MRI-only treatment planning: benefits and challenges

Abstract

Over the past decade, the application of magnetic resonance imaging (MRI) has increased, and there is growing evidence to suggest that improvements in the accuracy of target delineation in MRI-guided radiation therapy may improve clinical outcomes in a variety of cancer types. However, some considerations should be recognized including patient motion during image acquisition and geometric accuracy of images. Moreover, MR-compatible immobilization devices need to be used when acquiring images in the treatment position while minimizing patient motion during the scan time. Finally, synthetic CT images (i.e. electron density maps) and digitally reconstructed radiograph images should be generated from MRI images for dose calculation and image guidance prior to treatment. A short review of the concepts and techniques that have been developed for implementation of MRI-only workflows in radiation therapy is provided in this document.

Figure 1

Axial brain images of patient with a metastatic tumor in the brain. (a,c) CT image. No contrast between the tumor and the surrounding normal tissue. (b,d) T2-weighted Fluid-attenuated inversion recovery image (FLAIR) MR image. Higher soft tissue contrast of the MR image leads to more accurate delineation of the tumor.

Figure 2

Comparison of the transverse view of CT (left) and T2-weighted (right) images of a patient with prostate cancer; The volumes are as follows: prostate (magenta) and dominant intraprostatic lesion (cyan).

Figure 3

Comparison of CT and T2-weighted images of a patient with plastic needles and a plastic cylinder and tandem in place; Transverse view of CT (top row) and T2-weighted (bottom row) of a patient’s pelvis with axial view showed in left panel and sagittal and coronal views showed in middle and right panels, respectively. High-risk CTV volume shown in red.

Figure 4

MAR for CT-SIM (A) and MR-SIM (B) in a prostate cancer patient with bilateral hip implants.

Figure 5

Treatment planning CT and synthetic CT including dosimetric comparison for an average patient brain cancer patient. Dose planes at isocenter (percent dose) for the CT-SIM and synthetic CT. The corresponding dose histogram is also shown highlighting close agreement between dose calculations for a radiosurgery brain case.

Figure 6

Transverse view of CT (A), MRI (B) and synthetic CT (C) of a patient’s pelvis.

Figure 7

Anterior kilovoltage planar (A), CT (B), and synthetic CT (C) digitally reconstructed radiographs (DRRs) illustrating that while the skull is well-approximated by the synthetic CT, proper characterization of resection cavities are still a work in progress in the brain.