MRI-guided stereotactic ablative radiation therapy of spinal bone metastases: a preliminary experience

Abstract

Objective: MRI provides clear visualization of spinal cord, tumor, and bone for patient positioning and verification during MRI-guided radiotherapy (MRI-RT). Therefore, we wished to evaluate spine stereotactic ablative radiotherapy (SABR) feasibility with MRI-RT. Given dosimetric limitations of first generation Co-60 MRI-RT, we then evaluated improvements by newer linear accelerator (linac) MRI-RT.

Methods: Nine spinal metastases were treated with Co-60 MRI-RT. Seven received a single 16 Gy fraction, and two received three fractions totaling 24 or 30 Gy. After replanning with linac MRI-RT software, comparisons of organ at risk and dose spillage objectives between Co-60 and linac plans were performed.

Results: Spinal cord and cauda equina dose constraints were met in all Co-60 cases. Treatments were delivered successfully with real-time imaging during treatment and no treatment-related toxicities. While limits for dose spillage into surrounding soft tissues were not achieved due to the limitations of the Co-60 system, this could be corrected with linac MRI-RT delivery.

Conclusions: MRI-RT SABR of spinal metastases is feasible with Co-60 MRI-RT. Dose delivery is improved by linac MRI-RT.

Advances in knowledge: This is the first report of MRI-RT for SABR of spinal metastases. The enhanced visualization of anatomy by MRI may facilitate RT dose escalation for spine SABR.

Figure 1.

Example setup for a lumbar spinal metastasis treated with MRI-guided SABR. The vendor supplied flexible MRI coil array (black) is seen on the right overlying the abdomen. SABR, stereotactic ablative radiotherapy

Figure 2.

Example slices (sagittal and axial) from 3D setup volume at time of treatment for Co-60 MRI-RT SABR of a L1 spinal metastasis. The grayscale image represents the MRI simulation image. The red shaded box demonstrates the aligned setup imaging of the day. A green outline delineates the PTV which includes the vertebral body and left pedicle. The bright cerebral spinal fluid, spinal cord, and bony anatomy are clearly visualized in the spinal canal on the sagittal image. On the axial image, the tip of the spinal cord and cauda equina are observed posteriorly in the dural sac. 3D,three-dimensional; SABR, stereotactic ablativeradiotherapy; PTV, planning target volume.

Figure 3.

Example delivered Co-60 and replanned linac MRI-RT SABR plans for a T5 spinal metastasis at 16 Gy prescription dose. (a) Co-60 treatment plan isodose levels in color (legend on right) overlaid on grayscale MRI simulation. The treatment plan is not conformal due to Co-60 penumbra and MLC width. Hot spots of over 115% (18.4 Gy) are observed outside of the PTV in adjacent fat. However the spinal cord is only touching the 10 Gy isodose line. Additional OARs in the area (esophagus, aorta, trachea and lung) were contoured and maintained below TG-101 objective doses. (b) The MRI-linac SABR plan is much more conformal, with excellent coverage, and hot spots almost entirely within the PTV. (c) Co-60 (dashed line) and linac (solid line) DVH is shown for this plan for the PTV (orange) and partial cord (green). The linac plan has less low dose to the spinal cord and is hotter within the PTV target. DVH, dose–volume histogram; OAR, organ at risk; MLC, multileaf collimator; SABR, stereotactic ablative radiotherapy; PTV, planningtarget volume.