ESTRO 2020 Abstract Book

S703 ESTRO 2020

planning system (TPS). A phantom of height 42.20 cm was modelled as a master phantom that can be scaled to any age from infant to adult using the non-uniform age-scaling growth functions and equations. These scaling functions were developed from data published by the Society of Automotive Engineers (Fig 1A-C), which included body dimension measurements of more than 4000 children ages 3 months, 1, 3, 5, 10, 15 and 18 years (adult). Once modeled in DICOM format, we compared the percent difference between the heights of age-scaled DICOM phantoms and heights (50 th percentile) reported by the United States Centers for Disease Control and Prevention (CDC) for male and female children of the same ages. To validate the conversion of the phantom from FORTRAN to DICOM format across the full age range, we compared the percent difference between the volume of body regions (e.g. head, neck, and trunk) for 0.1, 0.5, 1, 2, 3, 5, 8, 10, 15 and 18 years for phantoms coded in the two formats. Lastly, we calculated the normalized mean square distance (NMSD) between the organs (heart, liver, stomach, lungs, and brain) of both phantom formats for the aforementioned ages. Results The heights of our age-scaled phantom and CDC reported heights agreed within 7% from infant to adult for both genders, with agreement better than 2% for ages five and older (Fig 1D/Table 1). The percent difference between the volume of head, neck and trunk of our phantoms in FORTRAN and DICOM formats were in good agreement, within. For all of the ages, the NMSDs were 0.0mm for each organ except brain where NMSDs are non-zero but less than 0.5mm agreement (Table 1).

commercial treatment planning system for whole body dosimetry studies. PO-1331 Monte Carlo assessment of the PTW-31021 Semiflex 3D performance under a 0.35 tesla magnetic field G.V. SanturiO 1 , S. Blak Nyrup Biancardo 1 , U. Bjelkengren 1 1 Herlev Hospital, Radiotherapy, Herlev, Denmark Purpose or Objective Magnetic resonance (MR) imaging systems in radiotherapy offer the possibility of acquiring high quality images with high soft-tissue contrast without adding extra dose to the patient. The MR system combined with a linear accelerator allows for on-table adaptive radiotherapy and also gated treatment using live images as the patient is being treated. This in turn may allow for margin reductions and dose escalation. However, the implementation increases the challenges for the medical physicist as the dosimetry under magnetic fields has not been incorporated in the currently used radiotherapy protocols (TRS-398, TG-51, TRS-483). The objective of this study is to investigate the performance of the ionization chamber PTW-31021 Semiflex 3D under a 0.35 tesla magnetic field for reference dosimetry using the magnetic field correction factor. Material and Methods The magnetic field correction factor ( k B ) was calculated using Monte Carlo simulations. The Monte Carlo based toolkit EGSnrc was used for the computations. The radiation source used was a 6 MV flattening filter free beam as this study aims to incorporate experimental measurements using a ViewRay MRIdian accelerator. The impact of the magnetic field was evaluated with the ionization chamber positioned perpendicular to the beam direction, and in parallel and perpendicular orientation with respect to the magnetic field. In addition, the ionization chamber was positioned parallel to beam orientation and parallel to the magnetic field. In order to compare to an often used ionization chamber for reference dosimetry the PTW-30013 Farmer was also simulated. This ionization chamber cannot be used parallel to the beam direction therefore the k B factor was computed for this detector merely perpendicular to the beam and with an orientation parallel and perpendicular to the magnetic field. Results The magnetic field correction factor for the Farmer ionization chamber agrees with those reported in the literature. The correction is approximately 0.3% in the parallel orientation and ~3 % for the perpendicular orientation. For the Semiflex (perpendicular to the beam) the corrections are higher than for the Farmer, yielding a correction of ~1.5% for the parallel orientation and ~3.5% for the perpendicular one. However, when using the Semiflex ionization chamber in a parallel orientation with respect the beam direction the correction is ~0.2%. Conclusion The performance of the Semiflex 3D ionization chamber for reference dosimetry purposes under a magnetic field of 0.35 tesla, was assessed. The results suggest that the Semiflex 3D ionization chamber has a better behavior than the Farmer chamber, for reference dosimetry, if used in a parallel orientation with respect the beam direction. Further investigation of the perturbation factors under magnetic fields for this detector can add a better understanding of the problem. PO-1332 Evaluation of IAEA small field correction factors using different detectors for FF and FFF energies G. Beyer 1 , G. Kidane 2 , R. Paiva 1 , V. Ganesan 2 , L. Crees 2 1 Medical Physics Services Intl Ltd, Medical Physics, Cork, Ireland ; 2 Queen's Hospital, Radiotherapy Department, Romford, United Kingdom

Conclusion We successfully developed the 3D model of the phantom in DICOM format and validated it with our previous model. The phantom in DICOM format can be imported into any