ESTRO 2020 Abstract book

S168 ESTRO 2020

differences in lung inflammation, SUVmean or NLG between pre- and post-treatment. The SUVmean of irradiated lungs and ILG were highly correlated with the ranges of arcs in the dynamic arc-based radiotherapy for EC. Conclusion SUVmean of irradiated lungs and ILG between pre- and post-treatment FDG PET/CT significantly increased in the lungs involved by arcs in the dynamic arc-based radiotherapy for EC.

breast was lower than TF by 19/11/35% respectively, namely 0.2/0.1/0.3Gy; integral dose was 7% lower than TF.

Poster discussion: PH: Treatment planning: applications 2

PD-0301 Virtual Tangential Arc Therapy (ViTAT): toward large scale auto-planning for breast cancer treatment P.G. Esposito 1 , R. Castriconi 1 , P. Mangili 1 , A. Fodor 2 , M. Pasetti 2 , N.G. Di Muzio 2 , C. Fiorino 1 , R. Calandrino 1 1 IRCCS Ospedale San Raffaele, Medical Physics, Milano, Italy ; 2 IRCCS Ospedale San Raffaele, Radiotherapy, Milano, Italy Purpose or Objective To study the feasibility of a new volumetric technique ViTAT (Virtual Tangential Arc Therapy) in mimicking tangential field (TF) irradiation for R-sided whole breast treatment, aiming to future implementation of large-scale knowledge-based (KB) automatic plan optimization. Material and Methods R-sided breast cancer planning (delivering 40Gy/15fr) was considered. ViTAT plans consisted of 4 arcs (6MV) optimized with a RA (Rapid Arc) technique, with start/stop angles consistent with TF geometry; arcs were completely blocked, apart the first and last 20° of rotation, being 20° the smallest deliverable angle in the RA modality. The start/end positions were obtained by an analysis of 47 clinical TF plans, considering the distribution of the beam angle positions. A virtual bolus was used during optimization: its properties were set based on a preliminary study on 5 patients, using 3 different thickness (1–1.5–2cm) with an equal PTV expansion and two values of density (0 and -500HU). Finally, 20 ViTAT plans were optimized with the RA module of the Eclipse Varian system (v13.6), imposing DVH constraints aimed to mimic the corresponding clinical TF plans; during optimization the system was also stressed to possibly improve PTV coverage/homogeneity and R-lung sparing, avoiding any additional low-dose bath to the lung and to the rest of the body. ViTAT performances were compared against clinical TF plans in terms of dose-volume parameters. Results The analysis of TF angles results in modal values of 55°/230° suggesting optimal values of 65°/220° to fully exploit the 20° open portion of the arcs. In order to limit any risk of collision, the medial starting angle was safely set at 60°. The 1.5cm thick bolus, -500HU, was chosen being associated with the lowest mean dose (Dm) and D2% for OARs and the best coverage of PTV. As for the study of the 20 patients, differences between TF and ViTAT were small and generally in slight favor of ViTAT, as summarized in Fig.1: PTV coverage was similar; PTV D1% was improved by 1% (p=0.04); dose homogeneity within PTV was improved (1SD: 0.1Gy, p=0.05) and R-lung Dm was similar (6.8 vs 6.9Gy) with a slight improvement/worsening in the 20-35Gy/2-15Gy range respectively. ViTAT better spared contralateral OARs: ViTAT Dm of hearth/left lung/left

Conclusion Results show that dose distributions obtained with ViTAT are comparable to TF or slightly better in terms of PTV dose distribution and Dm to body and contralateral OARs. The only slight worsening concerns the range 2-15Gy for the R-lung, although this should not be clinically relevant, without any increase of lung Dm. The results suggest that optimal TF-like dose distributions may be obtained with a properly guided arc technique, making easier the implementation of large-scale automatic optimization through KB models based on past and available TF plans. Extending ViTAT to L-sided breast cancer is currently ongoing. PD-0302 Biological uncertainties in proton therapy for paediatric mediastinal Hodgkin Lymphoma M. Aznar 1 , L.A. Rechner 2 , M.V. Maraldo 2 , E. Smith 1 , A. Lundgaard 2 , L.L. Hjalgrim 2 , A. Safwat 3 , R. MacKay 4 , A.H. Aitkenhead 4 1 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom ; 2 Rigshospitalet, Department of Oncology, Copenhagen, Denmark ; 3 Aarhus University Hospital, Department of Oncology and Danish Centre for Particle Therapy, Aarhus, Denmark ; 4 The Christie NHS Foundation Trust, Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective Proton Beam Therapy (PBT) is associated with different types of uncertainties than photon therapy. For example, it is unknown how much the increase in linear energy transfer (LET) and relative biological effectiveness (RBE) at the end of a proton’s range impact the doses to organs at risk (OARs). For paediatric patients with mediastinal Hodgkin lymphoma (HL), increased LET at the distal edge of the beam could, in theory, result in “hot spots” in the heart, oesophagus or lungs. Here, we investigate the impact of different beam arrangements on the LET distribution and dose to mediastinal OARs. In addition, we explore the impact of using a variable RBE in the dose calculation. Material and Methods Three paediatric patients with mediastinal HL previously treated with photon-based radiotherapy were selected for this study (ages 14-16 clinical stage 2A, one with bulky disease). We retrospectively created proton plans for