Radiation therapy for cancer has systemic consequences that are mediated through danger signaling in irradiated tissues and tumors and that engage the immune system. This proposal aims to understand how different dose-fractionation schedules in radiation therapy shape the immune system. Recent technological advances in physical dose delivery have encouraged innovative use of higher than conventional doses/fraction given over a shorter time. The optimal dose per fraction that generates anti-tumor as opposed to pro-tumor immune responses in humans is not known but preclinical data suggest that doses around 8Gy may be superior. This is the question addressed here by immune monitoring prostate cancer patients receiving hypofractionated and conventional regimens. The study is designed in a longitudinal fashion to detect changes in individual patients over time before, during and after treatment and to probe multiple aspects of their immune responses. Composing immune profiles with time for each patient circumvents some of the problems of individual variation. An additional benefit of this study comes from the fact that both dose-fractionation schedules will be compared side-by-side in one institution, which minimizes potential biases. The methods employed will be based on cutting-edge, sophisticated immune monitoring technologies to track the frequency, phenotype, and function of effector and suppressor lymphoid and myeloid-derived cells as well as humoral responses. They include Dextramer assay, ELISpot, multi-color flow cytometry, ELISA and multiplex technology. The aim is to paint a global immune picture that includes tumor-specific responses - qualitative as well as quantitative - for each patient as they undergo radiation treatment. The study will have high impact. It may provide data for a new paradigm for re-thinking radiation fractionation, the release of danger signals, and the development of tumor-specific immune responses. Knowing how different radiation dose delivery schedules affect the balance between pro- and anti-tumor immune responses is crucial if we are to engage the immune system in the context of cancer treatment, especially if we are to integrate it with other more systemic therapies, including chemo- (CT) and immunotherapy (IT). This study also has the potential to have an enormous impact as a fast-track translational application because it will ultimately allow us to harness the power of the immune system with an innovative approach that takes radiation oncology to the 21st century.