Ch24: Colorectal Diseases — Large Bowel Cancer & Rectal Disease（结直肠疾病）

Preparatory Mindset

Colorectal cancer (CRC) is the most common GI cancer; rectal disease adds benign conditions (hemorrhoids, fissure, fistula, abscess). The lectures (Jingge Yang: 8. Large Bowel Cancer 12.4KB + 9. Rectal disease 5.2KB) cover CRC presentation (left vs right), staging, screening, treatment (surgery + adjuvant), and the rectal disease spectrum (anatomy, bleeding, altered bowel habit, tenesmus; DRE/proctoscopy; hemorrhoids internal vs external; anal fissure/fistula/abscess).

Exam mindset: left vs right colon cancer presentations, Dukes/TNM staging, CEA, screening (FOBT, colonoscopy), TME for rectal cancer, adjuvant chemo for stage III, and the hemorrhoid classification (internal vs external by the pectinate line). The anatomy (rectal blood supply: superior/middle/inferior rectal arteries) links to the Anatomy chapters.

Core Concepts

1. Colorectal cancer — epidemiology & risk (MUST KNOW)

• The most common GI malignancy; third most common cancer overall
• Risk factors: age (>50), family history (FAP, Lynch/HNPCC), IBD (ulcerative colitis & Crohn's), diet (red/processed meat, low fiber), obesity, smoking, alcohol, prior adenomatous polyps (adenoma-carcinoma sequence)
• Genetic syndromes: FAP (APC gene — hundreds of polyps; prophylactic colectomy), Lynch syndrome (HNPCC — microsatellite instability, right-sided, younger)
• Adenoma-carcinoma sequence: normal → adenomatous polyp → dysplasia → carcinoma (over years) — screening & polypectomy prevent CRC

2. Clinical features — left vs right (MUST KNOW)

• Rectal cancer: bleeding, tenesmus (feeling of wanting to pass stool with nothing), altered bowel habit, mucus, weight loss; invasion → pelvic pain, urinary symptoms
• General red flags: any change in bowel habit >6 weeks in older patients, iron-deficiency anemia, rectal bleeding, palpable mass
• Metastasis: liver (most common — portal drainage), lung, peritoneum; Virchow's node (left supraclavicular)

3. Diagnosis & staging (MUST KNOW)

• Colonoscopy + biopsy = diagnosis (gold standard); CT colonography if colonoscopy incomplete
• Staging: TNM (T1-4 depth, N0-2 nodes, M0-1); Dukes (A: wall, B: through wall, C: nodes, D: mets); CT chest/abdomen/pelvis for staging; MRI rectum for TME planning (mesorectal fascia involvement); CEA baseline
• Tumor markers: CEA — monitoring & recurrence (not screening); CA19-9; MSI/MMR testing (Lynch screening, prognostic)
• Pre-op: full colonoscopy (synchronous tumors ~3%), CEA, staging CT, MRI for rectal; fitness assessment (elderly)

4. Treatment (MUST KNOW)

- Total mesorectal excision (TME) — the standard: sharp dissection within the mesorectal envelope, preserves the pelvic nerves (urinary/sexual function); low anterior resection (LAR) with anastomosis or abdominoperineal resection (APR) + permanent colostomy for very low tumors - Neoadjuvant (preoperative) chemoradiotherapy for locally advanced (T3-4/N+): downsizes → improves resectability & sphincter preservation, reduces local recurrence - Adjuvant chemo for node-positive - Local excision for very early (T1) favorable rectal tumors

• Colon cancer: surgical resection with en bloc lymphadenectomy: right hemicolectomy, left hemicolectomy, sigmoidectomy, (sub)total colectomy (FAP/Lynch); adjuvant chemotherapy (FOLFOX/CAPOX) for stage III (node-positive) & high-risk stage II; laparoscopic preferred
• Rectal cancer (MUST KNOW):
• Metastatic (stage IV): liver/lung resection (oligometastatic) ± perioperative chemo; palliative chemo/targeted (anti-EGFR if RAS wild-type; anti-VEGF); obstructing tumor → colonic stent or defunctioning stoma
• Emergency (obstruction/perforation): resection (Hartmann's if unprepared/peritonitis) or stent as bridge to elective
• Follow-up: CEA, colonoscopy (1 yr then 3-5 yr), CT; surveillance detects recurrence (liver mets resectable early)

5. Benign colorectal disease

• Diverticular disease: diverticulosis (asymptomatic) → diverticulitis (LIF pain, fever — "left-sided appendicitis") → complications (abscess, perforation, fistula, stricture); CT diagnosis; antibiotics ± drainage/surgery (Hartmann's for perforation)
• Colonic polyps: adenomatous (premalignant — remove), hyperplastic (benign); sessile vs pedunculated; polypectomy prevents CRC

6. Rectal disease — anatomy (MUST KNOW)

- Above (internal): superior rectal — PORTAL venous drainage; visceral innervation (pain-insensitive) - Below (external): inferior rectal — SYSTEMIC (internal pudendal→iliac); somatic innervation (painful)

