Sigmoid Colon Cancer - Complete Clinical Guide

1. ANATOMY AND BACKGROUND

2. ETIOLOGY AND RISK FACTORS

Modifiable / Environmental Risk Factors

• Diet: High saturated/polyunsaturated fat diet increases risk; high fiber and oleic acid (olive oil, fish oil) diet is protective. Increased red meat and decreased fiber specifically linked.
• Obesity and sedentary lifestyle: Dramatically increase cancer-related mortality
• Alcohol consumption: Associated with increased CRC incidence
• Smoking: Associated with increased risk of colonic adenomas, especially after >35 years of use
• Type 2 diabetes / insulin resistance
• Pelvic irradiation: May increase risk, particularly for rectal/sigmoid cancers

Non-Modifiable Risk Factors

• Age >50 (incidence rises sharply; screening starts at 45 per current guidelines)
• Personal history of polyps or CRC
• Family history of CRC
• Inflammatory bowel disease (IBD): Long-standing colitis (ulcerative colitis or Crohn's) increases risk - duration and extent of colitis correlate with risk; presence of primary sclerosing cholangitis further increases risk
• Acromegaly: Elevated IGF-1 increases risk
• Ureterosigmoidostomy: Increases risk of adenoma and carcinoma at the junction

Hereditary Syndromes

• FAP accounts for ~1% of all CRC but has provided the most insight into CRC molecular biology.
• Lynch syndrome accounts for ~5% of all CRC.

3. PATHOPHYSIOLOGY - MOLECULAR MECHANISMS

Pathway 1: Chromosomal Instability (CIN) - 65-70% of CRCs

• Step 1: Mutation of APC gene (chr 5q) - activates WNT/β-catenin signaling → dysplastic epithelium → early adenoma. Present in 80% of CIN tumors.
• Step 2: KRAS mutation - promotes growth and extension of the adenoma
• Step 3: Loss of DCC/SMAD4 (chr 18q)
• Step 4: TP53 mutation - promotes invasion into surrounding tissue
• Result: Late adenoma → carcinoma → metastasis

Pathway 2: Serrated / Methylator (CpG Island Methylator Phenotype - CIMP) - 15-20%

• Initiating event: BRAF mutation
• Subsequent hypermethylation of promoter regions of tumor suppressor genes and DNA repair genes → silenced gene expression
• Methylation of CG-rich areas (CpG islands) in the 5' region of genes
• More common in older patients, females, and the right colon (though serrated lesions can occur in sigmoid too)
• Can be CIMP+ MSS (~20%) or CIMP+ MSI-H (~13%)

Pathway 3: Microsatellite Instability (MSI/Mutator) - 12-15%

• Loss of mismatch repair (MMR) function via:
  1. Germline pathogenic variant in MLH1, MSH2, MSH6, or PMS2 (Lynch syndrome)
  2. Hypermethylation of the MLH1 promoter (sporadic)
  3. Double somatic mutation in an MMR gene
• Loss of MMR leads to accumulation of DNA base-pair mismatches in microsatellite repeats → MSI-H
• Pathologic features: Crohn-like reaction, mucin, lymphocytic infiltration, high tumor mutational burden (TMB)
• MSI-H tumors actually have better prognosis (~15% improvement in survival vs MSS)

CRC Molecular Subtypes (Consensus Molecular Subtypes - CMS)

Microbiome Contribution

4. PATHOLOGY

Macroscopic Types

• Polypoid/fungating (most common on the right, but occurs anywhere)
• Annular/constricting ("apple-core" lesion) - more common in sigmoid and left colon, causes obstruction
• Ulcerating
• Diffuse infiltrating (linitis plastica type)

Microscopic / Histological Types

• Adenocarcinoma - the vast majority (~95%)
• Mucinous adenocarcinoma - >50% extracellular mucin; worse prognosis
• Signet ring cell carcinoma - >50% signet ring cells; very poor prognosis (5-year survival 9-36%)
• Medullary carcinoma - associated with MSI-H; better prognosis
• Poorly differentiated carcinoma

