ESTRO 37 Abstract book

S1062

ESTRO 37

to high dose per fraction schemes. A proper balance should be achieved between high doses and toxicity to obtain the best clinical outcome for the patient. EP-1951 Evaluation of repainting for moving targets treated with continuous or pulsed scanned proton beams D. Dumont 1 , C. Ribeiro 2 , G. Janssens 3 , X. Geets 1 , A. Knopf 2 , E. Sterpin 4 , A. Meijers 2 1 Cliniques Universitaires Saint-Luc, radiotherapy, woluwe-saint-lambert, Belgium 2 University Medical Center Groningen UMCG, Department of Radiation Oncology, Groningen, The Netherlands 3 Ion Beam Applications IBA, Advanced Technology Group, Louvain-la-Neuve, Belgium 4 KU Leuven, Department Of Oncology- Laboratory of Experimental Radiotherapy, Leuven, Belgium Purpose or Objective Treatment of lung tumours with pencil beam scanned proton therapy (PBS-PT) entail specific challenges due to the high sensitivity of protons to density variations and the interplay between spot delivery and tumour motion. Repainting has been demonstrated as an effective strategy to mitigate the interplay effect. Proton facilities based on isochronous cyclotrons (IBA, ProteusPlus(PPlus)) with continouse beam extraction can deliver scaled repainting (further referred to as “controlled repainting”). Facilities based on synchrocyclotrons (IBA, ProteusOne (POne)) feature “intrinsic repainting” (uncontrolled) because their delivery is performed in multiple pulses. The aim of this study is to assess the intrinsic repainting and compare its performance to standard controlled repainting. Material and Methods For five non-small cell lung cancer (NSCLC) patients plans were created using RayStation. The prescription dose was 60 Gy. All plans were 4D robustly optimized based on the full inhale and full exhale phase, accounting for set-up errors of 5 mm and range uncertainties of ±3%. Three sets of plans were obtained per patient: (P1) PPlus without repainting, (P2) PPlus with 5 controlled repaintings, (P3) POne with intrinsic repainting. The treatment plans were reconstructed based on log file data and subplans, were each subplan corresponds to a specific 4DCT phase. For the splitting into subplans 5 s breathing cycle was assumed. To evaluate the plans, the voxel-wise minimum of the target (minimum dose obtained from all the scenarios in each voxel of this volume) was calculated with a dedicated tool [1], that simulates 14 scenarios of setup and range errors. As a result, effects of breathing motion, interplay, setup errors and range uncertainty were evaluated by analyzing the V 95 of the target. Results Target coverage was sufficient for all nominal plans (V95 > 99.5%). Results from the robustness evaluation are shown in Table 1 and Figure 1. All 4D plans recalculated for PPlus showed lower V 95 values than the initial plan. POne obtained adequate coverage for all patients just employing intrinsic repainting. The use of a range shifter, which increase the spot size in POne plans may explain partially these results.

Conclusion We confirmed that repainting can be used effectively to mitigate the interplay effect. Furthermore, the intrinsic repainting of POne shows to be more effective and re- liable in mitigating motion effects than the controlled repainting performed with PPlus. Future work includes the confirmation of these results for a different number of controlled repaintings. Also the use of a range shifter for the PPlus machine will be further evaluated to exclu- de any bias in the comparison between PPlus and POne. Ref: [1] C. Ribeiro, “Comprehensive prospective evaluation tool for treatments of thoracic tumours with scanned protons”, EP-1625, ESTRO 2017 EP-1952 CTV-PTV margin calculation for lung SBRT treatments. A. Prado 1 , G. Pozo 2 , A. Milanés 2 1 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain 2 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain Purpose or Objective To obtain CTV-PTV margins for lung SBRT considering inter-fraction and intra-fraction motion, error delineation, respiration, penumbra contributions, number of fractions, a 90% confidence level (α=2.5) and 95% prescription isodose (β=1.64). Material and Methods SBRT treatments were performed on a Varian Clinac iX (Varian Medical Systems, Palo Alto. CA) with an On Board Imager (OBI) unit. A cohort of 10 NSCLC patients was selected, with a dose prescription of 60 Gy in 8 fractions (7.5 Gy/fx). After a proper patient immobilization using a thermoplastic body mask, a cone beam (CBCT) was performed. Following the registration procedure a second CBCT was acquired to account for set up variations. At the end of the treatment delivery a third CBCT was made. Intra-fraction motion was estimated as the difference between pre and post treatment CBCT registrations (Δx lat , Δx vert , Δx long ). Inter-fraction motion was calculated as the difference between pre-treatment CBCT registrations of consecutive fractions. Delineation error was set to 1.5 mm. Respiration uncertainties were estimated as 1/3 of the standard deviation in each direction obtained as of the 10 GTVs delineated using every 4DCT phase. Penumbra standard deviation was obtained from published data (6.4 mm for lung). The expressions utilized for margin calculations were the following:

Figure 1: Equations to calculate different contributions of the margin recipe.