Reducing IMRT Delivery Times By Accelerating Segment Loading and MLC Positioning for the 1.5 T MR-linac
Abstract
Purpose
Intensity modulated radiotherapy (IMRT) is the current standard delivery technique on MR-linacs. IMRT deliveries cycle between radiating and moving segments, each starting with loading the segment. More complex plans usually contain more radiating segments, increasing the number of radiating-to-radiating intervals and typically doubling delivery time due to non-radiating overhead. This study investigates reductions in IMRT delivery time at the MR-linac achieved by accelerating segment loading and multileaf collimator (MLC) positioning through control system optimizations.
Methods
The delivery control system of the 1.5 T Unity MR-linac (Elekta, Sweden) was modified to (i) accelerate inter-component communication during segment loading, and (ii) optimize MLC positioning for higher travel speeds. Performance was tested using four representative clinical plans: prostate (727 MU), esophagus (596 MU), head&neck (691 MU), and liver (2357 MU), with 55, 83, 94, 33 radiating segments, respectively. Deliveries were performed in clinical mode and using either or both control system optimizations combined. Machine log files were extracted to estimate overall delivery time and its contributing components, and to compare MLC positions with planned values during radiating segments. Dosimetric validation was performed using the Delta4 Phantom+ MR (ScandiDos, Sweden).
Results
Overall delivery times were reduced by 0.5–1.8 min (liver–head&neck) across the treatment plans when both optimizations were enabled. Faster segment loading reduced radiating-to-radiating interval durations, with consistent median values of 600–640 ms across the four plans. Improved MLC positioning reduced the median interval duration by 200–440 ms, reflecting greater variation between treatment plans. As expected, the optimizations did not affect radiating segment durations, and MLC positions agreed with the planned positions within 0.2 mm. Dosimetric validation showed 2%/2mm global gamma passing rates of ≥97.5%.
Conclusion
Improved segment loading and MLC positioning can safely reduce radiating-to-radiating interval durations. Complex treatment plans characterized by low MU per segment benefit most from the presented optimizations.