Abstract Book

S316

ESTRO 37

conducted to investigate the full potential of the 3D range-modulator. OC-0602 Characterization of a novel breathing phantom for 4D applications in ion beam therapy N. Kostiukhina 1 , M. Clausen 1 , M. Stock 2 , D. Georg 1 , B. Knäusl 1 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective The in-house developed respiration phantom ARDOS (Advanced Radiation DOSimetry) was recently tested and benchmarked for 4D photon therapy [1]. The aim of this work was the extension towards 4D proton therapy and thus to (1) characterize suitable detectors for proton dosimetry, (2) to determine the dose calculation accuracy of state-of-the art treatment planning systems in simulated lung set-ups, and (3) to validate the CT calibration curve of stopping power ratios for ARDOS materials. Material and Methods ARDOS represents an average human thorax simulating breathing-induced tumor motion embedded in lung tissue of a torso featuring rib cage motion [1]. Treatment planning for scanned proton beams was performed using RayStation v5.99. A cross-calibrated pin point ionization chamber (PP) (TM31015, PTW) and TLDs (TLD-100, SNP14535, Thermo Fisher Scientific) were inserted in the lung tumor replica of the phantom. Three different scenarios were simulated in the static phantom: (a) ribs (b) no ribs and (c) rib/no rib interface (figure1) along the beam path. Dose calculations were conducted applying the Monte Carlo (MC) v4.0 as well as the Pencil Beam (PB) v4.1 algorithms on the basis of the clinical CT calibration curve. Additionally in the underlying treatment plans (TPs) the different phantom components were overwritten with the known material parameters (mass density, mean ionization energies and atomic mass composition) and dose recalculated. For the both objectives dose difference maps were analyzed and the average dose in the regions of interest (ROIs) corresponding to the detector volumes was extracted. Uncertainties in dose calculation were based on the beam model commissioning results revealing a tolerance level of 3%. Results The MC calculated dose in the delineated ROI agreed within 3.4% with the PP measurements for all three scenarios. The MC calculated dose was lower than the TLD measured dose by 2.6% for scenario a) and b) and by 5.3% for scenario c) (table1). Recalculation of the dose on the overwritten CT resulted in a dose difference below 3% within the tumor. Outside the tumor up to 40% (0.9 Gy) were reached (figure1). In order to achieve a homogenous dose in the tumor volume (ø=2cm) distal spots were put up to 3cm behind the target, which were most affected by the material override. MC was superior to PB when benchmarked against PP measurements. Conclusion The PP chamber is a suitable detector for future 4D dosimetry in scanned proton beams. TLD and MC dose values deviated more than PP due to the higher resolution of the TLDs and the sensitivity towards tissue interfaces. The dose in the detector ROIs was robust towards CT calibration. The investigation of the applicability of films especially in the penumbral area is currently ongoing. Systematic investigations in static phantom conditions are the foundation for an uncertainty budget determination in forthcoming 4D experiments with the ARDOS phantom. [1] Kostiukhina et al 2017 PMB

OC-0603 Spot scanning arc therapy for lung cancer: dosimetric improvement and interplay effect mitigation X. Li 1 , P. Kabolizadeh 1 , J. Zhou 1 , D. Yan 1 , C. Stevens 1 , T. Guerrero 1 , I. Grills 1 , X. Ding 1 1 Beaumont Health System, Radiation Oncology, Royal Oak MI, USA Purpose or Objective To determine whether spot scanning proton arc therapy (SPArc) can be safely and effectively implemented to treat mobile tumors in lung cancer patients while compared to the current multi-field robust optimized intensity modulated proton therapy (RO-IMPT). Material and Methods Six lung cancer patients with mobile tumors (motion from 0.5 – 1.4 cm) of average ITV volume of 330 cc (range 77 – 735 cc) were included in this study. Both SPArc and RO- IMPT plans were re-planned on average CT scan of 4DCT using robust optimization (RayStation version 6) with ±5mm setup uncertainties and ±3.5% range uncertainties in order to achieve an optimal coverage to 99% of the ITV with 66 Gy (RBE) in 33 fractions. The normal tissue sparing was performed on the static dose on the average CT scan, while the interplay effect was evaluated based on the ITV coverage using single-fractional 4D dynamic dose. This calculated the doses in synchronization to machine delivery characteristics and breathing cycle without any re-paintings, deformed and accumulated to the end of exhale phase 4DCT scan.