Emerging role of MRI in radiation therapy

Abstract

Advances in multimodality imaging, providing accurate information of the irradiated target volume and the adjacent critical structures or organs at risk (OAR), has made significant improvements in delivery of the external beam radiation dose. Radiation therapy conventionally has used computed tomography (CT) imaging for treatment planning and dose delivery. However, magnetic resonance imaging (MRI) provides unique advantages: added contrast information that can improve segmentation of the areas of interest, motion information that can help to better target and deliver radiation therapy, and posttreatment outcome analysis to better understand the biologic effect of radiation. To take advantage of these and other potential advantages of MRI in radiation therapy, radiologists and MRI physicists will need to understand the current radiation therapy workflow and speak the same language as our radiation therapy colleagues. This review article highlights the emerging role of MRI in radiation dose planning and delivery, but more so for MR-only treatment planning and delivery. Some of the areas of interest and challenges in implementing MRI in radiation therapy workflow are also briefly discussed. Level of Evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2018;48:1468-1478.

Keywords: MRI; cancer treatment; multimodality imaging; radiotherapy.

FIGURE 1:

Radiation treatment planning and delivery workflow.

FIGURE 2:

Various types of immobilization devices. Upper panels are reusable and customizable to patient (a) breast and thorax board, (b) Leg support used in prostate, (c) prone breast board. Lower panels are patient specific fixation devices (d) aquaplast mold for head and neck, (e) Vaclock fixation used for trunk and (f) solid aquaplast device for pelvic immobilization.

FIGURE 3:

Gross tumor volume (GTV) for head and neck cancer is delineated on pretreatment (a) MRI image which is registered to corresponding (b) CT dataset. MRI has higher contrast resolution, which enables tumor visualization and accurate GTV delineation, whereas CT images provide electron density information and are used for on-board registration with cone beam CT.

FIGURE 4:

(a) Treatment planning CT of the pelvis for prostate cancer. (b) T₂WI of the prostate. (c) Fused CT and MRI for external beam radiation therapy for prostate cancer.

FIGURE 5:

Workflow of a hybrid method for synCT generation from Dixon MR images. These methods were initially proposed for PET/MR attenuation correction. μ is linear attenuation coefficient (LAC). SynCT is generated by converting the μ-map to HU numbers.

FIGURE 6:

Planning CT (top row) and cone beam CT (bottom row) of a liver tumor in axial, coronal, and sagittal views. Cone beam CT images are of inferior quality with poor visualization of the tumor. Red curve is the contour of the tumor obtained on the planning CT and copied to the Cone beam CT after image registration.

FIGURE 7:

Example images of a pancreatic tumor. The poor soft-tissue contrast in (a) CT and (b) Cone Beam CT necessitates the use of implanted fiducials in order to allow accurate tumor positioning on the LINAC. MRI on the other hand allows (c) direct visualization and (d) motion tracking of the tumor as well as organs at risk, which is essential for online treatment monitoring. No implanted fiducials are needed.