Online Magnetic Resonance-Guided Radiotherapy (oMRgRT) for Gynecological Cancers

Abstract

Radiation therapy (RT) is increasingly being used in gynecological cancer management. RT delivered with curative or palliative intent can be administered alone or combined with chemotherapy or surgery. Advanced treatment planning and delivery techniques such as intensity-modulated radiation therapy, including volumetric modulated arc therapy, and image-guided adaptive brachytherapy allow for highly conformal radiation dose delivery leading to improved tumor control rates and less treatment toxicity. Quality on-board imaging that provides accurate visualization of target and surrounding organs at risk is a critical feature of these advanced techniques. As soft tissue contrast resolution is superior with magnetic resonance imaging (MRI) compared to other imaging modalities, MRI has been used increasingly to delineate tumor from adjacent soft tissues and organs at risk from initial diagnosis to tumor response evaluation. Gynecological cancers often have poor contrast resolution compared to the surrounding tissues on computed tomography scan, and consequently the benefit of MRI is high. One example is in management of locally advanced cervix cancer where adaptive MRI guidance has been broadly implemented for adaptive brachytherapy. The role of MRI for external beam RT is also steadily increasing. MRI information is being used for treatment planning, predicting, and monitoring position shifts and accounting for tissue deformation and target regression during treatment. The recent clinical introduction of online MRI-guided radiation therapy (oMRgRT) could be the next step in high-precision RT. This technology provides a tool to take full advantage of MRI not only at the time of initial treatment planning but as well as for daily position verification and online plan adaptation. Cervical, endometrial, vaginal, and oligometastatic ovarian cancers are being treated on MRI linear accelerator systems throughout the world. This review summarizes the current state, early experience, ongoing trials, and future directions of oMRgRT in the management of gynecological cancers.

Keywords: MR Linac; MR-guided radiotherapy; SBRT; cervical cancer; gynecological cancers; online MR guided radiation therapy.

Figure 1

POTD approach for oMRgRT for cervical cancer.

Figure 2

MRg Brachytherapy 3T Diagnostic MRI (A) Sagittal and (B) axial views compared to 0.35T MRIdian MRI (C) Sagittal (D) axial views.

Figure 3

Cervix cancer; MR linac boost of HR-CTV after pelvic EBRT. Left column (top to bottom) transversal and sagittal T2 weighted MRI at time of treatment planning and on-board CBCT scans in the first week of elective EBRT. Right column (top to bottom) MRI scans after 32.4 Gy of elective EBRT and first. MR Linac boost plans. For comparison reasons, the initial and boost HR-CTV contours are shown on the MRI scans (pink at time of treatment planning and yellow after 32.4 Gy EBRT). On the CBCT scans initial HR-CTV is shown. For the MR Linac boost plan, the online delineation of the first fraction for HR-CTV and rectum is shown. The images show the improved visualization of MRI compared to CBCT.

Figure 4

Endometrial cancer; MR Linac SBRT for concurrent vaginal vault recurrence and anterior abdominal wall metastasis. (A): T2 sagittal (B): ADC map derived from diffusion weighted acquired with MR Linac. Abdominal metastasis in pink and vaginal vault recurrence in blue. Target and OAR are clearly visualized on the MR images allowing for daily adaptation. (C, D): Typical daily MR Linac plans for both lesions (isodoses red 110%, orange 100%, blue 25%). (E): elective EBRT plan on planning CT (yellow 95%, green 82%, blue 52%) (F): overlay of elective and boost plans (range 0–70 Gy physical dose).

Figure 5

Ovarian cancer MR Linac SBRT for PAO oligometastatic relapse. (A) Contours delineated for pretreatment planning GTV1 and GTV2 yellow; duodenum pink, bowel orange. (B–E): Online dose distribution for fractions 1–5, GTV1 and GTV2 (red) and PTV1 and PTV2 (green) with 2 cm ring for online contouring and planning guidance orange. Within the ring structure, target structures were manually adapted for the duodenum (pink) and bowel bag (orange) after an initial automated deformation of the contours; (F) individual dose distribution with dose levels shown as percentage of 35 Gy (5 × 7 Gy) prescribed dose red 110%, orange 100%, light green 75%, and blue 50%.