A 68-year-old woman with diverticular disease presents with acute left lower quadrant pain and fever. CT abdomen shows pneumoperitoneum with gas predominantly in the paracolic gutter and pelvis. Which imaging feature best distinguishes perforation of a sigmoid colon diverticulum from perforation of a sigmoid colon carcinoma?

Explanation

## Distinguishing Diverticular Perforation from Carcinoma Perforation ### Pathophysiological and Imaging Distinctions **Key Point:** Diverticular perforation presents with **localized perforation site, visible diverticular disease, and minimal or normal wall thickness**, whereas carcinoma perforation shows **irregular wall thickening, shouldering, and mass effect** at the perforation site. ### Structural Features on CT | Feature | Diverticular Perforation | Carcinoma Perforation | |---------|--------------------------|----------------------| | **Wall thickness at perforation** | Normal to minimally thickened | Markedly thickened (>5 mm) | | **Visible diverticula** | Multiple diverticula present | Absent; mass replaces normal wall | | **Shouldering** | Absent | Present (abrupt transition) | | **Luminal narrowing** | Absent or minimal | Significant ("apple core" or "napkin ring") | | **Perforation localization** | At diverticular neck/dome | Within tumor mass | | **Associated findings** | Diverticulosis elsewhere | Lymphadenopathy, liver metastases | ### High-Yield Imaging Recognition **High-Yield:** The **presence of diverticular disease with a localized perforation site and absence of significant wall thickening** is the single best discriminator. Carcinoma perforation occurs through a thickened, irregular tumor mass. **Clinical Pearl:** Diverticular perforation typically occurs at the antimesenteric border of the colon (where the taeniae coli are absent), while carcinoma perforation occurs through the tumor itself, often with associated stricturing. **Mnemonic: DIVERT** — **D**iverticula visible, **I**ncidental finding, **V**ery localized, **E**arly wall (normal thickness), **R**arely shouldered, **T**ypically at dome. ### Diagnostic Algorithm ```mermaid flowchart TD A[Sigmoid perforation on CT]:::outcome --> B{Diverticula visible?}:::decision B -->|Yes + normal wall| C[Diverticular perforation]:::outcome B -->|No + thick wall| D{Shouldering present?}:::decision D -->|Yes| E[Carcinoma perforation]:::outcome D -->|No| F[Other cause]:::outcome B -->|Yes + thick wall| G[Diverticulitis with carcinoma]:::urgent ``` **Tip:** Always assess the wall thickness at the perforation site first — this single measurement often resolves the differential diagnosis.