ESTRO 37 Abstract book

S949

ESTRO 37

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. 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. model. Results

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.

Conclusion The use of AS1000 and Clinac to measure FFF beams is a success. The locks due to the generation of the system have been lifted. The very complex and high dose field checks in VMAT type stereo allowed to validate the modeling.This solution saves measurement time, facilitates use, and improves spatial resolution which can be better than that in available 3D phantoms. However, it was found that the Portal Dosimetry solution could not be used for large field, which is not problematic for stereotactic therapy. EP-1770 New RGB algorithm for dose measurements with EBT3 films tested in 6 MV x-ray beams up to 90 Gy doses K. Alikaniotis 1 , M. Severgnini 2 , G. Giannini 1 1 University/INFN of Trieste, Physics Department, Trieste, Italy 2 Riuniti Hospital ASUITS, Medical Physics Department, Trieste, Italy Purpose or Objective To study the dose response of Gafchromic EBT3 films in a 6 MV x-ray beam up to very high dose levels, 90 Gy, above the recommended 40 Gy value, with a modified function able to provide more accurate dose measurements by weighting the three RGB (Red Green