Colorectal Cancer
A malignancy that arises through a stepwise accumulation of mutations turning benign adenomatous polyps into invasive carcinoma, so screening and symptoms both follow directly from where in the bowel that clone happens to grow.
In a nutshell
Colorectal cancer develops through the adenoma-carcinoma sequence, so screening removes the polyp before it becomes cancer. Where the tumour sits determines the picture: right-sided lesions bleed occultly into liquid stool causing anaemia, left-sided lesions obstruct a narrower lumen and bleed visibly.
Classic presentation
An older adult with a change in bowel habit and rectal bleeding (left-sided), or unexplained iron-deficiency anaemia and fatigue with no gastrointestinal symptoms at all (right-sided).
Key points
- The adenoma-carcinoma sequence (APC, then KRAS, then p53) is why removing polyps at colonoscopy prevents cancer: this underpins the screening programme.
- Right-sided tumours present with occult bleeding and iron-deficiency anaemia because the caecum is wide and stool is liquid; left-sided tumours obstruct and bleed visibly because the lumen is narrower with formed stool.
- FIT testing detects occult blood invisible to the eye and is used both for screening and to triage symptomatic patients in primary care.
- 2-week-wait referral criteria include rectal bleeding, altered bowel habit, unexplained iron-deficiency anaemia, and an abdominal or rectal mass.
- TNM/Dukes staging tracks how far the tumour has progressed along the wall, nodes and distant organs (especially liver via portal drainage), and dictates whether cure is achievable by surgery alone.
First-line investigation
Colonoscopy with biopsy, guided into urgent 2-week-wait referral by red-flag symptoms or a positive FIT test.
First-line management
Surgical resection with curative intent once staged, with adjuvant chemotherapy or neoadjuvant chemoradiotherapy added according to stage and site.
Exam traps
- Unexplained iron-deficiency anaemia in an older adult is colorectal cancer until proven otherwise, even with no bowel symptoms: this is the classic right-sided presentation.
- Painless bright red PR bleeding is not automatically haemorrhoids in an older patient; red-flag features must be excluded before attributing bleeding to a benign cause.
- A normal FIT does not exclude cancer in a patient with red-flag symptoms; symptomatic red-flag criteria still warrant urgent referral regardless of FIT result.
- Left- and right-sided cancers are tested and taught as the same disease, but the exam expects you to predict the differing symptom pattern from tumour location, not memorise two unrelated lists.
Educational content pending clinical review. Not medical advice.