Abstract
PURPOSE: Abdominal compression can minimize breathing motion in stereotactic radiotherapy, though it may impact the positioning of dose-limiting normal tissues. This study quantified the reproducibility of abdominal normal tissues and respiratory motion with the use of an abdominal compression device using MR imaging. METHODS: Twenty healthy volunteers had repeat MR over 3 days under an abdominal compression plate device. Normal tissues were delineated on daily axial T2-weighted MR and compared on days 2 and 3 relative to day 1, after adjusting for baseline shifts relative to bony anatomy. Inter-fraction organ deformation was computed using deformable registration of axial T2 images. Deformation > 5 mm was assumed to be clinically relevant. Inter-fraction respiratory amplitude changes and intra-fraction baseline drifts during imaging were quantified on daily orthogonal cine-MR (70 s each), and changes > 3 mm were assumed to be relevant. RESULTS: On axial MR, the mean inter-fraction normal tissue deformation was > 5 mm for all organs (range 5.1-13.4 mm). Inter-fraction compression device misplacements > 5 mm and changes in stomach volume > 50% occurred at a rate of 93% and 38%, respectively, in one or more directions and were associated with larger adjacent organ deformation, in particular for the duodenum. On cine-MR, inter-fraction amplitude changes > 3 mm on day 2 and 3 relative to day 1 occurred at a rate of < 12.5% (mean superior-inferior change was 1.6 mm). Intra-fraction baseline drifts > 3 mm during any cine-MR acquisition occurred at a rate of 23% (mean superior-inferior changes was 2.4 mm). CONCLUSIONS: Respiratory motion under abdominal compression is reproducible in most subjects within 3 mm. However, inter-fraction deformations greater than 5 mm in normal tissues were common and larger than inter- and intra-fraction respiratory changes. Deformations were driven mostly by variable stomach contents and device positioning. The magnitude of this motion may impact normal tissue dosimetry during stereotactic radiotherapy.