Prostate cancer is the most frequently diagnosed non-skin cancer in the Western world, with a projected lifetime incidence of 11.6%. Standard treatment options include radical prostatectomy or definitive radiotherapy. After a radical prostectomy, up to 30% of patients will develop a biochemical relapse--at this point, the only curative intervention is salvage radiotherapy directed at the prostatic fossa. Even then, however, the pooled chance of biochemical control following salvage radiotherapy is on the order of 50% at five years. It is widely thought that these outcomes could be improved by escalating the radiation dose; however, the safe delivery of radiation is limited by the complicated local anatomy following radical prostatectomy and the need to radiate as little of the bladder and rectum as possible to preserve the quality of life. While enhanced radiation techniques allow high precision delivery, these necessitate confidence in targeting the radiation appropriately. In this project, we will perform a rigorous quantitative analysis of organ-at-risk deformation during the course of salvage radiation therapy, providing key information to help guide dose-escalated salvage radiotherapy. Specifically, we seek to quantify the interfractional deformation of the bladder and rectum during a course of dose-escalated salvage radiotherapy for biochemically recurrent prostate cancer. With these findings, we can determine whether planning margin volumes associated with prostatic fossa radiation can be optimized based on expected anatomical variations.