The Promise of Magnetic Resonance Imaging in Radiation Oncology Practice in the Management of Brain, Prostate, and GI Malignancies

Abstract

Magnetic resonance imaging (MRI) has a key role to play at multiple steps of the radiotherapy (RT) treatment planning and delivery process. Development of high-precision RT techniques such as intensity-modulated RT, stereotactic ablative RT, and particle beam therapy has enabled oncologists to escalate RT dose to the target while restricting doses to organs at risk (OAR). MRI plays a critical role in target volume delineation in various disease sites, thus ensuring that these high-precision techniques can be safely implemented. Accurate identification of gross disease has also enabled selective dose escalation as a means to widen the therapeutic index. Morphological and functional MRI sequences have also facilitated an understanding of temporal changes in target volumes and OAR during a course of RT, allowing for midtreatment volumetric and biological adaptation. The latest advancement in linear accelerator technology has led to the incorporation of an MRI scanner in the treatment unit. MRI-guided RT provides the opportunity for MRI-only workflow along with online adaptation for either target or OAR or both. MRI plays a key role in post-treatment response evaluation and is an important tool for guiding decision making. In this review, we briefly discuss the RT-related applications of MRI in the management of brain, prostate, and GI malignancies.

FIG 1

ROLE of MRI in radiation oncology workflow. CT, computed tomography; DWI, diffusion-weighted imaging; f-MRI, functional MRI; IGRT, image-guided radiotherapy; MRI, magnetic resonance imaging; MRS, spectroscopy; OAR, organs at risk.

FIG 2

Composite diagram of CT and MRI used for radiation planning for brain, prostate, and GI malignancies. Representative CT and MRI scans acquired during radiation planning process. (A) Axial CT and (B) corresponding axial MRI T1-weighted contrast-enhanced sequence for a patient with glioblastoma. The target volumes are seen over the left temporal lobe: GTV (magenta), CTV (blue), and PTV (red). The extension of PTV in the basifrontal region was a result of expansion from extension of CTV in the superior slices (not seen in this image). (C) Axial CT and (D) corresponding axial T2W MRI for a patient with prostate cancer. Dominant intraprostatic lesion clearly visualized (in red) on the MRI, whereas it could not be discerned on the planning CT images, thus facilitating dose escalation to the DIL. (E) Axial CT and (F) corresponding axial T2W MRI for a patient with locally advanced rectal adenocarcinoma. Target volumes include GTV (red), CTV (purple), and PTV (light blue). The extension of the gross disease into the prostate could be accurately seen only on the MRI. CT, computed tomography; CTV, clinical target volume; DIL, dominant intraprostatic lesion; MRI, magnetic resonance imaging; PTV, planning target volume.

FIG 3

Schematic workflow for potential MRI-guided adaptive radiotherapy undertaken in a course of fractionated RT MRI is performed at week 2 and week 4 during radiation. Anatomical adaptation is undertaken for morphological changes as detected on midtreatment MRI for target volumes and organs at risk. Biological adaptation with different dose levels is performed on the basis of findings from functional MRI, with areas of refractory disease escalated to a higher dose and areas with response de-escalated to a relatively lower dose level. MRI, magnetic resonance imaging; RT, radiotherapy.