Abstract Book

S1104

ESTRO 37

ITV 5 mm in all directions was done. For all cases, two VMAT plans with 6 MV FF and 6 MV FFF were calculated using a common prescription dose of 60 Gy in 5 fractions with the same optimization parameters. These plans were recalculated in a QA plan using OCTAVIUS 4D phantom (PTW, Germany) for the verification. For the QA treatment plan deliver, we used a new platform (Modus Medical, courtesy of the Hospital Clínico Universitario de Santiago de Compostela) that allowed move OCTAVIUS 4D in craniocaudal direction whereas the device is turning synchronized with the gantry. The period and range of platform movement were the same those GTV parameters acquire in the simulation for each patient. The ionization chamber array used was Octavius SRS1000 (PTW, Germany) with 977 liquid-filled ionization chambers 2x2 mm 2 active area and 2 mm of resolution. Trying to combine the experimental uncertainty with the possible deviations due to the movement, it was decided to use the gamma criterion. All plans were delivered in a TrueBeam STx HD120 MLC (Varian, CA) with 6DoF couch. Results In the graphs, the value of the percentage of points that fulfill the gamma criterion <1 is represented for the treatments calculated with FF and FFF in function of the amplitude of the movement of the GTV. Figure 1 shows the results with tolerance values for dose of 3% of central PTV dose and distance to agreement of 3 mm. For this tolerance, 5 of the 8 patients analyzed meet the criterion in more than 95% of the points. These 5 patients have amplitude of less than 6 mm. The results shown in Figure 2 are the analysis for a tolerance for dose of 2% dose and distance to agreement of 2 mm. In this case, 4 of the 8 patients analyzed, meet the gamma criteria with more than 90% of the points.

Results The relative differences of pixel values were 0.44 ± 0.22, 0.30 ± 0.13, and 0.11 ± 0.09% for 4,6 and 10 MV, respectively. The difference between fiducial marker position in actual and simulated image were 0.005 ± 0.002(LR), 0.060 ± 0.005(SI), 0.014 ± 0.004(AP) mm for 4MV, 0.011 ± 0.012(LR), 0.060 ± 0.003(SI), and 0.006 ± 0.006(AP) mm for 6 MV, and, 0.008 ± 0.009(LR), 0.067 ± 0.011(SI), and 0.026 ± 0.026(AP) for 10 MV. The differences of TMS were 0.10 ± 2.06% (-1.48%−3.10%), 4.88 ± 0.31% (4.54%−5.27%) and 2.54 ± 1.58% (0.26%−3.63%) for 4, 6 and 10 MV. Conclusion We have developed the simulation system to evaluate the tracking accuracy using kV imaging with MV scatter. It was suggested that it was possible to evaluate the tracking accuracy using this system. To archive RTRT with high dose rate, it is needed to evaluate the tracking accuracy in clinical case using this system. EP-2022 Interlay effect in SBRT VMAT lung FFF treatments using ionization liquid chambers array detector J. Bonaque-Alandi 1 , J. Bonaque-Alandi 2 , J.A. Bautista- Ballesteros 3 , J.A. Bautista-Ballesteros 2 , B. Ibanez- Rosello 2 , M. Adria-Mora 2 , J. Gimeno-Olmos 2 , V. Carmona- Meseguer 2 , F. Lliso-Valverde 2 , J. Perez-Calatayud 2 1 consorcio Hospitalario Provincial De Castellón, Radiofísica Y Proteccion Radiológica, Catellón De La Plana, Spain 2 hospital Universitario Y Politécnico La Fe, Oncología Radioterápica, Valencia, Spain 3 hospital Institut Curie, Radiotherapy, Paris, France Purpose or Objective To evaluate the interplay effect in SBRT VMAT lung treatments with flattering filter (FF) and flattering filter free (FFF) beams using a ionization liquid chambers array detector. Material and Methods Eight plans of different patients with 4D computed tomography (4DCT) were used in which each phase was used to generate the internal target volume (ITV). To create the treatment planning volume (PTV), expansion

Conclusion For amplitudes of movement of the lesion in the cranio- caudal direction less or equal to 6 mm, the verification with the system composed by by the experimental platform of movement and the 4D OCTAVIUS with the detector array 1000 SRS, gives the conclusion that the results for treatments of VMAT with flattering filter and without filter are equivalent, being able to make use of