Image IQ: 45-year old presenting with gradual abdominal swelling

Can you properly diagnose this 45-year old?

45-year-old presenting with gradual abdominal swelling.

Scanogram of the abdomen shows diffuse abdominal swelling with displacement of bowel loops displaced laterally. CT of the abdomen shows an ill-defined low density fat containing lesion occupying the entire abdomen and extending into the pelvis. Bowel loops are displaced and compressed anteriorly.

What is your diagnosis?