ESTRO 2021 Abstract Book

S1328

ESTRO 2021

Conclusion In the setting of PMRT, TD compared to TH reached a good target volume coverage, with an optimal sparing of OARs.

PO-1607 Evaluation of the Octavius 1600 SRS detector array for CyberKnife treatment plan verification A. Blin 1 , J. Legavre 1 , E. Trifard 1 , R. Lefeuvre 1 , C. Lafond 1,2,3 , J. Bellec 1 1 Centre Eugène Marquis, Radiotherapy, Rennes, France; 2 Inserm, U 1099, Rennes, France; 3 Rennes 1 University, LTSI, Rennes, France Purpose or Objective To evaluate the performance of the Octavius 1600 SRS (PTW, Frieburg, Germany) high resolution liquid-filled ion-chamber array for plan delivery quality assurance (DQA) for robotic radiosurgery. Materials and Methods The study was performed on a CyberKnife M6 (Accuray, Sunnyvale, USA). Intrinsic performances of the 1600 SRS detector array including reproducibility, dose linearity, beam-incidence-angle response dependence were evaluated. Output factors (OF) were measured using the central detector for fixed circular apertures ranging from 5 to 60 mm diameter and were compared to those determined with both a PTW 60017 diode and a PTW 60019 microdiamond. Thirty clinical MLC treatment plans DQA were performed and analysed using a gamma index method with a local absolute dose criterion of 3% and a distance-to-agreement criterion of 1.5 mm. Additionally, geometric deviations and mean dose difference between calculated and measured planar dose were reported. Results were compared to that obtained with our EBT3 film-based protocol for similar clinical treatment plans. Results The response reproducibility of the 1600SRS was 0.48%. Dose linearity was 0.14% for a dose range from 0,5 to 30 Gy. The beam incidence response dependence of the Octavius 1600SRS involved relative dose difference between measured and calculated dose from -2.3% to +5% for angles between 0° to 80° and reached up to 25% for 90° (with reference to the calculated dose). The mean dose difference was +0.48% considering all the available beams incidence. OF agreed at +/- 2% with reference to those determined with diode and microdiamond detectors for field sizes larger than 10 mm diameter. Larger discrepancies (up to 3.9%) were observed for field sizes equal to or smaller than 10 mm. The summary of DQA results were reporting in the table 1.

Conclusion The Octavius 1600SRS detector array is a reliable tool for performing CyberKnife treatment plan verifications when using field apertures larger than 10mm. The beam incidence angular response dependence of the 1600 SRS has no significant dosimetric impact for plan delivery quality assurance.

PO-1608 Determining equivalent squares for high energetic photon fields J. Ringholz 1 , S. Wegener 1 , O.A. Sauer 1 1 University of Würzburg, Department of Radiation Oncology, Würzburg, Germany