ESTRO 2021 Abstract Book

S1488

ESTRO 2021

with poor anatomical information is a limitation of this 2D-kV imaging system.

PO-1762 Feasibility study to achieve hypofractionated IMPT PBS within a single breath-hold for lung cancer V. Maradia 1 , S. Bertschi 1 , M. Krieger 1 , S. van de Water 1 , D. Meer 1 , J.M. Schippers 1 , A.J. Lomax 1 , D.C. Weber 1 , S. Psoroulas 1 1 Paul Scherrer Institute, Center for proton therapy, Villigen, Switzerland Purpose or Objective We study the feasibility of hypofractionated proton therapy treatments within a single breath-hold at PSI’s Gantry 2. Treatment delivery time in proton therapy depends on beam-on time and the dead time (time required to change energy layers and/or lateral position). We studied ways to reduce both beam-on and dead time, targeting a total treatment time of 5 seconds. We tested this approach on a small lung tumor patient plan (CTV volume: 65 ml), simulating a 3 Gy(RBE)/field hypofractionation scenario. Materials and Methods To reduce the beam-on time, we increased the beam current reaching the patient by developing new beam optics for PSI’s PROSCAN beamline and Gantry 2. Experimentally we obtained up to factor 5 higher beam current transmission. A more efficient tuning also achieved a factor 25 higher transmission but resulted in 1.5 times larger pencil-beam size at the patient. To reduce the dead time between the spots, we used spot reduced plan optimization [1]. This technique reduces the number of spots by 85% compared to the in-house clinical planning system for the lung case considered. Adding an energy layer reduction algorithm can further lower the dead time, by reducing the energy layers by 37% compared to the PSI plan. As such, we planned the same case using five different scenarios, based on three different treatment planning optimization methods (Table 1). Scenarios 1-3 were delivered with both the clinical and improved optics if PSI Gantry 2, whereas for scenarios 4-5 (with larger beam sizes and higher transmissions), we used only the improved optics. For all scenarios, we extracted treatment times, due to both beam-on time and dead time, from the log-files.

Results For traditional beam currents, the beam on time is the dominant factor and new beam optics with higher beam current bring a significant improvement. In figure 1, we showed results for a single field only, for the second field results are the same. A factor 5 higher-intensity beams deliver 3 Gy(RBE)/field in around 1- second beam-on time. With factor 25 high-intensity beams, beam-on time is below 0.5 seconds. Spot reduction and energy layer reduction bring down the dead time by 50% compared to the clinical plan. For our delivery scenario, the best combination is to use spot reduction and energy layer reduction, together with high- intensity beams, resulting in a total treatment time of 3 to 4 seconds.