ESTRO 2022 - Abstract Book

S199

Abstract book

ESTRO 2022

Conclusion Dose coverage degradation due to target motion during beam delivery is generally small in patients with adrenal gland metastases treated on an MR linac without gating options, but more noticeable degradation may be observed in patients with larger respiratory motion. The pre-treatment target respiration motion observed using 4DCT is conserved during treatment and is therefore valid for PTV margin estimation. The respiratory motion is reduced in patients treated with a compression belt; thus, the belt is a valid alternative to avoid the prolonged treatment time needed for gated treatments.

PD-0238 Experimental demonstration of conformal carbon ion therapy for irregularly moving tumors

T. Steinsberger 1,2 , M. Donetti 3 , M. Lis 1,4 , L. Volz 1 , M. Wolf 1 , M. Durante 1,2 , C. Graeff 1,5

1 GSI Helmholtzzentrum für Schwerionenforschung, Biophysics Department, Darmstadt, Germany; 2 Technical University of Darmstadt, Institute for Condensed Matter Physics, Darmstadt, Germany; 3 Fondazione CNAO, Research Department, Pavia, Italy; 4 Louisiana State University, Department of Physics and Astronomy, Baton Rouge, USA; 5 Technical University of Darmstadt, Department of Electrical Engineering and Information Technology, Darmstadt, Germany Purpose or Objective Treatment of locally advanced NSCLC is limited by toxicity and insufficient local control. Particle therapy could enable more conformal treatment than photon IMRT, but is challenged by irregular tumor motion, associated range changes and tumor deformations. Therefore, we propose a new strategy for robust online adaptive carbon ion therapy, which synergizes 4D treatment planning with online adaptive tumor tracking. The strategy was tested at a clinical carbon ion facility. We hypothesize that the technique is robust against irregular motion and limited motion imaging errors. Materials and Methods In multiphase 4D synchronized deliveries (MP4D), a separate conformal quasi-static treatment plan is created for each motion phase of a periodic 4DCT. During delivery, the motion phase is detected. For each energy slice, the dose delivery system (DDS) switches without delay between phase plans until all phase plans are completed. In the new method ‘MP4D with residual tracking’ (MP4DRT), the displacement of the tumor center-of-mass with respect to the same phase in the planning CT is compensated for by lateral beam tracking. We implemented this method into the DDS of CNAO and tested it experimentally for a patient plan based on a periodic 10 phase subset of a virtual XCAT 4DCT (planned motion amplitude 20mm). Treatments were delivered in a QA-like setting to a moving IC array. Log-file based dose reconstructions were performed on the full non-periodic XCAT 4DCT. MP4DRT was compared to MP4D, tracking and ITV plans. Tracking and ITV plans were rescanned 10x.We considered variable motion amplitudes and baseline drifts. The required precision of tumor imaging was evaluated by adding noise and latency to the motion signal. Results The table shows target coverage (D95), homogeneity (D5-D95) and conformity (CN) for deliveries with 25% amplitude variation +/- baseline drift of up to 8mm. Mode MP4DRT MP4D Tracking ITV Baseline stable drift stable drift stable drift stable drift D95 [%] 98.0 98.2 96.8 69.3 94.0 95.0 95.5 89.3 D5-D95 [%] 5.8 6.3 7.9 37.2 11.2 10.4 7.1 12.6 CN [%] 61.5 61.5 62.6 30.7 43.2 43.2 45.8 16.1 Fig. 1 shows dose distributions delivered for an irregular motion scenario with baseline drift. Fig. 2 compares measurements to planned dose distributions. The impact of imaging errors is shown by gamma pass rate and D95 on the CT reconstruction, where errors up to 2 mm led to clinically acceptable results.