Validation of a motion management and gating protocol on the Elekta Unity MR LINAC for margin reduction in prostate cancer

Abstract

Background: The recent installation of Elekta Unity MR-LINAC (MRL) at the Sir Anthony Mamo Oncology Centre (SAMOC) integrates magnetic resonance imaging (MRI) with radiotherapy delivery. The system offers superior soft tissue visualisation and motion management capabilities, particularly relevant for prostate cancer, where intrafractional motion compromises dose coverage and increased exposure to organs at risk (OAR). Objectives: This study aimed to evaluate the feasibility, performance, and limitations of a reduced margin protocol for prostate radiotherapy on the Elekta Unity MRL. Therefore, the validation of treatment margin reduction was essential to ensure safe margin reduction and effective treatment delivery in a typical clinical setting for prostate treatments. Research Methodology: The analysis of 360 treatment fractions was performed to characterise prostate motion using Unity log files. Then, a treatment planning study with a phantom was performed to simulate prostate displacements and evaluate dose–volume histogram (DVH) metrics using Monaco® TPS. Lastly, radiochromic film dosimetry validated treatment delivery under static and motion scenarios, with gamma analysis used for dose comparison. Results: The log file analysis showed minimal motion in the LR and AP directions, but higher variability in the SI axis, especially during the final 30 seconds of treatment. DVH analysis revealed that most treatment plans met dosimetric criteria, though extremely inferior displacements reduced target coverage. Film dosimetry and gamma analysis showed that the film exposed during motion with 2 baselineshifts (BLS) and the film exposed at 97% VOICE performed the best and poorest, respectively. Conclusions and Recommendations: This study shows that reducing prostate PTV margins to 2 mm is possible. Moreover, it was concluded that BLS can restore margin coverage. Clinically, combining margin optimisation with adaptive work-flows and quality assurance QA practices enhances safety and precision. Future research should validate results in larger cohorts, consider seminal vesicle motion, and apply such strategies across other anatomically mobile sites.