Differential diagnosis is duodenal diverticulum - in this case refuted by the presence of gross adjacent gastric wall edema (inflammation) and the mural defect location, i.e. first part rather than the more typical second for diverticulum. Contained duodenal peptic perforations are more common posteriorly (as compared with pneumoperitoneum in anterior perforation) and typically present with pain radiating to the back (in this case prompting a CT angiogram for clinically supected aortic dissection).