Spread

• Direct: Through the bowel wall layers (mucosa → submucosa → muscularis propria → serosa/peritoneum → adjacent organs)
• Lymphatic: To pericolic, intermediate, and apical nodes along the sigmoid arteries and IMA
• Hematogenous: Portal vein → liver (most common distant site); also lungs, adrenals, ovaries, bone
• Peritoneal: Transcoelomic spread → peritoneal carcinomatosis
• Implantation: At anastomosis (rare if adequate margins)

5. CLINICAL PRESENTATION

• Change in bowel habits: Alternating constipation and diarrhea, "pencil-thin stools" (classic for sigmoid obstruction)
• Rectal bleeding / hematochezia: Fresh or dark red blood mixed with stool
• Abdominal pain: Left iliac fossa or diffuse
• Microcytic (iron-deficiency) anemia: From chronic occult blood loss - may present as fatigue
• Obstructive symptoms: Increasing distension, obstipation, vomiting (in late/emergency cases)
• Perforation: Peritonitis, fever, acute abdomen (rare, emergency presentation)
• Palpable mass: Left iliac fossa in advanced disease
• Systemic symptoms: Weight loss, anorexia, cachexia in advanced disease

Many sigmoid cancers are identified on screening colonoscopy before symptoms develop.

6. INVESTIGATIONS

Blood Tests

• Complete Blood Count (CBC): Microcytic hypochromic anemia (iron deficiency from occult bleeding)
• Comprehensive Metabolic Panel: Liver function tests (for metastases), albumin (nutritional status)
• Serum CEA (Carcinoembryonic Antigen): Tumor marker. Pre-op CEA >5 ng/mL predicts worse disease-free survival. Used for monitoring recurrence post-resection, not for screening.
• CA 19-9: Less specific, used as adjunct
• Coagulation profile, Group & Save: Pre-operative
• LFTs, ALP: For liver metastases

Stool-Based Tests (for screening / initial detection)

• Fecal Occult Blood Test (FOBT) - annual
• Fecal Immunochemical Test (FIT) - annual
• Stool DNA-FIT (Cologuard) - every 1-3 years
• Any positive stool test → colonoscopy

Endoscopy

• Colonoscopy with biopsy: Gold standard for diagnosis. Visualizes the lesion, allows biopsy for histological confirmation, and detects synchronous polyps or cancers.
• CT Colonography (Virtual Colonoscopy): Every 5 years as a screening tool; useful if colonoscopy is incomplete
• Flexible Sigmoidoscopy: Every 5 years; detects sigmoid lesions specifically but misses proximal lesions

Imaging for Staging

• CT Chest/Abdomen/Pelvis with contrast: Primary staging modality. Assesses:
  • T stage (local tumor extent, peritoneal involvement)
  • N stage (regional lymphadenopathy)
  • M stage (liver, lung, peritoneal metastases)
• MRI Abdomen/Pelvis: Better soft tissue resolution; used for liver metastasis characterization; essential for rectal cancer (mesorectal margin assessment), less routine for sigmoid
• PET-CT: Not routinely indicated; reserved for patients being considered for definitive resection of oligometastatic disease (e.g., isolated liver or lung metastasis)
• Chest X-ray: Baseline lung assessment
• Transrectal Ultrasound: Not applicable to sigmoid (used for rectal cancer staging)

Pathological / Molecular Testing (Biopsy)

• Histology and grade: Adenocarcinoma type, differentiation
• MMR status (IHC for MLH1, MSH2, MSH6, PMS2) or MSI testing: Guides immunotherapy eligibility and prognosis; Lynch syndrome screening
• KRAS/NRAS mutation testing: Predicts resistance to anti-EGFR therapy (cetuximab, panitumumab)
• BRAF V600E mutation: Poor prognosis in metastatic CRC; specific therapy targets available
• Tumor Mutation Burden (TMB): High TMB predicts response to immune checkpoint inhibitors (ICIs)
• NTRK fusion testing: Rare (<1%), usually MSI-H, specific TKI therapy available

