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

Colorectal cancer (CRC) encompasses cancers of the colon and rectum, with approximately 60% arising in the colon and 40% in the rectum. It is the third most common cancer globally, with approximately 1.9 million new cases and 935,000 deaths annually. In the UK, colorectal cancer is the fourth most common cancer (42,000 cases per year). The vast majority (over 95%) are adenocarcinomas arising from glandular epithelium. Most colon cancers develop from pre-malignant polyps — particularly adenomatous polyps — through the well-established adenoma-carcinoma sequence, which takes 5-10 years. This long pre-malignant window makes colorectal cancer uniquely amenable to screening and early intervention. The 5-year survival for localised (stage I) disease exceeds 90%; for metastatic (stage IV) disease, median survival has improved to over 30 months with modern chemotherapy and targeted agents.

Colorectal cancer results from accumulated somatic mutations in the colonic epithelium, typically involving APC, KRAS, SMAD4, and TP53 in the classical chromosomal instability pathway. Lifestyle risk factors account for the majority of cases: red and processed meat consumption (over 500g red meat per week increases risk by 20-30%), obesity (abdominal adiposity increases risk), physical inactivity, alcohol (above 2 units/day), smoking, type 2 diabetes, and low fibre diet. Inflammatory bowel disease (Crohn's colitis, ulcerative colitis — cumulative risk of 2% at 10 years, 8% at 20 years, 18% at 30 years) significantly increases risk. Hereditary syndromes account for 5-10% of cases: Lynch syndrome (mismatch repair gene mutations — MLH1, MSH2, MSH6, PMS2 — causes 3% of all CRC, autosomal dominant, lifetime risk 25-80%), familial adenomatous polyposis (FAP — APC mutation, >100 adenomas, near-100% CRC risk without prophylactic colectomy). Family history of CRC in a first-degree relative doubles the population risk.

Early colon cancer is often asymptomatic — underscoring the importance of screening. Symptoms that develop include: change in bowel habit (persistent diarrhoea, constipation, or alternating — especially in people over 50); rectal bleeding (bright red blood on stool or mixed with stool — a red flag requiring urgent investigation, though most rectal bleeding is haemorrhoidal); dark, tarry stools (melaena — suggesting proximal colon or right-sided tumour with occult bleeding); unexplained iron deficiency anaemia (fatigue, pallor, dyspnoea — from chronic occult blood loss — particularly in right-sided colon cancer where bleeding may not be visible); abdominal pain, cramping, or bloating; unexplained weight loss; and palpable abdominal mass. Emergency presentations include bowel obstruction (vomiting, absolute constipation, abdominal distension) and perforation (acute abdomen with peritonitis). Any persistent symptom in adults over 40 warrants investigation — rectal bleeding in particular must never be attributed to haemorrhoids without excluding colorectal cancer.

Colonoscopy is the definitive diagnostic procedure — allows direct visualisation and biopsy of colonic mucosa throughout the entire colon; any suspicious lesion is biopsied. CT colonography (virtual colonoscopy) is an alternative for those unable to undergo traditional colonoscopy. Histopathology confirms adenocarcinoma and grade. CT scan of chest, abdomen, and pelvis is mandatory for staging — identifies lymph node involvement and distant metastases. MRI rectum is essential for rectal cancer staging (T stage, mesorectal fascia involvement, nodal status). TNM staging (Stage I-IV) defines treatment strategy and prognosis. Molecular testing of the tumour tissue is required for all metastatic CRC: KRAS/NRAS/BRAF mutation status (RAS-mutant tumours do not respond to EGFR antibodies cetuximab/panitumumab), microsatellite instability (MSI) or mismatch repair (MMR) status (MSI-H tumours respond to PD-1 inhibitor pembrolizumab). HER2 amplification testing is emerging as an actionable target.

Stage I-II (localised): surgical resection (colectomy with lymph node dissection) is curative in 80-90%. Adjuvant chemotherapy (6 months CAPOX — capecitabine + oxaliplatin, or FOLFOX — 5-FU/leucovorin/oxaliplatin) is recommended for high-risk stage II and all stage III disease, reducing recurrence by 20-25%. Stage III (lymph node positive): surgery + adjuvant CAPOX or FOLFOX — 5-year survival approximately 50-70% depending on number of nodes. Rectal cancer: preoperative chemoradiotherapy (long-course 5-FU or short-course radiotherapy) for stage II-III rectal cancer before total mesorectal excision (TME) surgery reduces local recurrence from 25% to below 10%. Stage IV (metastatic): median survival with modern treatment is over 30 months. First-line: FOLFOX or FOLFIRI plus bevacizumab (anti-VEGF) or cetuximab/panitumumab (anti-EGFR, only in RAS-wild-type). Second-line: FOLFIRI, TAS-102, regorafenib. MSI-H metastatic CRC: pembrolizumab (anti-PD-1) has become first-line in MSI-H metastatic CRC (KEYNOTE-177 trial — superior to chemotherapy). Liver-limited metastases: hepatic resection in selected patients produces 30-40% 5-year survival — potentially curative.

Undiagnosed or untreated colorectal cancer causes bowel obstruction (complete large bowel obstruction is a surgical emergency — 15-25% mortality), perforation with faecal peritonitis (20-30% mortality), and severe gastrointestinal haemorrhage. Liver metastases (present in 20% at diagnosis; 50% of patients will develop them) cause liver failure, jaundice, and portal hypertension. Peritoneal carcinomatosis causes malignant ascites and recurrent bowel obstruction. Five-year survival falls dramatically with stage: Stage I (90-95%), Stage II (75-85%), Stage III (50-70%), Stage IV (10-15%). Lynch syndrome-associated colorectal cancer carries elevated risk of endometrial, ovarian, urological, and other cancers in affected families — requiring cascade genetic testing of all first-degree relatives.

Colorectal cancer is highly preventable through lifestyle modifications and screening. Diet: increase fibre intake (whole grains, fruit, vegetables), limit red and processed meat, avoid alcohol excess. Exercise reduces CRC risk by 25%. Maintain healthy weight — obesity increases CRC risk by 30-40%. Regular aspirin use (75-150mg daily) reduces CRC risk by approximately 25% but is not routinely recommended purely for cancer prevention given bleeding risk. Bowel cancer screening: in the UK, the NHS offers FIT (faecal immunochemical test) home testing annually or biannually for those aged 50-74; positive FIT leads to colonoscopy. In the USA, colonoscopy every 10 years from age 45 is standard. Colonoscopy and polypectomy (removing adenomatous polyps before they become malignant) reduces CRC incidence by 60-80%. Lynch syndrome carriers require colonoscopy every 1-2 years from age 25. FAP families: prophylactic colectomy after polyp burden develops.

See your doctor promptly (within 2 weeks) for: any rectal bleeding combined with change in bowel habit in adults over 40; unexplained iron deficiency anaemia without an obvious cause; persistent change in bowel habit (6 weeks of looser stools or increased frequency) in adults over 60; or a palpable rectal or abdominal mass. Emergency presentation is required for signs of bowel obstruction (severe abdominal pain, distension, vomiting, absolute constipation) or peritonitis from perforation. Do not assume rectal bleeding is haemorrhoids without medical investigation — this is a common and dangerous error. Participation in national bowel screening programmes saves thousands of lives annually by detecting cancer at a curable stage.