Pathology from CT-guided biopsy: leiomyosarcoma. 

Primary inferior vena cava leiomyosarcoma is a rare malignant mesenchymal tumor which begins in the vessel wall and can have both extraluminal and intraluminal components. The largest portion of the mass is outside of the vessel wall, with tumor thrombus filling and nearly occluding the inferior vena cava.  

While this is a rare diagnosis, in this case, you can be fairly certain that you are dealing with a primary neoplasm of the inferior vena cava because that is the only structure into which the mass invades. It contacts the liver, right kidney, duodenum, and pancreas, but does not invade into these structures. 

The major differential item to rule out is adrenocortical carcinoma, which frequently invades into the inferior vena cava (the right adrenal is normal). 

See companion case with many similarities.