MRI has grown in prominence in the field of radiation therapy for its ability to explore the properties of tissues beyond that of standard x-ray-based contrast mechanisms. While innovations of MR in RT have led to the development of the synthetic CT for accurate high energy x-ray dose distribution calculations, proton therapy planning and range determination by synthetic CT may result in an unwanted increase in range uncertainty. This presentation will summarize our work to overcome limitations of the synthetic CT approach by employing a physics-based model to more directly calculate proton beam range with a multi-modal MR-based imaging process, potentially allowing for significant improvements in range uncertainty.