Primary retroperitoneal NB tumors frequently infiltrate behind the aorta and IVC (inferior vena cava) displacing these vessels anteriorly. In addition, tumors commonly encase the aorta, IVC, renal arteries, renal veins, celiac axis, and superior mesenteric artery. When compared with CT, MR appears superior in evaluating vascular displacement and encasement (Tanabe et al. 1993a, b). Measurement of tumor volume as a quantitative response to therapy has become important study endpoints, for prognosis and follow-up. In some therapeutic protocols, the percentage of tumor shrinkage as determined by CT scan or MR imaging is critical to the continuation or abandonment of specific therapies (Wheatley et al. 1995).
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