ESTRO 2021 Abstract Book

S1337

ESTRO 2021

pre-treatment verification and comparison 112 volumetric modulated arc therapy (VMAT) plans for 56 stereotactic patients. All plans used the flattening filter-free (FFF) photon beam. Materials and Methods The OCTAVIUS detector 1600SRS is a 2D-array with 1521 liquid-filled ionization chambers with an active volume of 2.5 x 2.5 x 0.5mm 3 (0.003cm 3 ). The main applications of the OCTAVIUS detector 1600SRS are pre- treatment plan verification in stereotactic body radiation therapy (SBRT) of small fields up to 15 x 15cm 2 . Detector layout in the center area is 6.5cm x 6.5cm with a spacing of 2.5mm. Initially, the new 2D-array 1600SRS was tested. The linearity, dose rate, angle dependence were checked, and cross-calibration for different field sizes was done. The study investigates verification of 56 VMAT SBRT plans (6FFF) located in lung, bone,or metastatic node. GTV volume ranged from 0.2 to 38.0 cm 3 . The fraction dose was 8 to 11.5Gy. The two measurement methods: the 2D perpendicular field by field (PFF) for planar dose distribution with Varian's Portal Dosimetry (PD) and the 3D dose distribution reconstructed from the field by field measurements (RFF) OCTAVIUS detector 1600SRS combine with Octavius Rotation Unit 4D were implemented. Gamma analysis with 5% of maximum dose threshold, local normalization with Dose Difference (DD) 2%, and Distance-to-Agreement (DTA) 2mm were done with passing rates more than 95%. The 2D gamma analysis for the Portal Dosimetry with ARIA 15.6 (Varian Med. Systems, USA) and 3D analysis for the 1600SRS detector with VeriSoft 8.0.1.0 (PTW, Freiburg) were obtained. Results The agreement of the tested parameters with the producer's specifications was demonstrated. For different sizes of fields, the cross-calibration factor was designated. Average value of cross factor for field 3x3cm 2 was 0.949, for field 4x4cm 2 was 0.956 and for field 5x5cm 2 was 0.962. For PD verification plans, there were 10.7% incorrect plans of all. Average volumetric 3D gamma evaluation (L2%/2mm, Th 5%) results was 98.6% (±1.05%) for the Octavius plans and 98.7% (±0.89%) for PD plans. The minimum passing gamma rate for PD verification plans was 92.7%, while this value for OCTAVIUS detector was 95.4%. The percentage difference between the individual fields analyzed based on the two methods ranged from ±3%. The greatest difference was obtained for the field that did not meet the gamma results, and it was 5.3%. Conclusion Both dosimetry methods showed comparable results and can be used as an alternative to each other for verification stereotactic plans. The OCTAVIUS detector 1600SRS is more precise, characterized by high resolution and the possibility of absolute measurement. The PD method is faster than the array, but it’s less accurate. If the result of the gamma analysis is lower than 95%, additional verification should be made by using OCTAVIUS detector 1600SRS. 1 1st Department of Radiology, Medical School, Aretaieion Hospital, National and Kapodistrian University of Athens, Medical Physics Unit, Athens, Greece; 2 1st Department of Radiology, Medical School, Aretaieion Hospital, National and Kapodistrian University of Athens, Radiation Oncology Unit, Athens, Greece Purpose or Objective The purpose of this study was to evaluate the use of Gafchromic EBT – XD film in the clinical dosimetry of MLC-based GRID radiotherapy. Materials and Methods The new Gafchromic EBT – XD film presents a high dose dynamic range and it was used to evaluate MLC-based GRID treatment field. Film pieces (3x3cm 2 ) were irradiated in different doses (50-4,000cGy). Doses were delivered in a 6MV photon beam from a SIEMENS ONCOR linac with 1cm MLC. After 24 hours all films were scanned in 24-bits RGB color channel with a resolution of 150 dpi. The EBT – XD calibration curve was created using the FilmQA Pro 2016 software by Ashland Inc. A sequence of 1x1cm 2 GRID fields was created using the Oncentra treatment planning system. The peak-to-peak distance was planned to be 2 cm. The prescribed dose at the central grid was 10 Gy at 10 cm depth. The GRID sequence was executed on a solid water phantom with an EBT – XD film placed at 10 cm depth. The film dose map was created in the FilmQA Pro 2016 software and a comparison between the film dose map and the treatment plan dose distribution was executed. The comparison criterion used was gamma index (3%/3mm). Finally, the peak-to-peak distance and the valley-to- peak ratio were calculated, both in the planned dose distribution and on the film dose map. Results The film dose map to plan comparison concluded in a γ index of 96.4% for 3.0% and 3 mm criteria. Regarding the geometry of the grid, the high dose areas on the EBT – XD film were found to have a peak-to-peak distance of 2 cm. The central GRID was measured 10.43 Gy on the film dose map with 4.3% difference from the prescription dose in the TPS. The mean valley-to-peak dose ratio from the EBT – XD film was measured 18.54%, including all the MLC interleaf leakage contribution, while the respective TPS valley-to-peak ratio was found 14.25%. PO-1616 Dosimetry of MLC based GRID radiotherapy using EBT – XD film C. Antypas 1 , N. Tzoumi 1 , C. Armpilia 1 , R. Avgousti 1 , A. Zygogianni 2