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MR-only radiotherapy planning is an area of active research and development. Technological advances in MR-guided delivery systems and availability of MR scanners in radiotherapy department have made the clinical integration of MR-only planning possible. The chapter outlines challenges and requirements for MR-only radiotherapy and gives background on the most important requirement of MR-only planning, namely, the development of synthetic CT. The chapter describes the simplest method for generating synthetic CT, mainly bulk density assignment, and sets the background and motivation for more advanced methods.

External beam radiotherapy for prostate cancer is in a period of rapid change and evolution as treatment paradigms shift toward hypofractionated regimens. Technological advances including image guided radiotherapy, volume modulated arc therapy and the use of magnetic resonance imaging for planning, treatment delivery and response assessment have made the move to hypofractionation possible. In this chapter, the treatment planning process applicable to hypofractionated, stereotactic radiotherapy for prostate cancer using a conventional linear accelerator is described with an emphasis on the increasing role of magnetic resonance imaging in simulation and planning. A simulation workflow using MR imaging only is presented and compared with one based on multi-modality imaging (CT and MRI). An overview of volume modulated arc therapy planning is provided with examples for extreme hypofractionation and simultaneous boost treatment of intra-prostatic disease. Further developments of MR-based planning methods to target dominant intra-prostatic lesions, perform on-line adaptive therapy and MR-guided radiotherapy delivery are discussed.