Impact of a vacuum cushion on intrafraction motion during online adaptive MR-guided SBRT for pelvic and para-aortic lymph node oligometastases

Highlights: • Vacuum cushion immobilization reduced intrafraction motion in posterior direction. • Most intrafraction motion occurred in the first 16 minutes. • On an MR-linac, vacuum cushion immobilization may not be needed for single targets. Vacuum cushion immobilization is commonly used during stereotactic body radiotherapy (SBRT) to reduce intrafraction motion. We investigated target and bony anatomy intrafraction motion (translations and rotations) during online adaptive SBRT on an MR-linac for pelvic/para-aortic lymph node metastases with and without vacuum cushion.