Metal nanoparticles with customized shapes, sizes, and surface modification have demonstrated tremendous potential for cancer therapy. It is well recognized that physical radiation dose deposited within tumors can be enhanced by passive accumulation of gold nanoparticles in tumors due to the remarkable increase in the fluence of photoelectrons when photons interact with high atomic number elements such as gold. However, the requirements of less clinically relevant radiation quality (low energy kilovolt x-rays) and clinically unachievable (without direct injection) gold concentration reduce enthusiasm for this approach. This proposal seeks to surmount these challenges by achieving a more tumor cell-specific concentration of gold nanoparticles within tumors by adopting an active targeting strategy using gold nanorods (GNRs) and providing photon source options optimized for specific clinical scenarios under active targeting. In preliminary data, we demonstrate that an active targeting strategy results in remarkable in vivo radiosensitization despite a much lower concentration of GNRs within tumors than the concentration previously believed to be necessary for radiosensitization following passive accumulation of gold nanoparticles in tumors. Active targeting also leads to radiosensitization in vitro, reduced repair of radiation-induced DNA double-strand breaks, and overcomes the inherent treatment resistance of tumors to traditional targeted therapies. Our central hypothesis is that active targeting significantly improves the efficiency of GNR-mediated radiosensitization by directly modulating tumor radiation response as a result of substantial microscopic dose enhancement in the vicinity of GNRs that reside in close proximity to tumor cells and vascular endothelial cells in vivo. A corollary hypothesis is that the GNRs serve as vectors for optimal delivery of the targeting moiety to an otherwise resistant tumor, a feature that can be further exploited for therapeutic payload delivery. Critical unanswered questions relate to the molecular mechanism of radiosensitization, biodistribution and kinetics of GNRs, and their fate at the whole animal, tumor and cellular levels. We will test our hypotheses and provide answers to the questions posed above by pursuing three Specific Aims: (a) to determine the molecular mechanism of GNR-mediated radiosensitization in vitro and in vivo, (b) to quantify the radiation dose enhancement by GNRs on a nano-/cellular-scale for different clinical irradiation scenarios, and (c) to determine the intratumoral concentration of GNRs. We anticipate that this proposal will lead to development of a comprehensive physically, biologically and clinically characterized radiation response modulation strategy that can be widely applied as a class solution across multiple tumor types, laying the foundation for more effective clinical radiotherapy with less toxicity.