7. STAGING

TNM Staging (AJCC 8th Edition)

Overall Stage Groups

8. TREATMENT

A. Surgery (Primary Treatment for Localized Disease)

• Extent: Segmental sigmoid resection following oncologic principles. The distal transection is at the upper rectum; proximal resection is at the distal descending colon.
• Vascular ligation: The IMA (or sigmoid arteries) are ligated at their origin (high ligation) to ensure harvest of the regional lymph nodes en bloc. A recent meta-analysis (2025) confirms the value of IMA ligation level in sigmoid/rectal cancer.
• Anastomosis: End-to-end colorectal anastomosis, typically with a double-stapled technique.
• Lymphadenectomy: Minimum 12 lymph nodes must be harvested for adequate staging.

• Laparoscopic / Robotic (preferred): Less postoperative pain, shorter hospital stay, faster return of bowel function, lower wound infection rate - with equivalent oncologic outcomes to open surgery (proven by COST, COLOR, CLASSICC trials).
• Open (laparotomy): When minimally invasive not feasible.

• Urgent surgery is common
• Segmental resection of the primary tumor
• If proximal bowel is perforated/ischemic: subtotal colectomy
• Hartmann's procedure (resection + end colostomy + closure of rectal stump): Historically preferred; now primary anastomosis may be done in hemodynamically stable patients
• Endoscopic stenting as bridge to surgery: Relieves obstruction, allows elective surgery with higher rates of primary anastomosis and laparoscopic completion; contraindicated if ischemia or perforation suspected

• Symptomatic primary: Address primary first (palliation)
• Asymptomatic primary: Systemic treatment first
• Resectable liver metastasis: Combined or staged resection (neoadjuvant chemotherapy improves progression-free survival)
• Resectable lung metastasis: Surgical resection improves survival
• Peritoneal metastasis (selected): Cytoreductive surgery + HIPEC (hyperthermic intraperitoneal chemotherapy) after multidisciplinary discussion

B. Chemotherapy

• Stage I: No adjuvant chemo needed
• Stage II (low risk): Observation; chemotherapy generally not beneficial, especially in patients >70
• Stage II (high risk): Consider adjuvant chemo. High-risk features: poorly differentiated histology, LVI, bowel obstruction, <12 nodes sampled, localized perforation, positive/close margins, T4, high tumor budding
• Stage III: Adjuvant chemotherapy recommended

• CAPEOX (capecitabine + oxaliplatin): 3 months sufficient for low-risk stage III (T1-3 N1); 6 months for high-risk
• FOLFOX (leucovorin + 5-FU + oxaliplatin): 6 months for high-risk stage III (T4/N2); 3 months inferior to 6 months in this group
• Addition of oxaliplatin does NOT improve survival in stage II or patients >70

• First-line: Fluoropyrimidine (5-FU/leucovorin or capecitabine) + oxaliplatin (FOLFOX/CAPEOX) or irinotecan (FOLFIRI)
• Biological agents added based on molecular profile:
  • Bevacizumab (anti-VEGF): Combined with FOLFOX or FOLFIRI in RAS-mutant CRC; objective response ~70%
  • Cetuximab / Panitumumab (anti-EGFR): Only in RAS/NRAS/KRAS wild-type (no mutation) tumors; ineffective with KRAS or NRAS mutations
  • BRAF V600E inhibitors (e.g., encorafenib + cetuximab): For BRAF-mutant metastatic CRC
  • Pembrolizumab / Nivolumab (anti-PD-1): For MSI-H/dMMR metastatic CRC - pembrolizumab approved first-line
  • TRK inhibitors (larotrectinib, entrectinib): For NTRK fusion-positive CRC (rare)

C. Radiation Therapy

• Sigmoid colon cancer: Radiation is NOT routinely used for colon cancer (unlike rectal cancer) due to the mobile nature of the colon and radiosensitivity of adjacent bowel.
• Exception: Locally advanced/fixed sigmoid cancers invading adjacent structures (T4b) may receive preoperative chemoradiotherapy.
• Recurrence: In selected recurrent cases, intraoperative radiation therapy (IORT/brachytherapy) may be added.

