Key Facts

Bowel cancer screening aims to detect colorectal cancer at an earlier, more treatable stage and to prevent CRC by detecting and removing premalignant adenomatous polyps. The NHS BCSP is one of the most established cancer screening programmes in the UK.

Epidemiology

CRC screening is offered because: CRC is common (~42,900 cases/year), has a long premalignant phase (adenoma-carcinoma sequence ~10 years), effective screening tests exist, and early detection dramatically improves survival (stage I 5-year survival >90% vs stage IV ~10%). Uptake of bowel cancer screening in England is approximately 65-70% — lower in men, younger age groups, and deprived populations.

Aetiology

Screening is offered to the average-risk population. Higher-risk groups have separate surveillance pathways:

• Family history: first-degree relative with CRC <50 or 2+ first-degree relatives → earlier/more frequent colonoscopy
• Lynch syndrome: colonoscopy every 2 years from age 25
• FAP: annual sigmoidoscopy/colonoscopy from age 12-14
• IBD: surveillance colonoscopy from 8 years after onset of extensive colitis

Pathophysiology

Screening exploits the biology of the adenoma-carcinoma sequence:

• Adenomas take ~10 years to progress to cancer — screening interval of 2 years provides multiple opportunities for detection
• Adenomas and cancers bleed intermittently — FIT detects haemoglobin in stool
• Polypectomy at screening colonoscopy interrupts the adenoma-carcinoma sequence, preventing cancer development
• Earlier stage detection at screening → better prognosis and less intensive treatment needed

Screening Pathway

• Invitation: automatic letter with FIT kit sent to home address; ages 56-74 every 2 years
• FIT completion: collect small stool sample at home; return by post
• Results: within 2 weeks; normal (<120 μg Hb/g) — routine recall in 2 years; abnormal (≥120 μg Hb/g) — invitation for screening colonoscopy
• Specialist screening practitioner (SSP) consultation: pre-colonoscopy assessment

Screening Colonoscopy Outcomes

• No abnormality: return to routine screening
• Adenomatous polyps: polypectomy + enter surveillance programme
• Cancer detected: staging and MDT referral
• Incomplete colonoscopy: CT colonography offered

Red Flags (Outside Screening)

• Symptomatic patients should NOT wait for screening — investigate per NICE NG12
• FIT can be used as a triage tool in symptomatic patients (threshold ≥10 μg Hb/g — NICE DG30)
• Screening is for asymptomatic average-risk population only

Bedside

• FIT (faecal immunochemical test): quantitative measurement of human haemoglobin; specific to lower GI bleeding (unlike guaiac which detected any haem)
• Advantages over guaiac FOBt: single sample, no dietary restrictions, automated analysis, quantitative result, higher sensitivity for CRC (~95% vs ~60%)

Bloods

• FBC: iron deficiency anaemia (in symptomatic patients)
• CEA: not used for screening — used for monitoring after treatment

Imaging

• CT colonography: alternative if colonoscopy incomplete, contraindicated, or patient preference; detects polyps >6 mm with >90% sensitivity

Special Tests

• Colonoscopy: gold standard investigation following positive FIT; allows biopsy and polypectomy
• Flexible sigmoidoscopy: previously used in bowel scope screening (one-off at age 55); being phased out
• Capsule endoscopy: for complete colonic assessment if colonoscopy not possible (emerging role)
• ctDNA (circulating tumour DNA): emerging blood-based screening technology under investigation (e.g. multi-cancer early detection tests)

Non-pharmacological

• Population-level screening: FIT every 2 years, ages 56-74 (extending to 50-74)
• Informed choice: screening information leaflet with invitation; consent required
• Quality assurance: BCSP has rigorous QA standards for colonoscopy (caecal intubation rate >95%, adenoma detection rate >25%, polyp retrieval rate >90%)
• Address health inequalities: targeted interventions to improve uptake in underserved populations (GP endorsement, text reminders, translated materials)

Pharmacological

• Aspirin chemoprevention: evidence of CRC risk reduction (~20% with regular aspirin use); not currently recommended as primary prevention for average-risk population but considered in Lynch syndrome (CaPP2 trial — aspirin 600 mg/day reduced CRC in Lynch syndrome)

Surgical/Interventional

• Polypectomy: at screening colonoscopy — curative for adenomas
• Surgery: if screen-detected cancer (see CRC management)
• Post-polypectomy surveillance: per BSG guideline based on risk stratification

Referral Criteria

• Positive screening FIT → automatic referral to screening centre for colonoscopy
• Symptomatic patients with FIT ≥10 → 2WW referral per NICE NG12
• High-risk family history → genetics/high-risk surveillance programme
• Lynch syndrome → 2-yearly colonoscopy from age 25

Bowel cancer screening reduces CRC mortality by approximately 25% in participants. The UK Flexible Sigmoidoscopy Trial showed 30% reduction in CRC incidence and 35% reduction in CRC mortality with one-off sigmoidoscopy at 55. Screen-detected cancers have significantly better stage distribution: >70% Dukes A/B vs ~50% in symptomatic presentation. 5-year survival for screen-detected CRC is approximately 75-80% vs ~55% overall. National uptake of ~65-70% means significant numbers are not benefiting — improving uptake is a key public health priority.

NHS BCSP Key Parameters

High-Risk Surveillance Pathways

Wilson & Jungner Screening Criteria Applied to CRC