ESTRO 2021 Abstract Book

S1319

ESTRO 2021

Octavius-based dose distributions were compared using the 3D gamma analysis tool of the Verisoft software. The global (G) criteria used for the gamma analysis were 3%(G)/1 mm and 3%(G)/2 mm, for 10% and 70% dose thresholds. The 70% threshold focused the analysis in the high dose regions that approximately correspond to the targets, while the 10% one is the typical dose cut off used for the gamma analysis. 3D gamma passing rates (GPRs) and confidence levels (CLs) for the GPR metric were derived.

Results Gamma passing rates (GPRs) are shown in Table 1. CLs for 3%(G)/1 mm criteria were 90.5% and 57.8% for the 10% and 70% dose thresholds, respectively. CLs for 3%(G)/2 mm criteria were 95.9% and 95.3% for the 10% and 70% dose thresholds, respectively. Conclusion Acceptable CLs were obtained for 3%(G)/2 mm criteria. 1 mm distance-to-agreement seems too tight to be used as tolerance with the Octavius Detector 1600 SRS. Based on our evaluation, we have set our departmental criteria at 3%(G)/2 mm for a 90% pass regardless the dose threshold (10% and 70%), when the Octavius Detector 1600 SRS is used. PO-1596 Virtual phantom-based pre-treatment verification of single-isocenter multiple brain targets SRS J. Calvo-Ortega 1 , P.B. Greer 2 , S. Moragues-Femenia 1 , C. Laosa-Bello 1 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain; 2 1. Calvary Mater Newcastle Hospital. 2. University of Newcastle, 1. Department of Radiation Oncology, Calvary Mater Newcastle Hospital 2. School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, Australia Purpose or Objective To establish the action levels (ALs) for pre-treatment QA of single-isocenter multitarget (SIMT) stereotactic brain radiosurgery (SRS) plans using the VIrtual Phantom EPID dose Reconstruction (VIPER) application (Phys Med Biol. 2017 Jun 7;62(11):4293-4299). Materials and Methods Twenty clinically treated SIMT SRS plans were included in this study. Number of lesions per plan ranged from 2 to 6, with a median volume of 1.5 cm 3 . Distance from target center to treatment isocenter ranged from 0.3 to 8.7 cm. Prescribed dose ranged from 5 to 24 Gy. These plans were designed with the Eclipse TPS (AAA version 10.0.28) using non-coplanar IMRT technique (11-15 fields), and delivered using 6 MV photons from a Varian Clinac 2100 CD linear accelerator with a Millennium 120 MLC. For verification purpose using the VIPER software (version 3.40_Rev2), each plan was recalculated on a virtual spherical water phantom of 20 cm-diameter created in the Eclipse. 3D dose reconstruction on this sphere was done by VIPER from the in air EPID images acquired (without any phantom) for the fields of the SRS plan. All fields were delivered on the EPID at couch angle of 0°, but the non-coplanar dose reconstruction was performed by VIPER. Eclipse and VIPER-based dose distributions on the sphere phantom were compared using the 3D gamma analysis tool of the VIPER software. The global (G) criteria used for the gamma analysis were 3%(G)/1 mm and 3%(G)/2 mm, for 10% and 70% dose thresholds. The 70% threshold focused the analysis in the high dose regions that approximately correspond to the targets, while the 10% one is the typical dose cut off used for the gamma analysis. 3D gamma passing rates (GPRs) and ALs (calculated following the AAPM-TG218 statistical process control approach) for the GPR metric were derived. A minimum GPR of 90% is required for an acceptable plan.