Colorectal Neoplasms

Epidemiology & Risk Factors - Cancer's Coordinates

• Incidence: 3rd most common cancer worldwide (men), 2nd (women).
• Age: Peak incidence >50 years; rising in younger adults.
• Geography: Higher in developed countries (Western diet/lifestyle).
• **Risk Factors (Modifiable):
  • Diet: ↓Fiber, ↑Red/processed meat, ↑Fat.
  • Lifestyle: Obesity, physical inactivity, smoking, alcohol.
• **Risk Factors (Non-Modifiable):
  • Personal history: Adenomatous polyps, IBD (UC > Crohn's).
  • Family history: CRC or adenomatous polyps in 1st-degree relative.
  • Hereditary syndromes: FAP, Lynch syndrome (HNPCC).

⭐ Lynch syndrome accounts for 2-4% of all CRCs; associated with multiple extracolonic cancers (endometrial, ovarian, gastric).

Pathogenesis & Polyps - From Polyp to Problem

• CRC develops via adenoma-carcinoma sequence (commonest) or serrated pathway.
• Key Genetic Pathways:
  • Chromosomal Instability (CIN) Pathway (~85%):
    • Sequential APC (gatekeeper), KRAS, TP53 mutations.
    • Linked to FAP.
  • Microsatellite Instability (MSI) Pathway (~15%):
    • Defective DNA Mismatch Repair (dMMR) genes (e.g., MLH1, MSH2).
    • Linked to Lynch Syndrome (HNPCC).
    • Often right-sided, mucinous.
• Polyps as Precursors:
  • Adenomatous (Neoplastic):
    • Tubular: Most common (~75%), lowest malignant risk.
    • Villous: Highest malignant risk (~40%).
    • Tubulovillous: Intermediate.
  • Serrated:
    • Hyperplastic: Small, benign, usually left-sided.
    • Sessile Serrated Adenoma/Polyp (SSA/P): Premalignant, right-sided, BRAF mutation.

⭐ Villous adenomas exhibit the highest malignant potential among adenomatous polyps, approaching 40%.

Clinical Features & Diagnosis - Detecting Danger

• Often asymptomatic; symptoms depend on tumor location:
  • Right-sided: Occult bleeding → iron deficiency anemia, fatigue, vague abdominal pain.
  • Left-sided: Altered bowel habits (constipation/diarrhea), visible rectal bleeding (hematochezia), tenesmus, obstructive symptoms.
• General: Unexplained weight loss, anorexia.
• ⚠️ Alarm symptoms: Persistent change in bowel habit (>6 wks), PR bleeding, unexplained weight loss, iron deficiency anemia (especially in males/post-menopausal females), palpable abdominal mass.
• Diagnosis:
  • History, Physical Exam (incl. Digital Rectal Exam - DRE).
  • Labs: CBC (anemia), LFTs. Carcinoembryonic Antigen (CEA) for prognosis & recurrence monitoring (not screening).
  • Colonoscopy: Gold standard for diagnosis & biopsy.
  • Imaging for staging:
    • CT (Chest/Abdomen/Pelvis) for TNM staging.
    • MRI Pelvis: Essential for rectal cancer local staging (esp. mesorectal fascia).
    • Endorectal Ultrasound (ERUS): T-staging in early rectal cancers.

⭐ Carcinoembryonic Antigen (CEA) levels >5 ng/mL pre-operatively are associated with a worse prognosis in colorectal cancer patients and can indicate residual disease if elevated post-operatively.

Staging & Management - Battle Blueprint

• Staging Workup:
  • Colonoscopy + Biopsy (diagnostic)
  • CT Chest/Abdomen/Pelvis (metastasis screen)
  • MRI Pelvis (essential for rectal cancer: T-stage, N-stage, Circumferential Resection Margin - CRM)
  • CEA (baseline & prognostic/monitoring marker)
• TNM Staging (AJCC): Core to treatment planning.
• Colon Cancer Management:
  • Primary Treatment: Segmental colectomy + adequate lymphadenectomy (aim for ≥12 nodes).
  • Adjuvant Chemotherapy: For all Stage III and high-risk Stage II (e.g., T4, perforation, poor differentiation).
• Rectal Cancer Management:
  • Early (e.g., cT1N0 low-risk): Local excision (TEMS/TAMIS) or Total Mesorectal Excision (TME).
  • Locally Advanced (cT3/T4 or N+): Neoadjuvant Chemoradiotherapy (NCRT) → TME → Adjuvant Chemotherapy.
  • Very low tumors may require Abdominoperineal Resection (APR).
• Metastatic Disease (Stage IV):
  • Palliative chemotherapy +/- targeted agents (based on KRAS, NRAS, BRAF, MSI status).
  • Surgery for resectable metastases (liver, lung) in select cases.

⭐ For rectal cancer, achieving a clear Circumferential Resection Margin (CRM > 1mm) on pathology post-TME is a critical determinant of local recurrence and survival; pre-operative MRI helps predict CRM involvement.

High‑Yield Points - ⚡ Biggest Takeaways

• FAP (APC gene) & Lynch syndrome (MMR genes) are major hereditary risks.
• Right-sided lesions: anemia, occult blood. Left-sided: altered bowel habits, obstruction.
• Colonoscopy is gold standard for screening & diagnosis.
• CEA is for prognosis & recurrence monitoring, not screening.
• Liver is the most common site for distant metastasis.
• Rectosigmoid junction is the most frequent location.
• Surgical resection is the primary curative treatment for localized disease.