ESTRO 37 Abstract book

S985

ESTRO 37

Republic of 2 Cancer Institute, Radiotherapy Oncology, Tehran, Iran Islamic Republic of 3 Teb Parto Sadra, Radiotherapy Department, Qoom, Iran Islamic Republic of Purpose or Objective MR-linac machines are being developed for image-guided radiation therapy but the magnetic field of such machines could affect dose distributions. The purpose of this work was to evaluate the effect of magnetic field on linac beam dosimetric parameters including penumbra for circular cones used in radiosurgery. Material and Methods Monte Carlo simulation was conducted for a linac machine with 1, 2 and 3 cm diameter circular cones at 6MV beam. A homogenous magnetic field of 1.5 T was applied transversely and parallel to the radiation beam. Percentage depth dose (PDD) curves and beam profiles in a water phantom (16 ×16 ×16 cm 3 ) with and without the magnetic field were calculated using Geant4 Monte Carlo toolkit. A patient’s head CT-DICOM images were also input into Geant4 and dose distributions for a treatment plan using 30 mm diameter of circular cone and three beam angles of 6 MV beam (0, 90 and 270 degrees) were calculated with and without the magnetic field. Results The results have showed that when the magnetic field is applied transversely, the PDDs in the water phantom differ in the buildup region and distant part of PDD curves, which is due to the electron return effect. The differences at distal part of PDDs are within 20 to 29% depending on the field size. The beam profiles at three different depths are all significantly different from that without the magnetic field. The penumbra is grater when magnetic field has been applied. However, when the magnetic field is applied parallel to the direction of the radiation beam, there is no change in the beam profile nor in PDDs as well as penumbra. The results of calculated dose distributions for a patient plan show significant dose perturbation due to the presence of magnetic field. Conclusion Linear accelerator based SRT and SRS use small circular cones. The beam penumbra for these cones can change in the presence of magnetic field. Perturbation of dose distribution has been also observed in a patient plan due to the presence of magnetic field. The results of this study show that dose distributions in the presence of magnetic field must be obtained for MR-guided radiotherapy treatments. EP-1827 Dose errors due to lung density variations in SBRT VMAT MONACO treatment planning system E. Corredoira 1 , Z. Aza 1 , C. Huertas 1 , F. Sanchez 1 , C. Huerga 1 , C. Ferrer 1 1 Hospital universitaria La Paz, Medical Physics and Radiation Protection Service, Madrid, Spain Purpose or Objective To evaluate the dosimetric impact of lung density variation on a stereotactic body radiotherapy plan with a Monte Carlo dose calculation system. Material and Methods We use computed tomography images to calculate dose distributions. Those images are acquired in two systems; a nuclear medicine PET/CT (GE Discovery LS) and a radiotherapy CT (Toshiba Activion S16). We scanned a electron density (ED) phantom (CIRS Model 162) with seventeen objects of known composition in the PET/CT with 140 kVp and 50 cm field of view (FOV) and in the radiotherapy CT with two kVp: 120 and 100 kVp and two FOVs 39 and 32 cm. The mean relative ED of each phantom insert in each image series was obtained selecting regions of interest with the MONACO treatment

those measured with an IBA Razor Chamber (RC), a Sun Nuclear EDGE diode and Gafchromic EBT3 films (International Specialty Products, Wayne, USA). The RC was positioned both vertically (RCV) and horizontally (RCH) to assess any differences in response. Polarity (k pol ) and saturation (J s ) factors were determined for each field size both for the RNC and the RCV. Results The OFs measured with the RNC uncorrected exhibited an overestimation (5%) for field size smaller than 1 cm and an underestimation for field size bigger than 2 cm (<3%). Deviations of k pol were observed, particularly for field size less than 1 cm and greater than 2 cm (Fig. 1). Variations of J s below 0.5% were recorded. The corrected OFs for the smallest beam showed an underestimation for both the RNC and the RCV with respect to films (3.5 and 5% respectively), while they are in agreement for field size ³ 0.8 cm (deviations from films within 1%). The EDGE and the RCH showed good agreement with films for field size ³ 1 cm, displaying instead an important underestimation for the very small fields. Overall OFs are shown in Fig. 2.

Conclusion OFs corrected for the RNC and the RCV were in agreement with films, while an underestimation has been observed for the smaller beam connected to a small volume averaging effect. The similar behavior of the two ionization chambers is probably due to the same size of the electrode diameter (2 mm). The RCV and the RCH showed an important difference in response, linked to the different sizes of the active length and the electrode diameter. Due to its very small spherical volume, the RNC is an attractive tool to measure OFs and its potential use for absolute dosimetry of SRS beams and patient-specific SRS quality assurance will be explored. EP-1826 An Assessment of SRS Linac based dosimetric parameters in the presence of magnetic fields H.A. Nedaie 1 , S. Gholami 2 , H. Mohssen 3 , H. Farimah 2 1 Cancer Institute Tehran Univ. of Medical Science, Radiotherapy Oncology Department, Tehran, Iran Islamic