Mass under sternum?

This patient is presenting with a 4 year h/o insidious in onset retrosternal noncardiac chest pain. Can anyone tell me what this is directly under this patient's sternum. Don't mind the air, he had just had a cholecystectomy. The air does however however help to demarcate the mass under the sternum which appears to be just outside the peritoneum. Looks the same on MRI and does not light up with contrast View attachment 223172
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums