Colon Cancer (Colorectal Cancer) — Causes, Staging & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Colorectal cancer (CRC) includes cancers of the colon and rectum. It is the 3rd most common cancer and 2nd leading cause of cancer death globally, with 1.9 million new cases and 935,000 deaths annually. Over 90% of CRCs develop from adenomatous polyps (adenomas) — benign growths on the colon lining that can transform into cancer over 10-15 years. This adenoma-carcinoma sequence makes CRC highly preventable through colonoscopic polypectomy. CRC staging by TNM classification (Stages I-IV) is the most important prognostic factor: Stage I 5-year survival 90%; Stage II 80%; Stage III 60%; Stage IV 14%. The adenoma-carcinoma sequence — the step-by-step molecular and histological transformation of a benign adenomatous polyp into invasive colorectal carcinoma over 10-15 years — provides a uniquely long window for prevention through colonoscopic polypectomy, making colorectal cancer one of the most preventable major cancers when organised population screening is effectively implemented.

Modifiable risk factors: low dietary fibre (under 15g/day), high red and processed meat consumption, obesity, physical inactivity, smoking, excess alcohol, Type 2 diabetes, and chronic inflammation (IBD — ulcerative colitis and Crohn's disease). Non-modifiable risk factors: age over 50 (incidence doubles every 10 years), personal or family history of CRC or adenomatous polyps, inflammatory bowel disease, and hereditary syndromes. Lynch syndrome (mismatch repair gene mutations — MLH1, MSH2, MSH6, PMS2) accounts for 3-5% of CRC — associated with early-onset cancer (age 40-45) and other cancers (endometrial, ovarian, gastric, urinary). Familial adenomatous polyposis (FAP — APC gene mutation) causes thousands of polyps and near 100% CRC risk if untreated.

Early CRC is often asymptomatic — detected on screening colonoscopy. Symptoms of more advanced disease: rectal bleeding (bright red blood with stools or dark altered blood), change in bowel habit (diarrhoea, constipation, narrowing of stools — persistent over 4 weeks), abdominal pain or cramping, sensation of incomplete bowel emptying (tenesmus), unexplained weight loss, iron deficiency anaemia (from chronic slow bleeding — a common presentation of right-sided colon cancer), and abdominal mass. Complications of advanced disease: bowel obstruction (usually left-sided tumours), bowel perforation (causing peritonitis), and fistula formation. Any rectal bleeding or persistent change in bowel habit in adults over 40 warrants investigation.

Colonoscopy is the gold standard diagnostic test — allows direct visualisation and biopsy of the entire colon and rectum. Flexible sigmoidoscopy examines only the left colon and rectum. CT colonography (virtual colonoscopy) is an alternative for those unable to tolerate colonoscopy. Biopsy provides histological diagnosis. Staging CT chest/abdomen/pelvis assesses regional lymph nodes and distant metastases (liver and lung — most common). MRI pelvis for rectal cancer (definitive local staging for surgical planning). CEA (carcinoembryonic antigen) tumour marker — elevated in 60% of CRC; useful for monitoring treatment response and recurrence. Molecular testing: MSI (microsatellite instability) status — MSI-H predicts immunotherapy response (pembrolizumab) and is found in 15-20% of CRC. RAS/BRAF mutation testing — guides anti-EGFR therapy (cetuximab/panitumumab only effective in RAS wild-type).

Stage I: endoscopic polypectomy or colectomy — surgery curative in over 90%. Stage II: colectomy (right or left hemicolectomy or sigmoid resection based on location); adjuvant chemotherapy for high-risk features (bowel perforation, T4 tumour, inadequate node sampling). Stage III: colectomy + adjuvant FOLFOX chemotherapy (oxaliplatin + fluorouracil + leucovorin) for 6 months — reduces recurrence by 25-30%. Stage IV (metastatic): FOLFOX or FOLFIRI (irinotecan + fluorouracil + leucovorin) plus biologics: bevacizumab (anti-VEGF — for most patients) or cetuximab/panitumumab (anti-EGFR — only for RAS wild-type tumours). Trifluridine-tipiracil (Lonsurf) or regorafenib for refractory disease. Pembrolizumab is highly effective for MSI-H/dMMR CRC — first-line for MSI-H metastatic CRC. Liver metastases: hepatic resection is potentially curative in 20-30%; combined with systemic therapy and interventional procedures (ablation, SIRT).

Untreated CRC progresses from local invasion to regional lymph node metastasis to distant (Stage IV) disease — most commonly to the liver, lung, peritoneum, and ovaries. Bowel obstruction from untreated large tumours causes severe abdominal distension, vomiting, and is a surgical emergency. Bowel perforation causes life-threatening peritonitis and sepsis. Chronic bleeding causes severe iron deficiency anaemia. Peritoneal carcinomatosis (seeding of cancer throughout the abdominal cavity) causes intractable ascites and bowel obstruction. Screening colonoscopy and polypectomy are highly effective at preventing CRC — removing adenomas before they transform into cancer. An estimated 90% of CRC is preventable or detectable at an early curable stage with regular screening.

CRC screening is highly effective at reducing mortality: NHS Bowel Cancer Screening (UK) — faecal immunochemical test (FIT) every 2 years from age 50-74; FIT-positive results lead to colonoscopy. USA: colonoscopy every 10 years from age 45 (or FIT annually). Lynch syndrome patients: colonoscopy every 1-2 years from age 20-25. Dietary prevention: increase dietary fibre (at least 30g/day from whole grains, legumes, vegetables, fruit); reduce red and processed meat; maintain a healthy weight; exercise regularly (150-300 minutes/week of moderate activity reduces CRC risk by 15-25%). Stop smoking. Limit alcohol. Aspirin (75-150 mg daily) reduces CRC risk in people at elevated risk — discuss with GP, as benefits must be weighed against GI bleeding risk.

See your GP urgently for: rectal bleeding (blood in or on stools) that is not explained by haemorrhoids, especially if dark or mixed with stool; persistent change in bowel habit lasting more than 4 weeks (looser stools, increased frequency, or alternating constipation and diarrhoea); unexplained iron deficiency anaemia — which may indicate occult bowel bleeding from a right-sided tumour (causing tiredness, breathlessness, pale skin); unintentional weight loss with abdominal pain or change in bowel habit; a palpable abdominal or rectal mass. NICE recommends urgent 2-week-wait (2WW) referral for colorectal cancer if aged 40 or above with rectal bleeding plus change in bowel habit; aged 50 or above with unexplained rectal bleeding or change in bowel habit; or any age with unexplained iron deficiency anaemia, positive FIT test, or low rectal mass. Attend Emergency Department for: complete bowel obstruction — severe abdominal distension, inability to open bowels or pass wind, vomiting; bowel perforation — sudden severe abdominal pain with peritonism (rigid abdomen). Complete your NHS bowel cancer screening test when invited — FIT testing from age 50 saves lives.