There is a large amount of fat density appreciated along the left lateral chest. There is a diaphragmatic defect appreciated on the left side, and likely this represents a diaphragmatic herniation of omentum into left chest.
This has been seen on multiple prior CT studies. the oldest study available is from 12/2006 and the appearance is stable since that time.
Differentials include: pleural/extrapleural lipoma, diaphragmatic lipoma, dermoid tumor, and less likely liposarcoma considering its been stable since 2006.