ESTRO 38 Abstract book

S565 ESTRO 38

and treatment delivery; the Sicily Dosimetric Project was born with this aim, using an approach very similar to external audits procedure. Material and Methods Thirteen centers from Sicily with a good experience in lung IMRT/VMAT treatments and with a great heterogeneity in terms of technologies, have participated to the project. In the first step, shared CT images (with CT calibration curve), with Target and OARs defined for a thorax anthropomorphic phantom, have been used to perform a treatment plan using 6MV energy beam with a prescription dose of 66 Gy in 33 fractions. Imposed planning constraints have been: at least 98% of the PTV received 95% of the prescribed dose and 2% of the PTV received 107% as maximum value; Heart, V 25Gy <10%, D average < 26Gy; Lungs ((Dx+Sx)-GTV): V 20Gy ≤30%; V 5Gy ≤65%; D average ≤20Gy. Dosimetric analysis of treatment plans have been performed considering cumulative DVH curves for PTV and OARs. In the second step, OSL detectors, commissioned and analyzed from a pilot center, have been used. Preliminary measurements have been performed using RW3 slab phantom (30x30x10 cm 3 ) and reference conditions. One OSLD, allocated in apposite slab, situated at depth of 5 cm, has been irradiated at 2 Gy dose, with 10x10 cm 2 field and using SAD set up; the average over three measurements has been compared with expected dose. Verification plans have been generated and delivered in the same RW3 slab phantom using both ionization chamber and OSL dosimeters (repeating measurements three times). Finally, treatment plans have been delivered on the anthropomorphic phantom with end-to-end approach and OSLD allocated in the apposite inserts. Dose has been verified in three points: PTV, Heart and Mediastinum. To check the positioning of the phantom, CBCT, MV or kV, have been used. The anthropomorphic phantom without OSLDs inside has been reseated over the couch 5 times and the average positions for all directions, longitudinal, vertical and lateral, have been taken into account to obtain corrected dose values from TPS. Results Results related to the first step of the project satisfy imposed constraints by most of all centers; mean values for PTV: D 98% =63,50Gy (61,25-65,83), D 2% =69,18Gy (67,10- 70,85); Heart: V 25Gy =9,33% (7,60%-11,65%), D average =9,87Gy (8,45-11,45); Lungs: V 20Gy =22,74% (18,50-27,90), V 5Gy =51,39% (39,48-58,28), D average =12,77Gy (10,71-14,80). Results related to the second step of the project are showed in Tab.1. Fig.1 shows the correlation between algorithms involved in the project and final results related to the anthropomorphic phantom; as it is expected Monte Carlo based algorithms give better results respect to CCC and AAA algorithms.

Fig. 1

Conclusion The Siciliy Dosimetric Project has been able to identify local problems for advanced treatments and could provide indications for future audit approache. PO-1021 Influence of beamline and scanning magnets on the magnetic fringe field at a proton PBS nozzle S. Gantz 1,2 , L. Riemann 2 , J. Smeets 3 , J. Pawelke 1,2 , A. Hoffmann 1,2,4 1 Helmholtz-Zentrum Dresden-Rossendorf, Radiooncology- OncoRay, Dresden, Germany ; 2 OncoRay - National Center for Radiation Research in Oncology, Medical Radiation Physics, Dresden, Germany ; 3 Ion Beam Applications SA, Advanced Technology Group, Louvain- la-Neuve, Belgium ; 4 Faculty of Medicine and University Hospital Carl Gustav Carus at the Technische Universität Dresden, Radiotherapy and Radiation Oncology, Dresden, Germany Purpose or Objective Real-time soft-tissue image guidance is a desirable concept to improve the targeting precision of proton therapy. In 2017, the first prototype of an MR-integrated proton therapy setup was realised at our horizontal fixed research beamline. Moving towards a clinical application this in-beam MRI system shall be transferred to a pencil beam scanning (PBS) research beamline that provides volumetric dose spot delivery. A magnetic survey was performed to quantify the effects of beamline and scanning magnets on the environmental magnetic field. Material and Methods The magnetic fringe field at the PBS nozzle was measured by a tri-axial Hall-probe magnetometer (THM 1176-LF, Metrolab) at two positions: (P1) at a lateral position 700 mm from the center of the scanning magnets, and (P2) at the planned magnetic isocenter of the in-beam MR