Abstract Book

S948

ESTRO 37

EP-1769 First PDIP validation for stereotactic treatment control using FFF beams and EPID aS1000 on Clinac S. Muraro 1 , D. Trauchessec 1 1 Clinique Clementville, radiotherapie, Montpellier, France Purpose or Objective Varian Medical System® (Palo Alto, CA) proposes Portal dosimetry as an integrated solution to control modulated beam treatment. Portal dosimetry is composed of an Amorphous Silicium detector, Portal Dose Image Prediction (PDIP) algorithm, and analysis software. Among all Varian Electronic Portal Image Detectors (EPID), only the Portal Vision aS1200 allows verification of Flattening Filter-Free (FFF) treatment plans with PDIP algorithm. Many centers with a previous EPID, such as aS1000, are forced to use other methods for treatment verification which requires additional investment. Indeed the use of the PDIP algorithm is not possible with the EPID aS1000 for FFF beams and the integrated image acquisition is locked with this kind of beams. The purpose of this study is to verify the feasibility of setting up these controls by PDIP concerning the as1000 EPID model and modulated FFF beams performed on Varian Clinac. Material and Methods The EPID aS1000 response was studied for X6 FFF beams by analyzing source detector distance (SDD), linearity, field size, remanence and backscattering arm effects. Repeatability tests were performed in each case. PDIP algorithm was configured for pre-treatment verification of FFF beams at 1400 MU/min dose rate by placing the detector at SDD 150cm instead of SSD 100cm to avoid saturation. The use of the Varian Portal Dosimetry software was validated using test plans provided by constructor including 8 open static fields and 4 modulated beams according to the specific criteria. A retrospective study was carried out on 34 stereotactic treatment plans in VMAT with X6 FFF corresponding to 140 beams. The plans were irradiated with the Varian Clinac 2300iX. Gamma analysis was performed on a Sun Nuclear ArcCheck® (Sun Nuclear, FL) plan control phantom. The gamma criteria of 2% 2mm and 2.5% 2.5mm for both global and local methods analysis were used. Results Imager response showed that SDD higher than 130 cm avoids the saturation phenomenon. Dose linearity was within 0.99%. The backscattering arm effect results in asymmetries on the vertical dose profiles of 0.88±0.36 %. Integrated dose images had showed a remanence dose resulting on an overstatement of the dose of 0.38 % at the maximum. The repeatability of the measurement showed mean dose deviations of 0.06±0.17 % and measured maximum dose deviations of 0.45±0.36%. The results of the retrospective study carried out from treatment plans under stereotactic conditions are presented in the figure below.

ray in TrueBeam™, and the surface dose in the case of the presence of a magnetic field tends to be close to the surface dose of high energy X rays in TrueBeam when the irradiation field was 10.5 × 10.5 cm² or more. Conclusion The surface dose in MRIdian® becomes lower by the presence of the magnetic field. It was demonstrated that the surface dose in MRIdian® was comparable to that of high energy in conventional linac. EP-1768 Therapeutic Analysis of the MR-Linac Radiation Therapy for Stereotactic Radio-Surgery Treatments S. Gholami 1 , F. Longo 2 , H.A. Nedaie 3 , A. S.Meigooni 4 1 Cancer institute- Radiotherapy Oncology research centre, Tehran University of Medical Sciences, Tehran, Iran Islamic Republic of 2 University of Trieste and INFN Trieste, Department of Physics, Trieste, Italy 3 Cancer institute- Tehran University of Medical Sciences, Radiotherapy Oncology research centre, Tehran, Iran Islamic Republic of 4 Comprehensive Cancer Centers of Nevada, Radiotherapy, Nevada, USA Purpose or Objective MR-Linac machines are being developed for image-guided radiation therapy, particularly in stereotactic radio- surgery (SRS) treatments. However, the magnetic field of such machines could affect the direction of the secondary electrons, hence change the pattern of dose distributions. The purpose of this project was to evaluate the impact of magnetic fields on therapeutic advantages of SRS treatments with circular cones. Material and Methods The Geant4 toolkit (Version 10.1.p02) was used to simulate 6 MV photon beam from a Varian2100C linear accelerator that is being used for SRS treatment with circular cones. A homogenous magnetic field of 1.5 Tesla was applied in perpendicular directions relative to the radiation beam. Percentage depth dose (PDD) curves and beam profiles in a water phantom (20 × 20 × 20 cm 3 ) were calculated with and without presence of the magnetic field. The equivalent uniform dose (EUD) and the therapeutic ratio (TR) for SRS cones with diameters of 10 mm, 20mm and 30 mm were calculated. These calculations were performed for different dose prescriptions using the Hug–Kellerer (H-K) radiobiological The results indicate that the applied magnetic field lead to the higher value of therapeutic ratio in the presence of the magnetic field compared to its absence. The TR values of a semi-sensitive tumor for applying magnetic field in the transverse direction for SRS cones with diameters of 10, 20 and 30 mm, for a prescribed dose of 10 Gy were 15%, 37.7% and 41.5% higher than the TR values from non-presence of magnetic field, respectively. For the above conditions, the EUD values have improved 34% , 72% and 84% at presence of the magnetic field for SRS cones with diameters of 10, 20 and 30 mm, respectively. Moreover a higher therapeutic response was obtained for more radio-resistant tumors undergone stereotactic radio-surgery technique using MR-Linac. On the other hand, for the radio-sensitive tumors the therapeutic response remains unchanged. Conclusion In conclusion stereotactic radio-surgery technique at presence of the magnetic field can provide a possible therapeutic advantage on sparing more normal tissue, especially for the tumors with more radio-resistant characteristics. This effect could be due to a higher EUD value for MR-Linac configuration at presence of the magnetic field. model. Results