A 72-year-old woman with a history of colorectal cancer presents with acute abdominal pain, distension, and constipation. CT abdomen shows dilated colon with a transition zone at the sigmoid region. Which imaging feature best distinguishes large bowel obstruction due to a malignant stricture from obstruction due to diverticulitis?

Explanation

## Distinguishing Malignant Stricture from Diverticulitis-Related Obstruction ### Key Radiological Features **Key Point:** Shouldering (also called "shelf-like" or "apple-core" appearance) at the transition zone is the hallmark of malignant stricture and best distinguishes it from diverticulitis-related obstruction. ### Comparison Table | Feature | Malignant Stricture | Diverticulitis-Related Obstruction | | --- | --- | --- | | **Transition zone appearance** | Shouldering, shelf-like, abrupt shelf edges | Tapered, gradual transition | | **Wall appearance** | Shouldering with asymmetric narrowing | Concentric narrowing | | **Pericolonic findings** | Minimal fat stranding (tumor confined) | Marked fat stranding, inflammation | | **Associated inflammation** | Absent or minimal | Prominent pericolonic inflammation | | **Length of narrowing** | Short segment (2–5 cm) | Variable, often longer | | **Haustra** | Loss in narrowed segment | Loss in narrowed segment | ### Pathophysiology 1. **Malignant stricture:** Tumor infiltrates the bowel wall asymmetrically, creating an abrupt, shouldered transition with shelf-like edges. The tumor is usually confined to the bowel wall with minimal surrounding inflammation. 2. **Diverticulitis-related obstruction:** Inflammation from diverticulitis causes concentric, tapered narrowing with prominent pericolonic fat stranding and inflammation extending well beyond the bowel wall. **High-Yield:** The shouldering sign (asymmetric shelf-like edges at the transition zone) is highly specific for malignancy and is a key discriminator on CT. Diverticulitis produces a more gradual, tapered narrowing with extensive surrounding inflammation. **Clinical Pearl:** Malignant strictures are typically shorter (2–5 cm) and located at the rectosigmoid junction, whereas diverticulitis-related obstruction often shows longer segments of involvement with prominent pericolonic changes. ### Why Other Features Are Less Discriminating - **Pericolonic fat stranding:** While more prominent in diverticulitis, it can also occur with perforated or complicated malignancy. - **Haustra loss:** Occurs in both conditions and is not discriminating. - **Air in small bowel:** Non-specific sign of any proximal obstruction.