Adenocarcinoma of the Colon

Definition

Adenocarcinoma of the colon is a malignant epithelial tumor arising from glandular epithelium, accounting for the majority of colorectal cancers.

Epidemiology and Risk Factors

Epidemiology:
- Most common malignancy of the GI tract.
- Peak incidence: 60–70 years.
- Higher incidence in developed countries.
Risk Factors:
- Dietary: Low intake of un-absorbable vegetables fibre, high intake of refined carbs and fat, and red/processed meat intake.
- Lifestyle: Smoking, alcohol consumption, and obesity.
- Genetics: Familial adenomatous polyposis (FAP), Lynch syndrome.
- Inflammatory Conditions: Long-standing ulcerative colitis or Crohn’s disease.
- Age: Incidence increases with age.
- Radiation exposure.

Symptoms

Early: Often asymptomatic.
Late:
- Altered bowel habits (diarrhea or constipation).
- Hematochezia or melena.
- Iron-deficiency anemia (common in right-sided tumors).
- Abdominal pain, obstruction, or bloating.
- Rectal tumors: Tenesmus, urgency, or bleeding.

Pathogenesis

Adenoma-Carcinoma Sequence (APC/β-Catenin Pathway):
- Accounts for 80% of sporadic colon tumors.
- Involves stepwise accumulation of multiple mutations .
- Key Steps:
  1. APC Mutation:
    - Germline (inherited) or somatic ( acquired) mutation of cancer suppressor genes (first hit)
    - Both copies of the APC gene must be functionally inactivated by mutation or epigenetic silencing.
    - APC is a key regulator of the WNT signaling pathway.
    - APC protein binds and promotes degradation of β-catenin.
    - Loss of APC function → Accumulation of β-catenin → β-catenin translocates to the nucleus → Activates transcription of oncogenes like MYC and cyclin D1, promoting proliferation.
  2. KRAS Mutation:
    - Activating mutations in KRAS → Increased cell proliferation and resistance to apoptosis.
  3. Mutations in SMAD2/SMAD4:
    - These encode factors in the TGF-β signaling pathway, which normally inhibits the cell cycle.
    - Loss of function → Uncontrolled growth.
  4. TP53 Mutation:
    - Found in 70–80% of colon cancers.
    - Typically occurs in late stages of tumor progression.
    - Loss of TP53 function often involves chromosomal deletions, reflecting chromosomal instability.
Microsatellite Instability (MSI) Pathway:
- Associated with defective DNA mismatch repair genes (e.g., MLH1, MSH2).
- Results in accumulation of microsatellite repeats.
- Common in Lynch syndrome (HNPCC) and 15% of sporadic cases.
- More common in right-sided tumors.

Microscopic and Gross Morphology

Gross Morphology:
- Proximal Colon:
  - Tumors grow as polypoid, exophytic masses that extend along one wall of the ascending or large-caliber colon.
  - Rarely cause obstruction.
- Distal Colon:
  - Tumors grow as annular, "napkin-ring" lesions.
  - Cause luminal narrowing and can lead to obstruction.
Microscopic Morphology:
- Tumors are composed of columnar cells resembling dysplastic epithelium found in adenomas.
- Invasive tumor component elicits a strong desmoplastic response, giving tumors a firm consistency.
Some variants:
- Poorly differentiated tumors:
  - May form few glands.
- Some produce abundant mucin (mucinous carcinoma) that accumulates within the intestinal wall.
- Signet-ring cell carcinoma: Tumors composed of signet-ring cells, similar to gastric carcinoma.
- Mucinous and signet-ring cell carcinomas are associated with poor prognosis.

Prognostic Factors

Stage: Most important determinant (TNM classification).
Histological Grade: Poorly differentiated tumors → Worse prognosis.
Lymphovascular Invasion: Indicates aggressive behavior.
MSI Status: Better prognosis in MSI-H tumors.
Molecular Profile: Chromosomal instability vs. MSI pathway involvement.

Complications

Obstruction: Common with left-sided tumors.
Perforation: Can lead to peritonitis.
Metastasis: Liver, lungs, and peritoneum are common sites.
Paraneoplastic Syndromes: Rare but possible.

High-Yield Differences: Left vs. Right-Sided Tumors

Feature	Left-Sided Tumors	Right-Sided Tumors
Location	Descending colon, sigmoid colon	Ascending colon, cecum
Pathogenesis	APC/β-Catenin Pathway	MSI Pathway
Symptoms	Obstruction, narrow stools, hematochezia	Anemia, fatigue, melena
Gross Appearance	Napkin-ring constrictions	Polypoid exophytic masses

High-Yield Exam Points

Key Mutations: APC → KRAS → SMAD2/4 → TP53.
APC Function: Regulates β-catenin in the WNT signaling pathway.
MSI Pathway: Loss of mismatch repair genes (e.g., MLH1, MSH2).
Right vs. Left Tumors: Differ in pathogenesis, presentation, and morphology.
Microscopy: Look for desmoplastic response, mucin production, and poorly differentiated cells.