ESTRO 2023 - Abstract Book

S1529

Digital Posters

ESTRO 2023

significant differences. The most particular difference is the special ring configuration in Halcyon where it was no possible to get no-coplanar beams.

PO-1804 Carbon ion arc therapy with continuous gantry/patient rotation

L. Volz 1 , L. Blechschmidt 1,2 , Y. Sheng 1,3 , M. Durante 1,4 , C. Graeff 1,2

1 GSI Helmholtz Center for Heavy Ion Research GmbH, Biophysics, Darmstadt, Germany; 2 TU Darmstadt, Department of Electrical Engineering and Information Technology, Darmstadt, Germany; 3 Shanghai Proton and Heavy Ion Center, Department of Medical Physics, Shanghai, China; 4 TU Darmstadt, Institute of Condensed Matter Physics, Darmstadt, Germany Purpose or Objective Carbon ion arc therapy (C-Arc) promises a high dose averaged LET (LETd) focus inside the target, relevant for overcoming tumor radioresistance. To ensure delivery time efficiency, a continuous gantry/patient rotation (e.g., with a chair) is preferential over a step-and-shoot delivery. In order to estimate the dosimetric impact of such an approach, we developed a framework for 4D (3D + rotation) biological dose calculation for carbon ion arcs. Materials and Methods Robust biologically optimized C-Arc plans were generated with the TRiP98 research treatment planning platform. Since energy changes are time consuming for synchrotron generated carbon ion beams, the plans featured a single energy per angular control point. For calculating the delivered dose under continuous patient rotation, we expanded an in-house beam delivery simulation toolkit to C-Arc, which enabled to compute the delivery time stamp for every beam spot. We considered a beam time structure similar to that of the Heidelberg Ion-Beam Therapy (HIT) accelerator, with spill pauses of 4s and maximum spill extraction times of 5s. To maximize the efficiency, adjacent angular control points were delivered in the same spill, when possible. Spill pauses were added, if the maximum spill extraction time was exceeded, or when there were not enough particles left, or when the beam energy changed. The dynamic intensity control system used at HIT, replicated in the simulation tool, automatically set the beam intensity for each spot. The delivered biological dose was calculated with TRiP98 based on a constant gantry/patient rotation during the spill, and a rotation stop during spill pauses. The geometry was updated every 0.1° step. Results Step-and-shoot C-Arc took ~10 minutes to deliver. For energy selection schemes with mono-energetic arc segments arriving at similar plan quality, the treatment time could be reduced to approximately 7 minutes with continuous rotation during delivery (Figure 1a). For continuous delivery, the dosimetric accuracy suffered slightly, with D95 = 96.3% compared to the 98.5% of the nominal plan (Figure 1b-d). Similar V95 decreased to 97.5% from 99.4%. Continuous rotation during delivery had negligible effect on the LETd distribution (Figure 2).

Figure 1: Comparison of the time efficiency and dosimetric quality of step-and-shoot and continuously rotating delivery.