ESTRO 2020 Abstract Book

S790 ESTRO 2020

consensus guidelines. The priority of the planning was to achieve optimal target coverage (> 98% of the CTV covered with 95% isodose) with acceptable sparing of the heart, LAD and ipsilateral lung. Prescribed dose was 50Gy for photons and 50Gy(RBE) for protons in 25 fractions. Statistical analysis was performed with Wilcoxon signed rank test. Results Proton DIBH and FB plans did not differ significantly in terms of heart, LAD and ipsilateral lung dose reduction. IMPT FB technique allowed for substantial sparing of the heart comparing to photon DIBH IMRT. Mean heart dose (MHD) was 0.14Gy(RBE) vs. 2.12Gy; p=0.04; respectively. Mean ipsilateral lung dose (7.2 Gy(RBE) vs. 12.6Gy; p=0.08) and lung V20 (15,1 Gy(RBE) vs. 22.4Gy; p=0.08) were not significantly different between proton and photon plans. Regarding analyzed LAD parameters mean LAD dose was significantly lower in proton plans comparing to photons (1.16Gy(RBE) vs. 5.57Gy; p=0.043; respectively), whereas LAD maxium dose and D max 0.2cm 3 were not different. Conclusion DIBH does not lead to further improvement of organs at risk dose reduction in proton radiotherapy. Proton FB technique comparing to photon DIBH IMRT ensures significant sparing of the heart in patients with left-sided breast cancer treated with regional lymph nodes. It also allows reducing dose to the left coronary artery. Proton beam radiotherapy could be a promising solution for breast cancer patients demanding locoregional adjuvant irradiation who have particularly high heart and LAD doses and do not benefit from photon DIBH radiotherapy technique. PO-1473 On the clinical implementation of conformity index for radiotherapy of prostate J. Scherman 1 , E. Wieslander 1 1 Skåne University Hospital, Radiation Physics- Department of Hematology- Oncology and Radiation Physics, Lund, Sweden Purpose or Objective The purpose of this study was to evaluate if any difference was observed in the conformity index for prostate volumetric arc therapy (VMAT) treatment plans after an implementation of a clinical guiding value. Material and Methods The conformity index used was Paddick’s Conformity Index (PCI) [Paddick 2000]. PCI is a measure of how well a prescribed isodose volume conforms to the size and shape of a target volume and is calculated according to: PCI = (TV PIV ) 2 /(TV·PIV), where TV PIV is the target volume covered by the prescriptions isodose volume, TV is the Target Volume and PIV is the prescription isodose volume. A retrospective analysis of conformity index for four pelvic VMAT- treatments was performed by the authors [Scherman and Wieslander 2018], where a specific PCI clinical guiding value of 0.89 was implemented for prostate VMAT-plans. A small script was created using Eclipse Scripting API (Varian Medical Systems) where treatment planners could calculate the PCI during the treatment planning process. Treatment plans were retrieved by extracting data for prostate patients 7 months prior to and 7 months after the implementation of the guiding value. A total of 575 prostate VMAT-plans were included in this current study, 284 prior and 291 after the clinical implementation. PCI distributions were compared prior and after implementation using a two-sided unpaired Wilcoxon signed-rank test, where p < 0.05 was considered statistically significant. Results Mean PCI values for the two distributions were both 0.91. No statistically significant difference was observed between the two distributions (p = 0.81, using a two-sided unpaired Wilcoxon signed-rank test). Comparing the two

Image 1: Typical beam arrangement used (135°, 180° and 225°) Conclusion We selected a three beam arrangement rather than two to improve plan robustness to breathing motion and to reduce the effect of anatomical changes along any one beam path at the expense of greater lung dose. Despite this, PBS still showed significant dosimetric advantage in most parameters with acceptable coverage of ITV. Lung V5 is shown to be an independent dosimetric factor in multivariate analysis in reducing the incidence of post- operative pulmonary complications. This study shows lung V5 reduction of over 50% in all 4 cases. Similiarly, both DVH parameters (V40, V25) for heart dose showed a significant reduction which may lower the incidence of post-operative cardiac toxicity. Further considerations will be required during treatment delivery to account for intra/inter- fraction tumour motion, breathing motion and interplay effects. This study indicates that delivering NACRT with PBS for oesophageal cancer may lead to a meaningful reduction in post-op pulmonary and cardiac toxicity. However, there remains a paucity of published clinical data for PBS in this setting. We recommend that NACRT with PBS for distal oesophageal cancer is tested prospectively in a randomised controlled trial. PO-1472 Cardiac dose reduction in proton vs. photon DIBH breast and regional lymph nodes radiotherapy P. Winczura 1 , K. Czerska 2 , J. Wejs-Maternik 3 , A. Blukis 3 , R. Mężykowski 3 , P. Olko 4 , R. Kopeć 2 , A. Badzio 5 1 Radiotherapy Center Elblag, Departent of Radiotherapy, Elblag, Poland ; 2 Institute of Nuclear Physics PAN, Cyclotron Centre Bronowice, Kraków, Poland ; 3 Radiotherapy Center Elblag, Radiotherapy, Elblag, Poland ; 4 Institute of Nuclear Physics PAN, Proton Radiotherapy Group, Kraków, Poland ; 5 Medical University of Gdańsk, Department of Oncology and Radiotherapy, Gdańsk, Poland Purpose or Objective Deep inspiration breath-hold (DIBH) is an effective and widely used technique to reduce the dose to the critical structures in left-sided breast cancer radiotherapy. In this study we compared photon Intensity Modulated Radiation Therapy (IMRT) versus Intensity Modulated Proton Radiotherapy (IMPT) with deep inspiration breath hold (DIBH) and free breathing (FB) on dose reduction to the heart and left anterior descending coronary artery (LAD). Material and Methods Five patients with left-sided breast cancer were planned on DIBH and FB computed tomography datasets. For each patient 6MV photon IMRT and proton IMPT with 2 or 3 fields plans were created. Patients were planned to the CTV which consisted of the left breast and regional lymph nodes: axilla level 1-3 and supraclavicular region. Contouring was performed according to the ESTRO