• Rectum: from sigmoid at the sacral promontory to the anorectal junction (12-18 cm); upper/middle/lower thirds; ampulla (distal dilation)
• Blood supply: superior rectal artery (from inferior mesenteric — portal system!), middle rectal (internal iliac), inferior rectal (internal pudendal); the pectinate line divides:
• Lymphatics: above pectinate → superior rectal → mesorectal/IMA nodes; below → inguinal nodes
• Examination positions: left lateral, knee-elbow, lithotomy; inspection → digital rectal examination (DRE) → proctoscopy/sigmoidoscopy

7. Hemorrhoids (痔) (MUST KNOW)

- Grade I: bleed, no prolapse - Grade II: prolapse on straining, reduce spontaneously - Grade III: prolapse, manual reduction - Grade IV: permanently prolapsed/irreducible (± thrombosis) - Symptoms: painless bright-red bleeding (on wiping), prolapse, mucus; thrombosed → acute pain

- Mild (I-II): fiber/fluids, stool softeners, avoid straining, topical (steroid/analgesic), sitz baths; rubber band ligation, sclerotherapy, infrared coagulation - III: banding/stapled hemorrhoidopexy/hemorrhoidectomy - IV/thrombosed/refractory: hemorrhoidectomy - Thrombosed external: excision within 72 h or conservative (analgesia, topical)

• Definition: abnormally dilated vascular cushions (internal hemorrhoidal plexus) — not "varicose veins" per se
• Internal hemorrhoids (above pectinate line — painless bleeding):
• External hemorrhoids (below pectinate line — painful): acute thrombosis → painful perianal lump (within 72 h → excision); skin tags
• Diagnosis: inspection + DRE + proctoscopy; exclude colorectal cancer (bleeding must be investigated — colonoscopy)
• Treatment:

8. Anal fissure, fistula & abscess (MUST KNOW)

• Anal fissure: painful tear, pain on defecation + bright bleeding; usually posterior midline (anterior = suspect Crohn's); treatment: stool softeners, topical nitroglycerin/calcium-channel blocker, botox; lateral internal sphincterotomy for chronic (be aware of incontinence risk)
• Perianal abscess: painful fluctuant lump (anorectal sepsis) → incision & drainage (I&D) — antibiotics alone insufficient
• Fistula-in-ano: chronic track from anal canal to skin after abscess — discharge, recurrent abscess; Goodsall's rule (anterior = radial; posterior = opens midline); treatment: fistulotomy (low), seton (high/sphincter involvement — protect continence); MRI for complex
• Pilonidal disease: natal cleft hair-containing sinus → abscess/drainage, excision
• Rectal cancer symptoms overlap — any bleeding/tenesmus needs exclusion of malignancy

High-Yield Points

• Right colon: anemia, occult bleed, mass; Left colon: obstruction, altered bowel habit, bright blood
• Diagnosis: colonoscopy + biopsy; staging CT (+MRI rectum); CEA monitoring
• Colon: resection + lymphadenectomy; adjuvant chemo for stage III
• Rectal: TME (nerve preservation); neoadjuvant chemoradiotherapy for T3-4/N+; APR + colostomy for low tumors
• Dukes: A wall, B through wall, C nodes, D mets; TNM
• Pectinate line: internal (portal, painless) vs external (systemic, painful) hemorrhoids
• Hemorrhoid grades I-IV; bleeding always needs colonoscopy (exclude cancer)
• Anal fissure: posterior midline, painful; nitroglycerin/botox; sphincterotomy if chronic
• Perianal abscess → I&D; fistula → fistulotomy/seton (Goodsall's rule)
• Diverticulitis: LIF pain, "left-sided appendicitis"
• FAP (APC, polyps), Lynch (MSI, right-sided, young) (CM exam tested); screening prevents CRC

Topic Summary

Colorectal cancer: most common GI cancer; risk (age, IBD, polyps, FAP/Lynch, diet). Right colon = anemia/mass (polypoid); left = obstruction/change in bowel habit (annular). Diagnosis: colonoscopy + biopsy; CT staging; MRI for rectal (TME planning); CEA. Treatment: colon — hemicolectomy + node clearance + adjuvant chemo (stage III); rectal — TME (nerve-preserving), neoadjuvant chemoradiotherapy for locally advanced, APR for low tumors; metastatic → resection of liver/lung ± chemo. Benign: diverticulitis (LIF, CT, ± drainage/surgery), polyps (remove). Rectal anatomy: superior/middle/inferior rectal arteries; pectinate line divides portal/painless internal from systemic/painful external hemorrhoids; hemorrhoid grades I-IV treatment ladder; anal fissure (posterior, nitroglycerin/botox/sphincterotomy), perianal abscess (I&D), fistula (fistulotomy/seton, Goodsall's rule). Always investigate rectal bleeding (colonoscopy) to exclude cancer.