ESTRO 37 Abstract book

S1057

ESTRO 37

different flattening filter free (FFF) photon energy for VMAT prostate cancer. EP-1943 Is dose escalation in intracranial pediatric ependymoma feasible with advanced radiation techniques? F. Tensaouti 1 , A. Ducassou 2 , L. Chaltiel 3 , S. Bolle 4 , J.L. Habrand 5 , C. Alapetite 6 , B. Coche-Dequeant 7 , V. Bernier 8 , L. Claude 9 , C. Carrie 9 , X. Muracciole 10 , S. Supiot 11 , A. Huchet 12 , J. Leseur 13 , C. Kerr 14 , G. Hangard 2 , A. Lisbona 11 , F. Goudjil 6 , R. Ferrand 2 , A. Laprie 15 1 ToNIC- Toulouse NeuroImaging Center- Université de Toulouse- Inserm- UPS-, ToNIC- Toulouse NeuroImaging Center- Université de Toulouse- Inserm- UPS-, Toulouse, France 2 Institut Universitaire du Cancer de Toulouse Oncopole, Department of Radiation Oncology, Toulouse, France 3 Institut Universitaire du Cancer de Toulouse Oncopole, Department of biostatistics, Toulouse, France 4 Institut Gustave Roussy, Department of Radiation Oncology, Paris, France 5 Centre Francois Baclesse, Department of Radiation Oncology, Caen, France 6 Institut Curie, Department of Radiation Oncology, Paris, France 7 Centre Oscar Lambret, Department of Radiation Oncology, Lille, France 8 Centre Alexis Vautrin- Vandoeuvre, Department of Radiation Oncology, Nancy, France 9 Centre Leon Berard, Department of Radiation Oncology, Lyon, France 10 CHU La Timone, Department of Radiation Oncology, Marseille, France 11 Institut de cancerologie de l'ouest, Department of Radiation Oncology, Nantes, France 12 CHU Bordeaux, Department of Radiation Oncology, Bordeaux, France 13 Centre Eugene Marquis, Department of Radiation Oncology, Rennes, France 14 Institut regional du Cancer Montpellier- Val d'Aurelle, Department of Radiation Oncology, Montpellier, France 15 Institut Universitaire du Cancer de Toulouse Oncopole, Department of Radiation Oncology-, Toulouse, France Purpose or Objective For pediatric ependymoma, previous publications described a dose effect on survival [1] and a majority of relapses within the high dose regions [2]. A survival benefit was described with SBRT on residue [3]. MacDonald et al [4] found that proton beams and IMRT have similar target coverage, but normal tissue sparing was better in proton. This prompted us to perform a dose escalation approach with an in-silico dosimetric comparison between Volumetric Modulated Arc Therapy (VMAT) photon therapy and Intensity Modulated Proton Therapy (IMPT). Material and Methods The cohort included 101 patients (60.4 % with posterior fossa), from the national multicentric PEPPI database [2].The dose to PTV59.4 was 59.4/1.8 Gy and the dose to simultaneous integrated boost volume, PTV67.6 was 67.6/2.05 Gy (this dose being chosen as an equivalent to sequential SBRT boost in the European trial NCT02265770). The Gross Total tumor (GTV) was defined as the tumor bed plus residual tumor, CTV59.4 was GTV+5 mm, PTV59.4 = CTV59.4 +3 mm. PTV67.6 was GTV+ 3 mm. Ballistic choices were made regarding the risk on brainstem, incorporating the possibility of a higher RBE at the end of the range of the spread out Bragg peak. All treatment plans were optimized with the physical dose-volume objectives/constraints (for targets as well as OARs). All plans were created by the same physicist using RaySearch System planning (Raysearch laboratories,

Stockholm, Sweeden, v5.0) and validated by an experienced radiation oncologist. Doses to target volume and 11 OARs were extracted from the HDV and treatment plans quality indices were derived. Results The mean integral dose was much lower with IMPT compared with VMAT (p<0.001). Planned target volume coverage (V95%) was similar. Conformity (CI) and homogeneity indices (HI) were better with protons for most cases (p<0.001). Mean and maximum doses to organs at risk were significantly lower with protons (p<0.001). In cases of posterior fossa tumor, with OARs in close vicinity to the PTV, a significant gain with protons was observed on dose for the brainstem, on V59 Gy, mean and D2% near maximum doses. Conclusion Dose escalation in pediatric ependymoma is feasible with both techniques but there is a potential significant gain on dose to OARs, including brainstem, with protons. One perspective for this study is LET based optimization. The other one is to stratify the population that could undergo such dose increase based on residue presence [3], advanced imaging factors [5, 6] and molecular biology [7] . References 1 - Ducassou et al, ESTRO 2014 2 - Tensaouti, et al. Radiother Oncol. 2017 3 - Massimino et al, Neuro Oncol. 2016 4 - MacDonald, et al.Int J Radiat Oncol Biol Phys. 2008 5 -Tensaouti et al, BJR 2016 EP-1944 Brain stereotactic Radiotherapy : four versus three table rotation position R. Garcia 1 , G. Francois 1 , E. Jaegle 1 , V. Bodez 1 , C. Khamphan 1 , M. Alayrach 1 , A. Badey 1 , P. Martinez 1 1 Institut Sainte Catherine, Physics, Avignon, France Purpose or Objective The implementation of stereotactic treatments with the help of VMAT irradiation were configured in the way to be close to the initial practice based on cones. To optimize the dose distribution, four table rotation positions were fixed in the dose planning process. Even if the dose distributions were not comparable, VMAT offered the benefit of accessing the entire sphere around the cranium and new advantages appeared covering multiple targets with the MC. However the four table position brought to a long session duration. The purpose was to consider removing one table position and validate that the dose distributions remained acceptable. Material and Methods The brain targets are irradiated with the use of VMAT on four table rotations : 0°, 45°, 270° and 315°. The stereotactic brain plans of 15 patients were re- planned, removing the 270° table position and using the same inverse planning parameters. The prescribed total dose was between 24 Gy and 33 Gy. The number of targets were 1 to 3 with volumes between 2 and 41 cc. The collimator rotations were adapted to optimize the MLC effect on the modulation. The comparisons were established with the use of indexes: Homogeneity, Conformity, healthy tissue coverage and target coverage. Other parameters were collected : MU amount, CTV Dmax, PTV Dmax, Brainstem Dmax, Braim Dmean and V10 Gy. The session duration was evaluated for both configurations. Results The dose distributions were compared and found with similar clinical acceptability. This first analyze was confirmed with the different indexes and parameters. 6 - Tensaouti et al, ESTRO 2016 7 – Pajtler et al, cancer cell, 2015