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Colorectal Cancer: Diagnosis & Staging

Clinical Presentation & Initial Diagnosis

• Right-sided tumors: Tend to be occult — vague abdominal discomfort, unexplained iron-deficiency anemia, fatigue, or weight loss. The wider caliber of the right colon makes obstruction less common.
• Left-sided tumors: More likely to cause obstructive symptoms — change in bowel habit, decreased stool caliber ("pencil stools"), and bright rectal bleeding.
• Rectal tumors: Hematochezia, tenesmus, urgency, and pain. Many patients — particularly with right-sided tumors — remain asymptomatic and are detected only on screening.

• Colonoscopy with biopsy — the gold standard for diagnosis and histological confirmation
• CEA (carcinoembryonic antigen) — elevated (>5.0 ng/mL) is a poor prognostic marker; also used for monitoring recurrence
• Complete blood count — to detect anemia
• For rectal tumors: digital rectal exam (DRE) and rigid/flexible proctoscopy to assess size, location, morphology, and fixation

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Staging Overview

TNM Components

Stage Groups & 5-Year Survival

Key AJCC 8th edition change: T1–T2 N2 was downstaged from IIIC → IIIA/IIIB; T4b N1 was moved from IIIB → IIIC, reflecting more nuanced survival data. — Mulholland & Greenfield's Surgery, 7e

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Preoperative Staging Workup (per NCCN Guidelines)

All CRC

• CT chest/abdomen/pelvis with contrast — standard first-line imaging to detect distant metastasis
• Colonoscopy — to exclude synchronous tumors (present in ~5% of patients)
• Serum CEA

Rectal Cancer (Additional)

• Pelvic MRI — primary tool for local staging; best assessment of tumor depth (T stage), nodal involvement, and circumferential radial margin (CRM)
  • CRM involvement (≤1–2 mm clearance) = very high local recurrence risk
• Endorectal ultrasound (EUS) — excellent for early T-stage (T1–T2) discrimination; useful for identifying submucosal invasion
• Rigid proctoscopy — for precise distance from anal verge

PET-CT (Selective Use)

• Not indicated for routine initial staging of resectable (stages I–III) CRC or pedunculated/sessile polyps with invasive cancer
• Consider when CT/MRI is equivocal, to guide biopsy, or in suspected metastatic disease
• Limitations: mucinous tumors (~17% of CRCs) have no FDG uptake; sensitivity limited for lymph nodes <10 mm and micrometastases
• Improves N-staging accuracy from 48% to 66% vs. CT alone; can alter management in ~26% of rectal cancer cases — Current Surgical Therapy, 14e

Liver Metastasis

• Liver MRI has greater accuracy than CT for hepatic metastases and may be used as first-line in equivocal cases — Yamada's Gastroenterology, 7e

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Pathologic Staging Nuances

Lymph Node Assessment

• A minimum of 12 lymph nodes must be examined for adequate pathologic staging
• The total number of nodes examined independently predicts survival — more nodes examined = better outcome across all T/N stages
• Lymph node ratio (positive nodes ÷ total nodes retrieved) is an independent survival predictor but not yet in formal staging criteria
• Peritumoral tumor deposits in the mesocolon/mesorectum (not associated with lymph node tissue) are classified as N1c

Tumor Regression Grading (post-neoadjuvant, AJCC 8th ed.)

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Key Prognostic Factors Beyond Stage

• Advanced TNM stage
• Poorly differentiated histology
• MSI-low (MSI-L) status — paradoxically, MSI-H is associated with better prognosis and predicts immunotherapy response
• KRAS and BRAF mutations — especially BRAF V600E (poor prognosis, identifies serrated pathway CRC)
• CEA >5.0 ng/mL at presentation
• Bowel obstruction or perforation at presentation
• Mucinous or signet-ring cell histology
• Vascular/perineural invasion
• Right-sided vs. left-sided — right-sided tumors more often MSI-H, female, older, advanced grade at diagnosis; different molecular profile affects response to therapy
• CMS4 (consensus molecular subtype 4) — mesenchymal, worst prognosis

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Molecular Diagnostics

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Historical Reference: Dukes Classification (Rarely Used Now)

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