ESTRO 2023 - Abstract Book

S1577

Digital Posters

ESTRO 2023

P. Borman 1 , P. Woodhead 1 , P. Uijtewaal 1 , B. Raaymakers 1 , M. Fast 1

1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands

Purpose or Objective MR-guided radiotherapy systems offer an ever growing range of (real-time) adaptation techniques, resulting in the more widespread use of hypo-fractionation. Delivering complex, high-dose IMRT plans may stretch treatment time slots and impacts patient comfort. Delivery efficiency is thus very important. The Unity 1.5 T MR-linac (Elekta AB, SWE) features gantry rotations of 6RPM, enabling fast beam-to-beam switching during IMRT. The multi-leaf collimator (MLC), however, is limited to move at 6cm/s. For plans with many segments, the ‘move-only’ parts between two segments induce a significant time overhead of up to 40%. Here, we investigate delivery efficiency improvements in a representative range of clinical MR-linac plans when the speed limit of the MLC is relaxed by means of an improved dynamic MLC controller in research mode. Materials and Methods The MLC software was modified to optimize the proportional–integral–derivative (PID) motor controller enabling faster leaf/diaphragm motion resulting in quicker segment switching. Ten clinical IMRT plans, five 7-beam 5x(2x3.125)Gy prostate and five 15-beam 8x7.5Gy lung plans, were delivered with and without the software optimization on the Delta4 Phantom+ (ScandiDos, SWE). In our clinic, most prostate fractions are split into two contiguous subfractions of 3.125Gy, of which one was selected for this study. The machine reports the actual MLC positions at 25Hz in a logfile. From there, the MLC positions were compared for each segment during beam-on, and the time spent in move-only parts was estimated. Additionally, a 3%/3mm gamma analysis was performed on the Delta4 dose maps with and without the software optimization. Results The average delivery speedup of the prostate subfraction plans was 32s and of the lung plans 36s, which is about 10% of the total delivery time. The move-only parts were shortened by 24% on average (fig. 1). During beam-on, the maximum position difference with and without improvement was 0.6mm for the leaves, and 0.3mm for the diaphragms (fig. 2). Therefore, exactly the same fluence modulation was achieved, which was corroborated by an average gamma pass-rate of 99.4%.

Figure 1: Speedup of the five prostate and lung plan deliveries using the improved MLC controller, as a percentage of the total delivery time (left) and as a percentage of the move-only, i.e. non-radiating time (right).

Figure 2: Difference of the leaf and diaphragm positions with and without MLC controller improvement. The positions of all segments of all ten plans are aggregated.

Conclusion