D. Endoscopic/Local Treatment

• Endoscopic mucosal resection (EMR) / Endoscopic submucosal dissection (ESD): For T1 lesions (confined to submucosa) without high-risk features (poor differentiation, LVI, deep submucosal invasion). Curative in select early-stage cases.
• Palliative colonic stenting: For malignant obstruction in patients unfit for surgery or as bridge to surgery.

9. PROGNOSTIC FACTORS

• Serum CEA >5 ng/mL (pre- and post-op): Worse disease-free survival
• Lymphovascular invasion (LVI): Worse DFS and OS in stage II/III
• Perineural invasion (PNI): Independent poor prognostic factor
• Tumor budding (single cells/small clusters in stroma): Intermediate/high budding → worse OS and DFS
• Mucinous histology: Worse progression-free and overall survival
• Signet ring cell histology: Very poor prognosis (5-year survival 9-36%)
• BRAF V600E mutation: Poor prognosis in metastatic CRC
• KRAS mutation: Worse prognosis; resistance to anti-EGFR agents
• MSI-H/dMMR status: Better prognosis (15% improvement in survival vs MSS); excellent immunotherapy response
• Number of lymph nodes sampled: <12 nodes = inadequate staging; associated with worse outcomes
• Surgical margin status: Positive margins = high recurrence risk

10. SURVEILLANCE AFTER CURATIVE RESECTION

• Physical examination + CBC + LFTs + CEA: Every 3 months for first 2 years, then every 6 months to 5 years
• CEA every 3 months: Sensitive marker for detecting otherwise undetectable recurrence
• Colonoscopy: 1 year post-resection, then every 3 years (patients have 3-5% risk of second CRC and >15% risk of adenoma)
• CT chest/abdomen/pelvis: Semi-annually to annually for first 3 postoperative years
• Circulating tumor DNA (ctDNA): Under investigation as biomarker for residual/recurrent disease
• Most recurrences occur within first 2 years after surgery

11. SCREENING (for at-risk individuals)

• Colonoscopy every 10 years
• FIT every 1 year
• Stool DNA-FIT every 1-3 years
• CT colonography every 5 years
• Flexible sigmoidoscopy every 5 years ± annual FIT
• Any positive non-invasive test → diagnostic colonoscopy

12. SPECIAL SITUATIONS

Obstructing Sigmoid Cancer (Surgical Emergency)

1. Emergency segmental resection ± primary anastomosis (if stable)
2. Hartmann's procedure (resection + colostomy)
3. Endoscopic stent as bridge to elective surgery (preferred where available)

Perforated Sigmoid Cancer

Recurrent CRC

• Sabiston Textbook of Surgery, 21st Ed. - Colorectal Cancer Genetics, Surgical Anatomy, Adjuvant Therapy sections
• Schwartz's Principles of Surgery, 11th Ed. - TNM staging, Inherited CRC, Recurrence treatment
• Harrison's Principles of Internal Medicine, 22nd Ed. - Localized colon cancer management, surveillance
• The Washington Manual of Medical Therapeutics - Epidemiology, staging, treatment overview
• Bailey and Love's Short Practice of Surgery, 28th Ed. - Molecular analysis in CRC
• Fischer's Mastery of Surgery, 8th Ed. - Etiology and pathophysiology
• Meta-analysis on IMA ligation level in sigmoid/rectal cancer surgery (BMC Cancer, 2025) - PMID 41063015