A 68-year-old woman with a history of colorectal cancer presents with progressive abdominal distension, constipation, and colicky pain. Plain abdominal X-ray shows dilated colon with a transition zone at the sigmoid region and a 'bird's beak' appearance. Which imaging modality is most appropriate to confirm the diagnosis and assess for metastatic disease?

## Investigation of Choice for Large Bowel Obstruction with Suspected Malignancy **Key Point:** CT abdomen and pelvis with IV contrast is the investigation of choice for large bowel obstruction — it confirms the diagnosis, identifies the cause (malignancy, stricture, volvulus), and simultaneously assesses for metastatic disease and complications. ### Why CT is Optimal in This Case | Feature | CT with IV Contrast | Barium Enema | Colonoscopy | MRI without Contrast | |---------|-------------------|--------------|-------------|---------------------| | **Confirms obstruction** | Yes (100%) | Yes | No (cannot pass obstruction) | Yes | | **Identifies cause** | Excellent (mass, stricture) | Good | Cannot assess proximal to obstruction | Moderate | | **Staging malignancy** | Yes (T, N, M staging) | No | No | Limited (no IV contrast) | | **Detects metastases** | Yes (liver, peritoneum, distant) | No | No | No | | **Assesses complications** | Yes (perforation, ischemia) | No | Risk of perforation | Limited | | **Speed** | Rapid (< 5 min) | Slow (30–45 min) | Variable | Moderate | | **Safety in obstruction** | Safe | Relatively safe | Risk of perforation | Safe | **High-Yield:** The 'bird's beak' appearance on plain X-ray is pathognomonic for **sigmoid volvulus or malignant stricture**. In a patient with colorectal cancer history, CT is essential to confirm malignant obstruction and stage metastatic disease — this guides surgical vs. palliative management. ### Clinical Pearl **The 'bird's beak' sign** represents an abrupt transition from dilated proximal colon to a narrowed segment, seen in: - Sigmoid volvulus (most common) - Malignant stricture - Diverticular stricture CT can differentiate these causes and assess for complications. ### Staging Information from CT in Colorectal Obstruction 1. **Tumor extent** — local invasion, depth (T stage) 2. **Lymph node involvement** — regional lymph nodes (N stage) 3. **Metastatic disease** — liver metastases, peritoneal carcinomatosis, distant nodes (M stage) 4. **Obstruction level** — proximal vs. distal to splenic flexure (affects surgical approach) 5. **Complications** — perforation, ischemia, fecal peritonitis **Mnemonic: COLT for CT findings in large bowel obstruction** - **C**ause identified (mass, volvulus, stricture) - **O**bstruction confirmed (transition zone) - **L**ocal staging (T stage, invasion) - **T**umor spread (metastases, N stage, M stage) ### Why Other Options Are Suboptimal **Barium enema:** While it can show the 'bird's beak' and obstruction, it does NOT stage the malignancy, detect metastases, or assess for ischemia. It is also contraindicated if perforation is suspected. **Colonoscopy:** Cannot pass a complete obstruction and risks perforation. Not diagnostic for the cause of obstruction in this setting. **MRI without contrast:** Cannot assess enhancement patterns needed to evaluate tumor vascularity, metastases, or complications. IV contrast is essential for staging. [cite:Harrison 21e Ch 297]