Abstract. Background: Advances in cancer therapeutics depend on reliable in vivo model systems. To develop biologically relevant xenografts, ultrasound was utilized for tissue-directed implantation of neuroblastoma (NB) cell line and patient-derived tumors in the adrenal gland, and for renal subcapsular engraftment of Ewing's sarcoma (ES). Materials and Methods: NB xenografts were established by direct adrenal injection of luciferase-transfected NB cell lines (IMR32, SH-SY5Y, SK-N-BE2) or NB patient-derived tumor cells (UMNBL001, UMNBL002In order to better understand tumor biology and to examine new therapeutic approaches, cancer researchers often utilize animal xenograft models for preclinical drug screening. Compared to cell-based in vitro studies, animal models more accurately replicate three-dimensional tumors and minimize utilization of artificial biological factors in culture. Early models relied on flank injection of cancer cells with subcutaneous heterotopic tumor engraftment, allowing for rapid assessment of growth and response to therapeutics. While these models have been broadly utilized in drug discovery because of their relatively low costs and ease of use, they are often inconsistent in later stages of clinical development and rarely demonstrate important disease characteristics such as the capacity to metastasize. The etiology of this discrepancy is multifactorial, but likely largely due to complexities in the interactions between tumors and the host microenvironment (1). In contrast, orthotopic cancer models have been found to closely replicate native tumor histopathology and enhance metastatic potential (2). It has now been established that engraftment of human cancer cells into the appropriate anatomical sites maintains original biological features of cancer growth with metastasis, and provides more accurate models than subcutaneous xenografts for drug discovery (3, 4). Orthotopic models have been further enhanced by implantation of fresh patient-derived cells, rather than established cell line-based xenografts. There is strong evidence that cell lines maintained in serum-containing growth media harbor greater genetic divergence from the primary tumor than direct xenografts, which may have functional implications in preclinical work (5). This provides evidence that development of reliable patient-derived orthotopic xenografts is an important improvement over standard models for investigation of new therapies and diagnostic imaging modalities.There is a particularly strong need to develop improved models for preclinical studies of neuroblastoma (NB), the 779 This article is freely accessible online.