Magnetic Resonance Imaging for Target Delineation and Daily Treatment Modification.

Magnetic resonance (MR) imaging has become a prevalent modality in radiation oncology owing to its excellent soft-tissue contrast and ability to provide functional information. Recent technological developments have combined MR imaging with treatment delivery systems, to provide in-room MR guidance for patient setup and treatment delivery. Availability of in-room MR imaging enables direct visualization of soft-tissue targets and nearby organs at risk, thus providing a platform for fast and accurate target and organs at risk delineation for plan adaptation and target tracking during treatment. This article describes the 2 clinically implemented MR image-guided radiotherapy systems and their role in target localization and in-room treatment adaptation. Clinical data from early adopters of these systems is reviewed.