Multiparametric Quantitative MRI as a Metric for Radiation Treatment Planning

Magnetic resonance imaging (MRI) provides excellent soft tissue contrast, and in combination with its quantitative functional imaging capability, this modality is ideal for use in radiotherapy. MRI images, either used directly or fused with CT, play an increasingly important role in contouring gross tumor volume (GTV) and organs at risk (OAR) in radiation treatment planning (RTP) systems. The soft tissue contrast of MRI images provides more accurate tumor delineation than CT, although CT images have sufficient geometrical stability and electron density information for accurate radiation treatment planning. Many vendors now offer 70 cm wide-bore MRI systems with dedicated radiofrequency (RF) coils and immobilization devices for RTP simulation comparable to CT simulators. Many groups are researching future applications of MRI simulation independent of CT and as a stand-alone modality for RTP and online guided radiotherapy. The aim of this paper is to discuss the major functional MRI pulse sequences used in RTP: dynamic contrast enhanced MRI (DCE-MRI), diffusion-weighted imaging (DWI) and magnetic resonance spectroscopic imaging (MRSI). We discuss the potential of these functional MRI techniques in different sites for tumor response evaluation and possible adaptive radiotherapy.