To Bag or Not to Bag? Comparative Evaluation of Patient Positioning and Motion Management In MR-Guided Prostate SBRT
Abstract
Purpose
Vacuum cradles/bags (VC) combined with head and leg immobilization are commonly used for prostate stereotactic body radiotherapy (SBRT) treatment setup. Removing the VC may offer practical advantages for efficient patient positioning, comfort, and high-quality imaging during MR-guided radiotherapy, including real-time cine-MR treatment monitoring. This study investigates the impact of removing VC on inter- and intrafraction motion in prostate SBRT patients treated on MR-Linac.
Methods
We retrospectively evaluated 40 prostate SBRT patients treated on MR-Linac, including 20 patient’s setup with VC and 20 without. Setup efficiency was quantified using the duty cycle, defined as the ratio of beam-on time to total gated treatment time. Patient positioning reproducibility and motion management were assessed through inter- and intrafraction motion analyses. Interfraction motion included setup errors from rigid registration between daily MRI vs. planning MRI. Intrafraction motion was evaluated using real-time cine-MR imaging to assess the 95th percentile of target displacement during treatment.
Results
Patients treated with vs. without VC had median (interquartile range) duty cycle of 94.0%[86.4%,97.8%] vs. 95.4%[64.7%,97.9%] respectively. Median interfraction motion (mm) with vs. without VC was 3.0[-2.9,8.1] vs. 1.4[-3.6,6.2] in lateral (LR) direction, 2.1[-4.9,9.3] vs. 0.9[-6.7,9.5] in superior–inferior (SI) direction, and 3.7[1.6,7.0] vs. 3.2[0.6,6.0] in anterior–posterior (AP) direction. Median intrafraction motion (mm) with vs. without VC was 0.4[0.3,0.8] vs. 0.7[0.3,1.1] (LR), 1.6[1.2,2.3] vs. 1.7[1.1,2.3] (SI), and 2.5[1.3,3.2] vs. 2.7[1.7,3.9] (AP), with corresponding median total resultant displacements (TRD) of 2.6[1.8,3.3] vs. 2.9[2.0,4.0]. Statistical testing using Mann–Whitney with Sidak-Bonferroni multiple comparison corrections demonstrated no statistically significant differences between the two cohorts except for AP (p=0.0167) and LR (p=0.0063) intrafraction motion, however the TRD (p=0.054) for intrafraction motion was not significantly different.
Conclusion
Prostate SBRT patients can be effectively positioned and treated on MR-Linac without VC, using only head and leg immobilization, without compromising setup efficiency, motion management, or patient comfort.