We aimed to determine the planning target volume margins applicable to magnetic resonance-guided stereotactic body radiation therapy (MRgSBRT) of localized prostate cancer on a 1.5 T MR-Linac.

A total of 10 patients with low- to intermediate-risk prostate cancer were retrospectively enrolled. All patients received five-fraction MRgSBRT treatments, and underwent daily endorectal ballooning and bladder control. Five patients were implanted with the hydrogel rectal spacer. Three MR scans at each fraction were acquired to evaluate the intrafractional prostate motion. The required planning target volume margin was determined using the van Herk margin recipe based on direct MR-MR prostate registration. The set-up margin for localization using bony anatomy was also estimated.

The mean and standard deviation of translational displacement in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were –0.3 ± 0.7, 2.0 ± 1.5, and 1.4 ± 1.1 mm, respectively. Planning target volume margins over the MRgSBRT fraction duration were 2.8 (LR), 5.3 (SI), and 3.9 mm (AP). Set-up margins were estimated to 2.6 (LR), 3.3 (SI), and 3.3 mm (AP). Prostate motion increased considerably with treatment duration. Adapt-to-position had smaller intrafractional motions than adapt-to-shape. The rectal spacer insignificantly (p > 0.1) reduced the prostate motion in SI and AP.

Online workflow efficiency enhancement and intrafractional motion monitoring are vital for planning target volume margin reduction and precision enhancement in MRgSBRT.