Can VMAT Reduce Treatment Time on the MR-Linac without Compromising Plan Quality?
Abstract
Purpose
This study investigates the feasibility of VMAT treatments on the MR-Linac by comparing planned and measured quality and deliverability of investigational VMAT plans with existing clinical IMRT plans to determine whether VMAT can maintain clinically acceptable plan quality while significantly reducing overall treatment times.
Methods
For 10 randomly selected patients receiving 5-fraction, 36.25 Gy prostate SBRT, VMAT plans were generated using the new research version of Monaco (v6.29.0.0) and compared with clinical IMRT plans (v6.2.1.0). All plans met the clinical target and OAR constraints. Differences were evaluated using the Wilcoxon signed-rank test (p < 0.05). Plans were delivered on a 1.5 T Unity MR-Linac (Elekta, Sweden) in investigational VMAT or clinical IMRT mode, measured with the Delta4 MR phantom (Scandidos, Sweden). Patient-specific QA was assessed using global gamma scores (3%/3 mm), and delivery efficiency was calculated as the percentage of maximum dose rate.
Results
IMRT and VMAT plans demonstrated comparable target coverage and OAR sparing, with all values well within clinical constraints. Wilcoxon testing showed slightly higher bladder and rectal volumes and a slightly higher mean sphincter dose for VMAT, alongside a significantly lower left femur dose, reflecting differences in delivery geometry, but differences were small and still within clinical limits. Patient-specific QA confirmed high delivery accuracy, with gamma passing rates averaging 99.9% (IMRT) and 99.1% (VMAT) and identical DTA scores of 99.7%. VMAT achieved significantly higher (p < 0.002) delivery efficiency (92.0% vs 61.5%), resulting in much shorter treatment times per fraction with an average treatment time improvement from 6.5 min to 4.0 min.
Conclusion
VMAT plans achieved substantially higher delivery efficiency than IMRT while maintaining comparable target coverage and OAR sparing, with all plans meeting clinical QA criteria. These results indicate that VMAT can significantly shorten treatment times with 38.4% without compromising plan quality for prostate SBRT.