Point: Principles of magnetic resonance imaging integration in a computed tomography-based radiotherapy workflow

Computed tomography (CT) is the primary imaging modality on which radiation therapy has been based, leading to a CT-based workflow that is well established. CT provides a geometrically accurate patient model on which accurate radiation planning occurs, and it is the basis of the present state-of-the-art image guidance systems, which are well integrated with the treatment unit. Magnetic resonance imaging (MRI) is becoming an increasingly important tool in radiation oncology, as it can provide anatomical and functional information regarding the tumor and normal tissues, which may be complimentary to information from CT alone. MRI may be integrated into a CT-based radiotherapy (RT) workflow, using image registration tools. Such tools are already an inherent part of the RT workflow, for multimodality and multiphasic image registration for radiation planning (for MRI, positron emission tomography, and other imaging) and for image guidance at the treatment unit. The rationale for MRI integration in a CT-based RT workflow is reviewed here, and technical challenges and solutions regarding image registration of MRI throughout the entire RT process are discussed.