## Investigation of Choice for Colorectal Cancer Diagnosis **Key Point:** Colonoscopy with biopsy is the gold standard for diagnosis of colorectal cancer. It allows direct visualization, tissue sampling, and histopathological confirmation — the only definitive diagnostic test. ### Role of Each Investigation | Investigation | Primary Role | Sensitivity | Timing | |---|---|---|---| | **Colonoscopy + biopsy** | Diagnosis & histology | 95–99% | First-line diagnostic | | **ERUS** | Local T-staging (T1–T4) | 85–90% | After diagnosis confirmed | | **CT abdomen/pelvis** | Metastatic staging (M-staging) | 70–80% | Staging after diagnosis | | **MRI pelvis** | Detailed local staging (T, N) | 90% | Rectal cancer T/N assessment | ### Diagnostic Algorithm ```mermaid flowchart TD A[Suspected colorectal cancer]:::outcome --> B[Colonoscopy + biopsy]:::action B --> C{Histology confirms malignancy?}:::decision C -->|Yes| D[Proceed to staging]:::action D --> E[CT chest/abdomen/pelvis for M-staging]:::action D --> F[ERUS or MRI for local T/N staging]:::action C -->|No| G[Alternative diagnosis]:::outcome ``` **High-Yield:** Colonoscopy is the only investigation that provides: 1. **Direct visualization** of the lesion 2. **Tissue diagnosis** (histology/grade) 3. **Assessment of synchronous lesions** (up to 5% have synchronous cancers) 4. **Therapeutic capability** (polypectomy if early lesion) **Clinical Pearl:** Once diagnosis is confirmed histologically, staging investigations (CT, ERUS, MRI) are ordered based on tumor location and need for neoadjuvant therapy planning. **Warning:** Do not order staging investigations (CT, ERUS, MRI) before histological confirmation — they are staging tools, not diagnostic tools. A negative biopsy does not exclude cancer; repeat colonoscopy or alternative imaging may be needed. [cite:Harrison 21e Ch 197]
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