ESTRO 38 Abstract book

S920 ESTRO 38

the corresponding static dose map through a local gamma analysis (2%-2mm, 30% dose threshold) and absolute dose difference evaluation for each diode of both detector matrices. Results The agreement between static and calculated dose maps showed gamma > 98.5% for all plans. Figa shows that gamma passing rate of dynamic dose distributions is always acceptable (>95%) for the 2 cm peak-to-peak amplitude breathing pattern, for dual half arc template, except for the 6 s period motion for which a value of 91.2% was found. At the extreme peak-to-peak amplitude of 4 cm the gamma passing rate decreased to 70%. As shown in Figb, the gamma passing rates as a function of the different arc templates for the worst breathing pattern scenario (A=4cm, T=6s) are always < 75%. Analyzing the absolute dose deviations between dynamic and static measurements for both detector plates, the blurring effect is clearly predominant at the edge of the fields, but discrepancies up to 4 Gy were observed in the central high dose area, suggesting a possible interplay effect (Fig. c).

Purpose or Objective Electronic portal imaging device (EPID) dosimetry with flattening filter free (FFF) beams is still challenging, and is not supported by Varian commercial solution for aSi- 1000 EPID model. Moreover, in order to develop a model for in vivo 3D verification of stereotactic body radiotherapy (SBRT) treatments, the continuous mode should be characterized for small field sizes and high dose- rates, which could saturate EPID response. This study aims at investigating the dosimetric response of an aSi-1000 EPID operating in continuous mode under these challenging conditions, for further in vivo SBRT verifications. Material and Methods An aSi-1000 EPID installed on a Varian TrueBeamSTx was irradiated with 6 and 10 MV FFF photon beams using the maximum available dose rates (1400 and 2400 MU/min, respectively), at variable source-detector distance (SDD) of 150 and 180 cm. Different EPID imaging sets were acquired in continuous mode (C.M.) and were also compared to the commonly used integrated mode (I.M.), in order to study relevant dosimetric characteristics, such as: dose linearity, repeatability and reproducibility of EPID response, ghosting effect and field-size dependence, where EBT3 film measurements were included for fields in the range of SBRT (0.5 2 –10 2 cm 2 ). Dynamic arc fields were also measured to study EPID dose response dependence, when subject to potential fluctuations in dose-rate and compared to the static irradiation. In-house Matlab software was implemented to automatically process all data, and handling different image formats. Results Saturation of EPID response occurred only for continuous mode at higher dose-rate exposures, conversely to integrated mode, due to different reading schemes. Therefore continuous EPID imaging for 10 MV FFF beams and for larger field sizes (>15 2 cm 2 ) of 6 MV FFF, should be done at 180 cm SDD. Dose response linearity was found to be similar for both modes, varying within 2.5% (I.M.) and 2.1% to 2.9% (C.M.) for static and dynamic arc fields, respectively, and for small MU values (≤5MU). Response repeatability was slightly better for I.M. and 6 MV FFF, due to lower dose-rate, being in all cases within 0.6%. Reproducibility (over 7 months) was well within 0.5% for both modes and energies. Field size dependence of EPID response in both modes agreed within 1%. However, when compared to EBT3 film, major discrepancies (>3%) were found for fields ≤ 1 2 and 1.5 2 cm 2 , in 6 and 10 MV, respectively. Signal increasing due to ghosting effect was within 1% to 1.2% for the configuration with the highest pre-irradiation dose (300MU), being comparable to the signal variations found between continuous acquisition frames (± 1.1, 1SD). Conclusion The dosimetric response of aSi-1000 EPID in continuous mode with FFF beams and high dose rates showed comparable results to the commonly used integrated mode, and dosimetric properties similar to those of FF beams. These results are promising to perform 3D verifications of SBRT with dynamic techniques using continuous EPID imaging. EP-1710 Update ADAM-pelvis phantom: New possibilities to simulate treatment scenarios in radiotherapy W. Johnen 1,2 , A. Runz 1,2 , N. Homolka 1,2,3 , N. Niebuhr 1,2,3 , P. Mann 1,2 , B. Beuthien-Baumann 4,5 , C. Gillmann 1,2 , A. Pfaffenberger 1,2 , A. Elter 1,2,3 , A.L. Hoffmann 5,6,7 , E. Troost 2,7,8,9,10 , S.A. Körber 11 , G. Echner 1,2 1 German Cancer Research Center, medical physics, Heidelberg, Germany ; 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany ; 3 University of Heidelberg, Faculty for Physics and Astronomy, Heidelberg, Germany ; 4 German Cancer

Conclusion Our preliminary results suggested that the DWA treatment could be safely implemented for lung SBRT without tumor tracking if the tumor motion does not exceed 2 cm. Further investigations are required to evaluate the impact of 3D irregular tumor motion on dose delivery, as well as the impact of the interplay effect inside the target. Nevertheless, we expect a low interplay effect due to the low dose rate, constant gantry and ring speed and high fraction doses. EP-1709 Dosimetric response of aSi1000 EPID continuous imaging of FFF beams for in vivo 3D SBRT verification A.R. Barbeiro 1 , L. Parent 2 , T. Younes 1 , L. Vieillevigne 2 , F. Chatrie 1 , F. Younan 3 , X. Franceries 1 1 UMR-1037- Inserm- Université Toulouse III Paul Sabatier, Centre de Recherches en Cancérologie de Toulouse, Toulouse, France ; 2 IUCT-Oncopole, Dpt. Ingénierie et physique médicale, Toulouse, France ; 3 Groupe Oncorad- Garonne- Clinique Pasteur, Service de Radiothérapie, Toulouse, France