceeds to oncological segmental colectomy.

Key points

Right-sided cancers often present with iron-deficiency anaemia, fatigue, weight loss or a mass; left-sided lesions more often produce altered bowel habit, bleeding or obstruction.
Rectal cancer may cause bright bleeding, tenesmus, urgency, incomplete evacuation or a palpable lesion on digital rectal examination.
In symptomatic adults, quantitative FIT supports referral decisions, but a rectal or abdominal mass, obstruction, persistent unexplained symptoms or strong clinical concern must override a reassuring result.
The symptomatic NICE referral threshold and the NHS bowel-screening threshold serve different populations and must never be interchanged; screening policy also varies between UK nations.
Colonoscopy with biopsy usually establishes diagnosis; CT colonography is an alternative when colonoscopy is unsuitable or incomplete but cannot provide tissue.
Stage with CT chest, abdomen and pelvis; add high-resolution pelvic MRI for rectal cancer and discuss every confirmed case at a specialist colorectal MDT.
Test colorectal tumours for mismatch-repair deficiency or microsatellite instability because results affect Lynch syndrome assessment, prognosis and selected systemic treatment.
CEA is useful as a baseline and for selected follow-up, but a normal value neither excludes cancer nor makes surveillance imaging unnecessary.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Sporadic precursor pathways

Most cancers arise through accumulated molecular changes in conventional adenomas or serrated lesions, progressing over time from dysplasia to invasion.

Inherited predisposition

Lynch syndrome, familial adenomatous polyposis and other pathogenic germline variants substantially increase risk and alter surveillance for relatives.

Inflammatory and lifestyle risk

Longstanding colitis, age, smoking, alcohol, obesity, inactivity and dietary patterns modify risk, while many affected people have no obvious single cause.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Clonal dysplastic growth
Driver alterations allow abnormal epithelial cells to escape normal growth control and expand within an adenomatous or serrated precursor.
2
Local invasion
Further molecular change permits malignant glands to breach the muscularis mucosae, invade bowel wall and access lymphatic or venous channels.
3
Regional and distant spread
Tumour reaches lymph nodes and commonly the liver through portal venous drainage, with later peritoneal, pulmonary or other metastases.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Right-sided pattern

Occult blood loss causes microcytic iron-deficiency anaemia, lethargy, dyspnoea or syncope; weight loss and a right-sided abdominal mass may appear before obvious bowel change.

Left-sided pattern

Progressive constipation, looser stools around narrowing, colicky pain, visible blood and abdominal distension suggest a distal lesion, especially when symptoms are new and persistent.

Rectal lesionRed flag

Tenesmus, urgency, mucus, bleeding and a sense of incomplete evacuation should prompt inspection and digital rectal examination where appropriate, with urgent referral even if FIT is low when a mass is felt.

Obstructing or perforated tumourRed flag

Absolute constipation, vomiting, marked distension, peritonism, fever, shock or free air indicate an oncological surgical emergency; closed-loop caecal dilatation can perforate remotely from a distal tumour.

Metastatic or systemic diseaseRed flag

Hepatomegaly, jaundice, ascites, respiratory symptoms, bone pain, cachexia or venous thromboembolism may be the first clue to advanced disease and require prompt staging rather than symptom-only treatment.

