Christopher Williams, PhD & Elizabeth Huynh, PhD
Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Boston

Superior soft-tissue contrast and  more accurate patient set-up

MRI-guided radiation therapy (RT) is a new treatment modality that is rapidly emerging. For many cancers, multiple imaging modalities (such as CT, MRI and/or PET) are used for identification and delineation of treatment targets, however current treatment machines rely solely on the use of x-ray imagers for guidance during RT delivery. X-ray imaging excels at visualizing dense materials such as bone, but struggles at distinguishing different soft-tissues. MRI provides superior soft-tissue contrast compared with x-ray imaging¹, which can improve RT guidance if integrated with a treatment delivery machine.

One major advantage of using MRI for guidance is the potential for more accurate patient set-up.  Traditionally, the patient is aligned based on their bony anatomy as a surrogate for their tumor, which adds uncertainty in target localization. MR-based setup allows the disease to be directly visualized and used for alignment, potentially enabling smaller treatment margins (thus minimizing radiation dose to healthy tissue or enabling dose escalation).

Furthermore, good visualization of soft-tissue on MRI also enables online adaptive RT where the treatment plan can be adjusted based on daily changes in a patient’s anatomy. In current clinical practice, the treatment plan is based on a simulation image that is acquired several days or even weeks before treatment.  Being able to adapt a treatment plan based on a patient’s current anatomy is especially important for tumors surrounded by anatomy that can change dramatically day-to-day (such as the bowel) or in cases where the tumor may be rapidly growing².

An additional benefit of MR guidance is the ability to perform continuous cine imaging throughout treatment for motion monitoring.  Rapidly acquired MRI slices can be used to track the motion of the target or critical organs to gate the treatment beam on or off.  Traditional methods often rely on using the external motion of the patient’s chest or abdomen, which may not correlate well with target motion³.  As MRI is a non-ionizing modality, this additional imaging is also not harmful to the patient.

With the advent of several commercially available MR linacs^4,5, the clinical potential of MR guidance is beginning to be realized.  For example, disease sites in the abdomen are commonly treated on MR linacs due to the daily variation in position of the gastrointestinal organs that can be accounted for using online adaptive RT⁶. This is also true for thoracic tumors (especially ultra-central tumors that may not otherwise be treatable), where the motion management capabilities can be used to reduce target size⁷. Integrating MR-guidance in RT has enabled us see anatomy on treatment that could never have been seen before, and to treat tumors that could not have been previously treated with radiation.

References

1. Khoo VS, Joon DL. New developments in MRI for target volume delineation in radiotherapy. Br J Radiol. 2006;79(SPEC. ISS.). doi:10.1259/bjr/41321492
2. Ménard C, van der Heide U. Introduction: Systems for Magnetic Resonance Image Guided Radiation Therapy. Semin Radiat Oncol. 2014;24(3):192. doi:10.1016/j.semradonc.2014.02.010
3. Keall PJ, Mageras GS, Balter JM, et al. The management of respiratory motion in radiation oncology report of AAPM Task Group 76. Med Phys. 2006;33(10):3874-3900. doi:10.1118/1.2349696
4. Mutic S, Dempsey JF. Treatment of patients with this system commenced at Washington University in. Semin Radiat Oncol. 2014;24:196-199. doi:10.1016/j.semradonc.2014.02.008
5. Raaymakers BW, Jürgenliemk-Schulz IM, Bol GH, et al. First patients treated with a 1.5 T MRI-Linac: Clinical proof of concept of a high-precision, high-field MRI guided radiotherapy treatment. Phys Med Biol. 2017;62(23):L41-L50. doi:10.1088/1361-6560/aa9517
6. Henke L, Kashani R, Robinson C, et al. Phase I trial of stereotactic MR-guided online adaptive radiation therapy (SMART) for the treatment of oligometastatic or unresectable primary malignancies of the abdomen. Radiother Oncol. 2018;126(3):519-526. doi:10.1016/j.radonc.2017.11.032
7. Henke LE, Olsen JR, Contreras JA, et al. Stereotactic MR-Guided Online Adaptive Radiation Therapy (SMART) for Ultracentral Thorax Malignancies: Results of a Phase 1 Trial. Adv Radiat Oncol. 2019;4(1):201-209. doi:10.1016/j.adro.2018.10.003