A 68-year-old woman with chronic constipation presents with acute left lower quadrant pain. CT abdomen shows free air in the peritoneal cavity. Which imaging feature most reliably distinguishes perforated sigmoid diverticulitis from perforated sigmoid colon carcinoma?

## Distinguishing Perforated Diverticulitis from Perforated Colon Carcinoma ### Critical Discriminating Feature **Key Point:** The presence of **visible diverticula** in the colon wall is the most specific imaging finding that distinguishes perforated diverticulitis from perforated colon carcinoma. ### Comparative Imaging Features | Feature | Perforated Diverticulitis | Perforated Colon Carcinoma | |---------|--------------------------|---------------------------| | **Colon wall appearance** | Normal or mildly thickened; **diverticula visible** | Focal mass, shouldering, irregular margins, stricture | | **Pericolic fat stranding** | Present (inflammatory) | Present (malignancy-related) | | **Abscess formation** | Common, loculated | Variable; may be infiltrative | | **Distribution of disease** | Segmental (sigmoid), can be multifocal | Focal lesion at perforation site | | **Proximal colon changes** | Normal or mild dilatation | Proximal obstruction, dilatation | ### Why Diverticula Are the Discriminator 1. **Perforated diverticulitis**: The presence of **colonic diverticula** on CT is pathognomonic. Diverticula are outpouchings of the colon wall that are visible as small rounded projections. When inflamed, they show pericolic fat stranding. The perforation occurs through a diverticulum. 2. **Perforated colon carcinoma**: A **focal mass lesion** with shouldering, irregular margins, and stricturing is the hallmark. Diverticula are NOT present; instead, there is an infiltrative tumor causing wall thickening and narrowing. **High-Yield:** **Visible diverticula + pericolic fat stranding = perforated diverticulitis**. **Focal mass + shouldering + stricture = perforated carcinoma**. ### Clinical Pearl In perforated colon carcinoma, you will often see **proximal bowel obstruction and dilatation** (upstream of the tumor), whereas in diverticulitis, the proximal colon is usually normal or mildly dilated. This proximal obstruction is a secondary finding that supports the carcinoma diagnosis. **Mnemonic:** **DIVERT** = **D**iverticula visible, **I**nflammatory stranding, **V**ariable location, **E**xcentric thickening, **R**ecurrent episodes, **T**ypically in sigmoid. **Warning:** Do not confuse the pericolic fat stranding seen in both conditions—it is a non-specific sign of inflammation or malignancy. The key is the **morphology of the colon wall**: diverticula vs. mass.