ESTRO 2023 - Abstract Book

S470

Sunday 14 May 2023

ESTRO 2023

The use of multiple intracranial targets irradiation with single isocenter is increasing due to potential reduction of delivery time. However, the technique requires both dosimetric and geometric accuracy verification before being implemented and as a periodic QA. The aim of the current study was to develop an end-to-end validation methodology for this technique. Materials and Methods CIRS Shane phantom representing head and shoulder part of human anatomy was used for validation in three different clinics having the following equipment: 1. Varian Edge with Eclipse HyperArc, 2. Elekta Versa HD with BrainLab Elements, 3. Varian TrueBeam with RayStaion. All clinics used multiple non-coplanar arcs with 6FFF beam. The phantom was CT scanned and three targets (spheres with diameter 1.2, 1.6, 2.0) were created on the same coronal, but different transversal and sagittal planes (Figure 1). The centres of targets coincided with three ion chamber plugs in the phantom. The treatment plans were made to cover the targets with 80% isodose of 20, 18 and 15 Gy, respectively. The measurements were made with PTW 3D pinpoint chamber (31022) and readings were corrected with detector specific output factor correction factor following published methodology (IAEA TRS 483). For dose distribution measurements Gafchromic EBT-XD film was positioned 1 cm below the ion chamber plane and the phantom was shifted anteriorly by 1cm to have the center of the targets corresponding with the film plane. The plan was also recalculate with the shift to account for it. The phantom was positioned using CBCT imaging and 6 DOF couch in each clinic. The analyses of the film was done using IBA MyQA Patient software in multichannel mode.

Figure 1. Film showing the location of multiple targets

Results The ion chamber measurements were within 0.2-5% of the calculated dose. The majority had the dose difference < 3%. The higher discrepancy was seen for the heavily modulated plan (HyperArc), while the lowest was seen for least modulated plan (BrainLab Elements). All films had a passing rate > 90% for 2D gamma criteria of 5%,1 mm (20% threshold). Conclusion The methodology was successfully tested at four clinics. It could be used as part of an audit to validate the dosimetric and geometric accuracy of multiple targets stereotactic dose delivery. 1 mm distance to agreement criteria is very sensitive to any misalignment, while 5% dose difference criteria allows for uncertainty of the film and small field ion chamber measurements. PD-0588 Performance characterization of the Blue Physics scintillator in the 1.5T MR-linac S. Oolbekkink 1 , B. van Asselen 1 , S. Woodings 1 , J. Wolthaus 1 , W. de Vries 1 , A. van Appeldoorn 1 , M. Feijoo 2 , M. van den Dobbelsteen 1 , B. Raaymakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 2 Blue Physics, Inc., Lutz, USA Purpose or Objective On the MR-linac, the treatment is adapted based on the daily anatomy. Dosimetric plan QA is usually based on integral dose verification. However, to verify real-time adaptive treatments, they should be measured with a high temporal resolution. Furthermore, the detectors should not be affected by, or distort, MR image acquisitions used to measure full adaptive workflows, including cine imaging for motion monitoring. As plastic scintillating dosimeters (PSD) potentially have these properties, we have tested the performance of the Blue Physics (Lutz, FL) PSD system for use in the Elekta 1.5T MR-linac (Stockholm, SE). Materials and Methods Besides the dose signal obtained from the scintillator in the PSD, Cherenkov radiation is generated by fast electrons inside the plastic fiber. To detect the amount of Cherenkov signal, a second adjacent fiber, without scintillator, is used. Several calibration procedures were tested in both a conventional linac and MR-linac. Performance of the PSD was tested in terms of detector stability, dose linearity, dose rate stability, axial rotation, PDD and output factors. Measurements were performed in a PTW BEAMSCAN MR (Freiburg, DE) water phantom with a PTW microDiamond as reference detector. Furthermore influence of the orientation of the detector relative to the magnetic field was determined using a PTW RW3 slab phantom. The application of patient plan verification was tested by irradiating an IMRT plan on the RTsafe Prime head phantom (Athens, GR). A comparison was made to the same plan measured with a PTW Semiflex 0.125cc detector. MRI acquisition dependence was tested with the Modus QUASAR MRI 4D motion phantom (London, ON, CA). Experiments were conducted with the PSD and Semiflex for gantry angles 225°, 315°, 45° and 135° with and without MRI acquisitions.