ESTRO 37 Abstract book

S948

ESTRO 37

1 National Cancer Center Hospital, Department of radiation oncology, Tokyo, Japan

Purpose or Objective To reveal surface dose of MRI-guided radiotherapy system (MRIdian®,ViewRay Inc. USA) by comparing with that of between conventional linac (TrueBeam,Varian, USA). Material and Methods MRIdian® consists of a 0.345T double-doughnut MRI coupled with a gantry that equips three Co-60 sources.Radiochromic films (Gafchromic film EBT3, Ashland) were used for surface dose measurements. An EBT3 film was placed on the surface of a 20 cm thickness of water equivalent phantom (Solid Water HE, Gammex) with a SSD of 100 cm. Irradiations were performed for a field size of 4.2×4.2 , 10.5×10.5, 14.7×14.7, and 21.0×21.0 cm² in MRIdian® and TrueBeam™. Measurements were made with and without a magnetic field of 0.345 T in MRIdian®. The photon energy was 4, 6, 10, 15MV X-ray. The irradiated film was then calibrated to convert the net optical density to the absorbed dose in water in advance. The region of interest in the film was within about 1×1 cm at central location of the irradiation field. The surface doses were obtained from each of the film using a film analysis software (DD-system Ver.10.3.2,R-Tech Inc, Japan). Results In all cases of MRIdian® with the magnetic field as well as without the magnetic field and TrueBeam™, the surface dose tended to increase as the irradiation field size increased. In MRIdian®, the surface dose decreased with the magnetic field compared with the no magnetic field. That is because the scattering electron is moving along direction of the magnetic field by influence of Lorenz force and it results in dose reduction. When the irradiation field was small, the rate of decrease of the surface dose was small, but as the irradiation field increased, the rate of decrease becomes large. The surface dose at the maximum field size of 21.0×21.0 cm² was less than half of that without the magnetic field. When there is no magnetic field, the surface dose of MRIdian® was comparable to the surface dose of 4 MV X- 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

Purpose or Objective To establish the commissioning procedure of MR-guided radiotherapy system (MRIdian®, Viewray Inc.) on Monte Carlo code GEANT4. Material and Methods MRIdian® allows us to perform step-and-shoot IMRT during cine-MRI with a superoinferior static 0.345 T. The several studies reported the dosimetric changes by Lorentz force under the presence of magnetic field, called 'electron return effect (ERE)”. The Monte Carlo code GEANT4 (Ver. 10.2 patch02) is used to assess the dosimetric impact. In this study, major parts such as the cylindrical shape of a 60 Co source (Φ 2 cm), a collimator, and inner surface of the wall of bore were modelled in the GEANT4. The commissioning for modelling MRIdian® on the GEANT4 was conducted by the following procedures: (I) To assess beam quality by comparison of the lateral and depth-dose profile between the GEANT4 and the film measurements (EBT3) or the BJR-25 data for a field size of 4.2 and 27.3 cm 2 . (II) To assess calculation accuracy under the magnetic field on the GEANT4 though three tests: Test (a). Larmor radius with 0.345 T was assessed by measuring trajectory of electron in vacuum on the GEANT4 and compared with the theoretical value. Test (b). Surface dose on the phantom was calculated and compared with the film measurements. That is because electron with gyration motion by Lorentz force occurred from the inner surface of bore is moving along the superoinferior static magnetic field, which results in reduction of the surface dose. Test (c). The dose distribution in water equivalent phantom having two air gaps with a thickness of 1 and 4 cm was calculated, and the dosimetric changes in the air gaps caused by ERE were assessed by comparison with the film measurements. Results (I) The comparison with the GEANT4 by the BJR-25 shows 0.0± 0.8% and 0.3± 1.5% in a range of 0 to 20 cm depth for a field size of 4.2 and 27.3 cm 2 , respectively. The penumbra of the GEANT4 for a field size of 4.2 and 27.3 cm 2 was 1.28 and 1.63 cm, respectively. The measured penumbra with the EBT film for those was 1.31 and 1.93 cm, respectively. Both lateral and depth dose profiles shows agreements with the measurements. (II) Test (a): The Larmor radius of 1 MeV of electron with 0.3, 1, and 1.5 T, calculated by GEANT4 was 1.58, 0.47, and 0.32 cm, respectively. The theoretical Larmor radius was 1.56, 0.46, and 0.30 cm, respectively. Test (b): In the EBT3 film measurement, about 50% reduction of the surface dose with the magnetic field of 0.345 T was observed due to the reason mentioned above, comparing by that without the magnetic field. The similar results on the GEATN4 were obtained. Test (c): The changes of dose distributions in air gaps by the ERE were observed both on the GEANT4 and in the film measurements. The gamma analysis with a criterion of 3%/3 mm shows the pass rate of 93.9% . Conclusion In this study, modelling procedures of MR-guided radiotherapy system on the Monte Carlo code GEANT4 was proposed. The GEANT4 provides high calculation accuracy under a magnetic field. EP-1767 Comparison of surface dose between linac and MRI-guided radiotherapy system by film measurement Y. Shibata 1 , H. Okamoto 1 , T. Sakasai 1 , Y. Miura 1 , J. Kuwahara 1 , C. Kuroki 1 , T. Kato 1 , Y. Abe 1 , J. Itami 1 1 National Cancer Center Hospital, Radiation Oncology, Tokyo, Japan