Magnetic resonance linear accelerator technology and adaptive radiation therapy: An overview for clinicians

Abstract

Radiation therapy (RT) continues to play an important role in the treatment of cancer. Adaptive RT (ART) is a novel method through which RT treatments are evolving. With the ART approach, computed tomography or magnetic resonance (MR) images are obtained as part of the treatment delivery process. This enables the adaptation of the irradiated volume to account for changes in organ and/or tumor position, movement, size, or shape that may occur over the course of treatment. The advantages and challenges of ART maybe somewhat abstract to oncologists and clinicians outside of the specialty of radiation oncology. ART is positioned to affect many different types of cancer. There is a wide spectrum of hypothesized benefits, from small toxicity improvements to meaningful gains in overall survival. The use and application of this novel technology should be understood by the oncologic community at large, such that it can be appropriately contextualized within the landscape of cancer therapies. Likewise, the need to test these advances is pressing. MR-guided ART (MRgART) is an emerging, extended modality of ART that expands upon and further advances the capabilities of ART. MRgART presents unique opportunities to iteratively improve adaptive image guidance. However, although the MRgART adaptive process advances ART to previously unattained levels, it can be more expensive, time-consuming, and complex. In this review, the authors present an overview for clinicians describing the process of ART and specifically MRgART.

Keywords: 0.35 Tesla MR Guidance; 1.5 Tesla MR Guidance; RT adaption; Unity; ViewRay; adaptive image guidance; adaptive radiation therapy; magnetic resonance (MR)-guided radiation; personalized radiation therapy.

FIGURE 1.

Historic Evolution of Radiation Technology Over the Past 30 Years: Image-Guidance Radiotherapy (IGRT), 2-Dimensional Radiotherapy (2D-RT), 3D-RT, Intensity-Modulated RT/Volumetric Modulated Arc Therapy (IMRT/VMAT), Particle Therapy, and Adaptive Therapy (ART).

FIGURE 2.

Illustrations of Radiation Treatment Given (A) Without Daily Computed Tomography (CT)-Based Image Guidance Using Plain Films and (B) Treatment Using CT on Rails.

FIGURE 3.

Limitations of Image Guidance Alone—Volume Change/Deformations Can Be Seen But Not Accounted for Dosimetrically: Treatment of the Prostate Gland Associated With Significant Rectal Movement and Gas Causing Distension. CT indicates computed tomography; RT, radiotherapy; MV, megavoltage.

FIGURE 4.

Traditional Radiotherapy (RT) Compared With Adaptive RT (ART). A comparative overview of the workflow associated with adaptive RT compared with traditional non-ART is presented. CT indicates computed tomography; MR, magnetic resonance.

FIGURE 5.

Detailed Overview of Workflow, Additional Steps, and Potential Dose Improvements Associated With Adaptive Radiotherapy (ART) Either Online or Offline. IGRT indicates image-guided radiotherapy; RT, radiotherapy.

FIGURE 6.

Detailed Example of Dose Distribution Differences Seen With Adaptive Therapy Using Magnetic Resonance Guidance. CT indicates computed tomography.

FIGURE 7.

Example of Cone-Beam Computed Tomography (CT), Which Is Typical for Daily Image Guidance on Linear Accelerators, Compared With Daily Magnetic Resonance Imaging.

FIGURE 8.

(A) Computed Tomography (Ct) to Magnetic Resonance (MR) Registration Is Compared With Daily MR Acquisition for the Purpose of MR Guidance. (B) MR-Guided Radiotherapy (RT) Acquires an MR Image Each Day. This enables clear visualization of targets as they change daily. MRI indicates magnetic resonance imaging; Sim, simulation; SV, seminal vesicle.

FIGURE 9.

Illustration of Bowel Movement Intrafraction on Close Proximity to High-Dose Radiotherapy (RT) Into an Otherwise Voided Space. CT indicates computed tomography; MRI, magnetic resonance imaging; sim, simulation.

FIGURE 10.

(A) The Electron-Return Effect. Dose was deposited at the lung/tissue interface (3600-centigray [cGy] line) and at the trachea/tissue interface (6600-cGy line). (B) The Electron Air-Stream Effect (White Arrow; 400-cGy Isodose Line). Dose was deposited to protruding anatomic structures from electrons swept away from the treatment field by the magnetic field.

FIGURE 11.

Example of Liver Imaging Improvement With Magnetic Resonance (MR) Compared With Computed Tomography (CT). 4D indicates 4-dimensional; Eovist, gadoxetate disodium; MRI, magnetic MR resonance imaging; MRL, magnetic resonance lymphangiography; RT, radiotherapy; T, Tesla; T1w, T1-weighted.