ESTRO 2021 Abstract Book

S21

ESTRO 2021

PH-0046 Benefit of 4D robustly optimized proton plans for NSCLC patients with intrafractional motion > 5mm S. Spautz 1 , L. Haase 1 , M. Tschiche 2 , S. Makocki 2 , E.G. Troost 1,2,3,4,5 , C. Richter 1,2,3,4 , K. Stützer 1,3 1 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany; 2 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; 3 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany; 4 German Cancer Consortium (DKTK), Partner Site Dresden, and German Cancer Research Center (DKFZ), Heidelberg, Germany; 5 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany, and; Helmholtz Association / Helmholtz-Zentrum Dresden - Rossendorf (HZDR), Dresden, Germany Purpose or Objective The inclusion of breathing-induced variation using 4D robust plan optimization (RO) may yield more robust proton plans for non-small cell lung cancer (NSCLC) patients. Here, we analysed the benefit of 4DRO in terms of robustness against different motion effects. Materials and Methods Five NSCLC patients with relevant intrafractional motion in the primary (CTVp; on average 3.2-11.1mm) and nodal clinical target volumes (CTVn; 0.9-7.4mm) were included. CTVs and organs at risk (OARs) were contoured on the planning (pCT) and up to two control 4DCTs (cCT). In RayStation 7.99 (RaySearch, Sweden), we optimized three robust normo-fractionated plans [dose: 66Gy(RBE)] with our clinical criteria of 5mm setup and 3.5%+2mm range uncertainty: RO on the average CT with density override of the primary integral gross tumour volume (3DRO); RO on the average, minimum, maximum and mid inspiration CT image (4DRO3); and RO on the average CT and all eight 4DCT phases (4DRO8). On each of the average, minimum and maximum inspiration pCT, 16 setup and range error scenarios were analysed. To assess the influence of intrafractional changes, a 4D dose was calculated for the pCT and compared to those for the cCTs assuming equal weights of all breathing phases. Interplay effects were simulated by 4D dynamic dose (4DDD) scenarios on the pCT using a logfile-based dose reconstruction with machine logfiles from mock treatments with and without 5 layered rescans and the breathing signals from 4DCT acquisition. To account for a possible fractionation effect within the first fractions, we accumulated 4DDD scenarios with 4 different starting times. Results All nominal plans fulfilled target coverage ( D 98% >95%) and OAR sparing; 3DRO achieved lower mean lung dose [up to 0.3 Gy(RBE)] in 4 patients and lower V 5Gy of contralateral lung (up to 4pp). CTVp/CTVn coverage failed setup and range robustness on average in 7%/17% (3DRO), 9%/10% (4DRO3) and 9%/12% (4DRO8) of the scenarios, respectively. 4D dose target coverage on the pCT remained >97% and within 0.5pp difference to the nominal results for both CTVp and CTVn for all planning strategies; however, interfractional changes in the cCTs reduced mainly the CTVp coverage by about 2.5pp, 2.7pp and 2.5pp in the case of 3DRO, 4DRO3 and 4DRO8 plans, respectively. Compared to the nominal plans, single 4DDD scenarios showed a larger mean loss of CTVp/CTVn coverage in 3DRO plans (2.9pp/2.0pp) than in 4DRO plans (4DRO3: 2.2pp/1.6pp, 4DRO8: 2.2pp/1.9pp). Rescanning improved the D 98% values by less than 1pp on average, but was even worse for single scenarios. Irrespective of rescanning, target coverage was restored to clinical acceptance (>95%) in all cases when considering potential fractionation on the pCT.