Hereditary signal

Young diagnosis, multiple colorectal tumours, endometrial or related Lynch-spectrum cancers, polyposis, or a strong multigenerational family history warrants genetics-led assessment rather than informal reassurance.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Quantitative faecal immunochemical testFirst step
Why
Triage eligible symptomatic people into the colorectal cancer pathway.
Interpretation and limitations
Use the current NICE and local symptomatic pathway; in England a result at or above 10 micrograms haemoglobin per gram supports suspected-cancer referral. Non-return, a result below threshold, persistent concern, a mass or obstructive features still require safety-netting or referral.
02
Full blood count, ferritin and iron studies
Why
Detect and characterise chronic gastrointestinal blood loss.
Interpretation and limitations
Microcytosis and low ferritin support iron deficiency, but inflammation may raise ferritin. Unexplained iron-deficiency anaemia, particularly in an adult without menstrual loss, warrants gastrointestinal evaluation rather than iron replacement alone.
03
Colonoscopy with targeted biopsy
Why
Visualise the whole colon, obtain histology and find synchronous lesions.
Interpretation and limitations
Histology confirms cancer type and grade. Record completeness and bowel preparation; an impassable stenosis or incomplete examination requires alternative imaging of the remaining colon and MDT planning.
04
CT colonography
Why
Evaluate the colon when optical colonoscopy is unsafe, incomplete or declined.
Interpretation and limitations
A suspicious lesion still needs tissue or definitive surgical correlation. Extracolonic findings can be useful but the test is not a therapeutic substitute for endoscopy.
05
CT chest, abdomen and pelvis
Why
Stage distant and nodal disease and detect emergency complications.
Interpretation and limitations
Assess liver, lung, peritoneum, nodes, local invasion, obstruction and perforation. Indeterminate lesions may need liver MRI, PET-CT or interval imaging decided by the MDT.
06
Pelvic MRI for rectal cancer
Why
Define local stage, mesorectal fascia and sphincter relationships.
Interpretation and limitations
Circumferential resection margin risk, extramural vascular invasion, nodal disease and tumour height shape neoadjuvant and surgical strategy; report through a rectal-cancer MDT.
07
Tumour MMR or MSI testing
Why
Identify mismatch-repair deficiency and guide downstream decisions.
Interpretation and limitations
An abnormal pattern triggers a defined pathway using tumour features, possible BRAF or methylation tests and germline genetics. It can also influence adjuvant and metastatic immunotherapy decisions.
08
Baseline CEA
Why
Provide a comparator for postoperative or treatment surveillance.
Interpretation and limitations
Elevation is neither specific nor universal. A normal baseline does not exclude recurrence; a rising trend requires clinical review and imaging under the surveillance pathway.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Benign anorectal bleeding

Bright blood with haemorrhoids or fissure may explain symptoms, but persistent bleeding, anaemia or altered bowel habit still requires colorectal assessment.

Inflammatory or diverticular disease

Pain, diarrhoea, fever and segmental inflammatory imaging suggest IBD or diverticulitis, although cancer can obstruct, perforate or coexist.

Functional bowel disorder

A longstanding fluctuating pain-stool pattern without bleeding, weight loss, anaemia or mass supports IBS; new alarm features override prior reassurance.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ReferralSymptomatic suspected-cancer routeFirst stepNew colorectal symptoms, unexplained iron deficiency, or a concerning examination finding.

1Take a site-focused history, examine the abdomen, inspect the anus where relevant and perform digital rectal examination for rectal symptoms when clinically appropriate.
2Request quantitative FIT according to the current symptomatic pathway, plus blood count and iron studies, without allowing testing logistics to delay referral for a mass, obstruction or marked concern.
3Refer on the applicable suspected-cancer pathway when criteria are met; document what happens after a low result and arrange active review if symptoms persist or evolve.
4Use colonoscopy with biopsy or CT colonography as selected by the diagnostic service, then stage confirmed cancer and present it to the colorectal MDT.

02EmergencyObstruction or perforation responseDistension, absolute constipation, peritonism, sepsis, free gas or impending caecal perforation.

1Resuscitate using an ABCDE approach, establish intravenous access, correct fluid and electrolyte losses, provide analgesia, keep nil by mouth and start antimicrobials when perforation or sepsis is suspected.
2Obtain urgent contrast-enhanced CT when the patient can tolerate it and involve senior colorectal, anaesthetic and critical-care teams before physiological collapse.
3Choose resection, diversion, stoma or selected colonic stenting through specialist assessment of site, perforation risk, stage, fitness and local expertise; do not promise a single universal operation.
4Send resection tissue for complete pathology and ensure recovery leads into oncological staging, MDT review, rehabilitation and stoma support where relevant.

03Definitive careStage-directed multidisciplinary treatmentDefinitiveBiopsy-proven colorectal adenocarcinoma after complete staging.

1For resectable colon cancer, assess fitness and plan oncological segmental colectomy with regional lymphadenectomy, using prehabilitation and enhanced recovery where appropriate.
2For rectal cancer, integrate MRI risk features, tumour height and patient goals to select upfront surgery, radiotherapy, chemotherapy or total neoadjuvant approaches under the current MDT protocol.
3Use pathological stage, margins, nodes, molecular markers and fitness to decide adjuvant therapy; metastatic disease requires specialist review for systemic therapy and possible liver, lung or peritoneal local treatment.
4After curative treatment, follow the current national and local surveillance schedule using clinical review, CEA, colonoscopy and cross-sectional imaging, while addressing bowel, sexual, urinary and stoma function.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Treat bacterial contamination when tumour perforation, peritonitis or sepsis accompanies the presentation.

