Maximum xenograft size depends upon tumor passage, immunodeficient mouse type, and tumor implantation site. A. Comparison of tumor passages (P0 vs. P1-2). B. Comparison of NSG and NS immunodeficient mice. C. Subgroup comparison of passages plus NSG or NS immunodeficient mice. D. Comparison of tumor implantation sites. E. Comparison of tumor implantation sites in P1-2/NS mice. F. Comparison of gross tumor size at necropsy with ultrasound findings obtained within 2 weeks of death.