Sci-Fri PM: Delivery - 03: Spine SBRT: Treating multiple vertebrae using cone-beam CT image-guidance and the hexapod...

The intent of spinal SBRT is to maximize the biological effective dose and improve local control, while sparing the adjacent spinal cord. We report on the spinal SBRT positional accuracy for multiple, consecutive vertebrae in a single course, using the Elekta Synergy-S and BodyFIX immobilization systems.

After initial patient adjustment, verification cone-beam CT (CBCT) images were acquired before, during and after treatment. These images were used to assess immobilization and correct any misalignment exceeding 1 mm or 1°, in all six degrees-of-freedom using the HexaPOD robotic couch. This analysis is based on 415 verification images from 67 consecutive courses of treatment. These treatment courses comprised 25 single thoracic vertebrae, 16 multiple thoracic vertebrae, 20 single lumbar vertebrae and 6 multiple lumbar vertebrae.

The absolute intra-fraction motion averaged over all directions (±std dev.) for the T-single, T-multiple, L-Single and L-Multiple was 0.54 (±0.73) mm, 0.54 (±0.88) mm, 0.36 (±0.57) mm, and 0.47 (±0.63) mm respectively. The percentage that exceeded the 1.5 mm planning margin was 3.8%, 4.0%, 1.0% and 0.85% respectively. T-spine treatments were out-of-tolerance more frequently than the L-spine. There was a statistically significant difference between single and multiple lumbar treatments (unpaired t-test, p<0.01), but this was not clinically significant as 99% were within our 1.5 mm margin.

Near-rigid immobilization with the acquisition of intra-fraction CBCT images and the correction of misalignments in all six degrees-of-freedom provides the necessary precision to safely perform SBRT of consecutive spinal metastases within one course of treatment.