This study evaluated the extent of prostate displacement during SBRT on an MRI Linac using comprehensive motion management (CMM) and identified variables associated with intrafraction motion (IFM).
212 patients with clinically localized prostate cancer were treated with 5-fraction SBRT on a 1. 5 T MR-Linac where IFM was continuously tracked and gated by CMM. Pre-beam positional shifts were identified from MRI registration prior to beam delivery. Intrafraction positional variability during beam delivery was evaluated, and multivariable analysis identified variables associated with IFM.
In 614 fractions (62.7%), a > 1.5 mm pre-beam positional shift led to an adapt-to-position (ATP) plan correction. Mean anterior-posterior and superior-inferior pre-beam shifts were 2.2 mm and 2.1 mm, respectively. For 962 evaluable fractions, the median beam-on-time was 13.7 min with a mean duty cycle of 95.8%. Sustained > 3 mm displacement was observed in 520 fractions (54.1%) with a median cumulative duration of 24 s; >5 mm displacement was observed in 209 fractions (21.7%) with a median duration of 12.4 s. The ATS + ATP workflow was associated with reduced odds of sustained > 3 mm motion (p = 0.035), while older age was associated with increased odds (p = 0.011).
Significant prostate shifts can occur immediately prior to and during radiation beam delivery, frequently exceeding applied margins and potentially leading to tumor underdosage. Continuous motion tracking and gating during prostate SBRT is an important tool in reducing IFM and enhance treatment delivery accuracy.
Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology. 2026 May 17 [Epub ahead of print]
Michael J Zelefsky, Ting Chen, Ryan D Walters, Cheongeun Oh, Nicholas W Colangelo, Hesheng Wang, Peter B Schiff, Siming Lu, Sangkyu Lee, Matthew Long, Allison McCarthy, Jeffrey D Cooney, Paulina Galavis, David Barbee
Department of Radiation Oncology, Department of Biostatistics, NYU Grossman School of Medicine, New York, NY, USA. Electronic address: ., Department of Radiation Oncology, Department of Biostatistics, NYU Grossman School of Medicine, New York, NY, USA.