ESTRO 38 Abstract book

S508 ESTRO 38

varying patient anatomy, and less sensitivity to planner experience and skills. [1] Powis et al., RadiatOncol 12 (2017) 81 PO-0942 Optimization of adaptive aperture margins in robustly optimized pencil beam scanning proton plans G. Vilches Freixas 1 , M. Unipan 1 , I. Rinaldi 1 , J. Martens 1 , C. Ares 1 , G. Bosmans 1 1 Maastro Clinic, Department of Radiation Oncology, Maastricht, The Netherlands Purpose or Objective At the end of the beam line of the Mevion S250i proton therapy system there is an adaptive aperture to reduce lateral penumbra, especially at low energies, by laterally trimming the spots at the edge of the treatment field. The goal of this study was to determine the optimal aperture margin (AM) from the CTV in a robust optimized plan for different anatomical sites, treatment volumes, and number of beams. Material and Methods In a water phantom, a L-shaped volume simulating a CTV with 10 cm the largest x, y and z dimensions was placed at different depths from 1 up to 15 cm from the phantom surface. In the first phase of the study, nominal non-robust Monte Carlo (MC) 60 Gy(RBE) (2 Gy(RBE) x 30 fractions) plans were created. Using the scripting module in Raystation 8.A, plans varying from single-beam up to 4 beams were generated for every target volume depth. In each plan, the AM margins from the target volume were varied from 2 mm to 10 mm in 2 mm steps. In the second part of the study, 120 robust optimized MC plans taking into account 4 mm setup uncertainty (SU) and 3.5% range uncertainty were created with different AMs. In a robust optimized plan, the SU uncertainties should also be taken into account in the aperture margin. For this reason, the AM in a robust optimized plan (AM_RO) are defined as the AM in a nominal plan plus the SU: AM_RO = AM_nominal + SU. Moreover, to check this hypothesis, extra 2 mm margins were added and subtracted to the AM_RO in a robustly optimized plan. Plan robustness, after plan optimization, was further evaluated with the previous robustness settings (4 mm setup and 3.5% range uncertainties, respectively) by calculating 28 different scenarios of combined uncertainties via an automated script. In both phases of the study, the following metrics were used to drive a decision: Homogeneity Index (HI), D98%, D2%, D95% and the Conformity Index (CI). The results were benchmarked against 6 head and neck and 9 central nervous system brain tumour patients. Results The effect of the AM was more predominant at shallow depths, as expected. More beams allow tighter AMs. The optimal AM was found to be: 4 mm + SU for 1 to 3 beams, 2 mm + SU for 4 beams, and 6 mm + SU for 1 to 2 beams in the case of shallow tumours (Fig. 1 and 2). For the last situation, AM had to be enlarged to improve target coverage. Furthermore, based on the clinical validation, an AM of 6 mm + SU was found to be optimal also for small tumours close to air cavities.

Conclusion Optimal AMs for robustly optimized pencil beam scanning (PBS) proton plans for the Mevion S250i were derived and validated for 15 patient cases. The use of apertures in PBS improves lateral penumbra and potentially reduces the dose to the organs at risk. PO-0943 Harmonization of proton planning for head and neck cancer using PBS: First report of the IPACS collaboration M. Stock 1 , J. Gora 1 , A. Perpar 2 , P. Georg 2 , G. Kragl 1 , E. Hug 2 , V. Vondracek 3 , J. Kubes 4 , C. Algranati 5 , M. Cianchetti 6 , M. Amichetti 6 , T. Kajdrowicz 7 , R. Kopec 7 , P. Olko 7 , K. Skowronska 7 , U. Sowa 7 , E. Gora 8 , K. Kisielewicz 8 , B. Sas-Korczynska 8 , T. Skora 8 , A. Bäck 9 , M. Gustafsson 10 , M. Sooaru 10 , P. Witt Nyström 11 , T. Björk Eriksson 12 1 MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria ; 2 MedAustron Ion Therapy Center, Radiation Oncology, Wiener Neustadt, Austria ; 3 Proton Therapy Center Czech, Medical Physics, Prague, Czech Republic ; 4 Proton Therapy Center Czech, Radiation Oncology, Prague, Czech Republic ; 5 U.O. di Protonterapia- Azienda Provinciale per I Servizi Sanitari Trento, Medical Physics, Trento, Italy ; 6 U.O. di Protonterapia- Azienda Provinciale per I Servizi Sanitari Trento, Radiation Oncology, Trento, Italy ; 7 Institute of Nuclear Physics Polish Academy of Sciences- Cyclotron Centre Bronowice, Medical Physics, Krakow, Poland ; 8 Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology, Radiation Oncology, Krakow, Poland ; 9 University of Gothenburg, Radiation Physics, Gothenburg, Sweden ; 10 Sahlgrenska University Hospital, Therapeutic Radiation Physics, Gothenburg, Sweden ; 11 The Skandion Clinic, Radiation Oncology, Uppsala, Sweden ; 12 Sahlgrenska University Hospital, Oncology, Gothenburg, Sweden Purpose or Objective Clinical evidence for the advantage of proton therapy (PT) is still limited. To show superiority larger scale clinical trials in a collaborative network must be performed. A collaborative network between PT centres in Trento in Italy, Poland, Austria, Czech Republic and Sweden (IPACS) was founded to implement those trials and harmonize PT.