First clinical application of Comprehensive Motion Management with 1.5T MR-linac on prostate cancer patients treated with radiotherapy: technical aspects and suggested workflow recommendations.

Purpose: MR-guided radiotherapy (MRgRT) on the MR-linac (MRL) with daily online plan adaptation enables better control of inter-fraction variability. Recently, Comprehensive Motion Management (CMM) was introduced for the 1.5T MRL. CMM halts dose delivery when the target moves outside its defined position and allows for target drift corrections, compensating for intra-fraction variability. This study aims to report the first clinical experience with prostate MRgRT using CMM, focusing on intra-fraction motion management and treatment delivery time.

Material and methods: Sixty patients with low- to intermediate-risk prostate cancer were treated with the 1.5T MRL using CMM. PTV margins were 5 mm in all directions, with a 3-mm margin posteriorly. Fifty patients received ultra-hypofractionated radiotherapy (SBRT), and 10 received hypofractionated radiotherapy (hypoRT). The CMM threshold was set to 100% of the GTV volume.

Results: A total of 450 treatment fractions were administered. The median beam-on time was 10.3 min, with a median duty cycle of 98.9% (95% CI 98.6-99.2%). Beam hold occurred in 158 fractions (35%), with an average of 24.5 instances per fraction (95% CI 16-32). Thirty-two baseline shift replans were performed. Both acute and late toxicities were low, with no Grade 3 or higher toxicities.

Conclusions: CMM has been successfully implemented in MRgRT. In-treatment corrections and baseline shift replanning did not significantly impact treatment time, enhancing treatment quality without compromising patient compliance or treatment feasibility. The low incidence of beam-hold events suggests that CMM could potentially allow for a safe reduction in PTV margins.