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

Abstract

Background and purpose: 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/paraaortic lymph node metastases with and without vacuum cushion. Materials and methods: Thirty-nine patients underwent 5x7 Gy SBRT on a 1.5T MR-linac, 19 patients were treated with vacuum cushion, 19 without and 1 patient sequentially with and without. Intrafraction motion was calculated for target lymph nodes (GTVs) and nearby bony anatomy, for three time intervals (pre-position verification (PV), pre-post, PV-post, relating to the online MRI scans) per treatment fraction. Results: Vacuum cushion immobilization significantly reduced anterior-posterior translations for the pre-PV and pre-post intervals, for bony anatomy and pre-post interval for GTV (p < 0.05). Mean GTV intrafraction motion reduction in posterior direction was 0.7 mm (95% confidence interval 0.3-1.1 mm) for pre-post interval (mean time = 32 min). Shifts in other directions were not significantly reduced. More motion occurred in pre-PV interval than in PV-post interval (mean time = 16 min for both); vacuum cushion immobilization did not reduce intrafraction motion during the beam-on period. Conclusion: A vacuum cushion reduces GTV and bony anatomy intrafraction motion in posterior direction during pelvic/para-aortic lymph node SBRT. This motion reduction was found for the first 16 min per session. For single targets this motion can be corrected for directly with an MR-linac. Intrafraction motion was not reduced during the second half of the session, the period of radiotherapy delivery on an MRlinac. Vacuum cushion immobilization may not be necessary for patients with single lymph node oligometastases undergoing SBRT on an MR-linac.

Progression-free survival in patients with 68Ga-PSMA-PET-directed SBRT for lymph node oligometastases

“…19. Median PFS (95% CI) was 21 (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)) and 8 (6-13) months for low and high risk groups, respectively (Figure 2). Similar differences between the risk groups were also observed for BPFS, WS-PFS and ADT-FS.…”

“…Twelve percent of patients had progression limited to lymph nodes in the same pelvic sub-region as before (classified as lower pelvic region left and right (comprised of internal and external iliac and obturator regions), presacral or mesorectal region and common iliac region left and right). A shorter PFS was observed after SBRT for repeat oligometastatic disease compared with newly diagnosed oligometastatic disease: median PFS (95% CI) was 9 (7-18) vs. 17 (13)(14)(15)(16)(17)(18)(19)(20)(21) months, respectively (Figure 3).…”

“…Since the clinical introduction of the MR-linac at our department in August 2018, patients were treated on the MR-linac except for patients with exclusion criteria for MR-linac delivery, such as an inability to lie still for 60 min. Vacuum cushion immobilisation was used for all patients until March 2019; we then investigated effect on target motion [21]. After March 2020, patients with single pelvic targets did not receive immobilisation when treated on MR-linac, as the immobilisation was found to offer no advantage during MR-linac treatment for these patients [21].…”

“…Vacuum cushion immobilisation was used for all patients until March 2019; we then investigated effect on target motion [21]. After March 2020, patients with single pelvic targets did not receive immobilisation when treated on MR-linac, as the immobilisation was found to offer no advantage during MR-linac treatment for these patients [21]. Follow-up after 3 months was at the discretion of the urologist for most patients; patients received a questionnaire every 6 months to register grade !3 late toxicities.…”

Stereotactic body radiotherapy of lymph node metastases under MR-guidance: First clinical results and patient-reported outcomes

“…However, data are only available from one other study group evaluating MR-guided SBRT using the 1.5 T Elekta MR-Linac: Winkel et al retrospectively compared high field (1.5 T) MR-guided irradiation of lymph node metastases with conventional CBCT-Linac treatment in 20 patients and demonstrated fewer unplanned violations of OAR constraints [19,22]. The same study group also successfully evaluated the utilization of a vacuum cushion for MR-guided pelvic/paraaortic lymph node SBRT to reduce intrafractional motion [23]. However, to the best of our knowledge, there has not yet been any study about clinical results and patient-reported outcome following MR-guided SBRT of lymph node metastases.…”