Intravenous broad-spectrum antimicrobials

Use the local emergency intra-abdominal sepsis regimen with renal adjustment and early review.

Take cultures where feasible without delaying therapy, check allergy and organ function, obtain source control urgently, and narrow or stop treatment with microbiology advice.

Forms the backbone of selected adjuvant and metastatic colorectal cancer treatment.

Fluoropyrimidine-based systemic therapy

Regimen, cycle timing and dose are calculated by the specialist oncology protocol.

Dihydropyrimidine dehydrogenase assessment, performance status, renal function, interactions and toxicity plans are essential; mucositis, diarrhoea, cytopenia and cardiotoxicity require prompt oncology contact.

Correct iron deficiency and support preoperative optimisation while cancer investigation proceeds.

Oral iron or intravenous iron

Replace documented deficiency using the current formulary and perioperative pathway.

Iron treatment must not postpone source investigation. Oral preparations may worsen gastrointestinal symptoms; intravenous products require monitored administration and selection based on timing and tolerance.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Obstruction and perforation

Circumferential tumour can block the colon, while pressure or local invasion causes perforation, peritonitis and emergency surgery with higher risk.

Bleeding and iron deficiency

Chronic occult loss, particularly from proximal tumours, causes iron-deficiency anaemia; brisk bleeding occurs less often but can destabilise.

Metastatic and cachectic disease

Hepatic, pulmonary or peritoneal spread causes organ dysfunction, pain, ascites and systemic weight loss that influence treatment tolerance and prognosis.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Before treatment, document performance status, frailty, nutrition, anaemia, renal and hepatic function, comorbidity, fertility concerns and the patient's priorities.
After surgery, monitor for ileus, anastomotic leak, infection, venous thromboembolism, urinary retention, stoma problems and nutritional decline using the local enhanced-recovery pathway.
During systemic treatment, check blood counts, renal and liver biochemistry and regimen-specific toxicity before each cycle; supply a clear 24-hour oncology contact route.
Surveillance must use the current NICE and local schedule rather than an improvised CEA-only plan; investigate symptoms between planned appointments.
Track late effects including low anterior resection syndrome, neuropathy, fatigue, sexual or urinary dysfunction, psychological distress and work impact, with rehabilitation referral where needed.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

FIT is a triage test

Quantitative symptomatic FIT estimates bleeding from lower gastrointestinal lesions; it does not prove or exclude cancer and is not a licence to ignore a palpable mass or progressive symptoms.

Two FIT thresholds coexist

The symptomatic referral cut-off and the population-screening cut-off answer different questions. England's screening programme changed sensitivity in 2026, while devolved programmes may use other rules.

Rectal examination changes pathways

A low rectal lesion can be directly palpable and may be missed if clinicians request remote tests without examination. Record lesion position and avoid traumatising a painful area.

Synchronous disease matters

A stenosing tumour can prevent complete colonoscopy; the unseen proximal colon still requires evaluation before or after urgent treatment because another advanced lesion may coexist.

Biology refines anatomy

MMR or MSI status is not an optional academic result: it informs Lynch investigation and can alter systemic treatment, especially in advanced disease.

Cancer care includes function

Bowel frequency, continence, stoma confidence, fertility, intimacy and return to work are meaningful outcomes alongside recurrence and survival and should be discussed early.

11Common pitfallsFrequent interpretation and management errors.

01
Attributing repeated rectal bleeding to haemorrhoids without examining the patient or assessing cancer risk.
02
Using a screening-programme FIT threshold to interpret a symptomatic patient's result.
03
Treating iron deficiency while failing to identify its colorectal source.
04
Ordering CEA as a diagnostic rule-out test for bowel cancer.
05
Failing to complete proximal-colon assessment after an incomplete examination caused by stenosis.
06
Planning rectal-cancer surgery before high-quality pelvic MRI and specialist MDT discussion.