Clinical applications of magnetic resonance imaging (MRI) with an endorectal coil (ERC) in the treatment planning of prostate cancer provides superior soft tissue contrast compared to computed tomography (CT) but may introduce geometric distortions that reduce their utility. While the ability to clearly see cancerous nodules in MRI is helpful in planning a simultaneous-integrated boost treatment where dose is boosted of a specific cancerous lesion, so too is the precise knowledge of the lesion's spatial location. The purpose of this research is to evaluate the deformable image registration of an MRI to a planning CT, and calculate the added margin for transferred contours based on registration uncertainty.