ESTRO 2021 Abstract Book

S1320

ESTRO 2021

Results Gamma passing rates (GPRs) are shown in Table 1. ALs for 3%(G)/1 mm criteria were 93.3% and 47.1% for the 10% and 70% dose thresholds, respectively. ALs for 3%(G)/2 mm criteria were 99.2% and 94.2% for the 10% and 70% dose thresholds, respectively. Conclusion Acceptable ALs were obtained for 3%(G)/2 mm criteria. 1 mm distance-to-agreement seems too tight to be used as tolerance with VIPER. Based on our evaluation, we have set our departmental criteria of SIMT SRS QA with VIPER at 3%(G)/2 mm for a 90% pass regardless the dose threshold (10% and 70%). 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 IMRT plans performed using the VIrtual Phantom EPID dose Reconstruction (VIPER) application. Materials and Methods VIPER is a software that permit to reconstruct the 3D dose in a virtual water phantom where the clinical plan is mapped from the TPS (Phys Med Biol. 2017 Jun 7;62(11):4293-4299). Dose reconstruction is done by VIPER using the in air EPID images acquired for the fields of the plan to be verified. A virtual cylindrical phantom of 30 cm-diameter was used in this study. The VIPER software (version 3.10beta, May 2019) was configured for the 6 MV beams from a Varian Clinac 2100 CD linac. The accuracy of the 3D dose reconstruction done by VIPER was estimated using the Monte Carlo- based PRIMO software (https://www.primoproject.net). The 3D doses calculated by the VIPER software were compared with the corresponding PRIMO-based doses, for the prostate, head and neck and C-shape cases of the AAPM TG-119 document. 50 clinically IMRT plans designed in the Eclipse TPS (AAA version 13.7.14) were verified using the VIPER software: simple breast (6), breast & nodes (8), head & neck (15), lung (3), prostate (1), prostate & nodes (6), gynecological pelvis (1), lung SBRT (3), abdominal node SBRT (2), hippocampal-avoidance whole brain (1), spine SRS (3), and palliative spine (1). The Eclipse doses distributions were compared to the corresponding VIPER doses using a 3D gamma analysis. Gamma passing rates (GPRs) with 3%(G)/2 mm criteria with a 10%- dose threshold, were calculated. VIPER vs Eclipse comparison was also done for each plan for the point dose in a region of high dose and low gradient. Departmental ALs were calculated following the AAPM TG-218 statistical process control approach. Results • 1) 3D GPRs greater than 98% were obtained when VIPER-doses were compared with the Monte Carlo calculations, using 2%(G)/2 mm criteria. • 2) VIPER vs Eclipse: average GPR of 99.4% (SD: 0.7%), average mean gamma index of 0.29 (SD: 0.05), and average point dose difference of 0.7% (SD: 1.4%) were obtained for the 50 IMRT clinical plans. • 3) ALs of 97.3%, 0.70 and 2.9% were derived for the GPR, mean gamma index and point dose difference metrics, respectively. PO-1597 Establishing action levels for virtual phantom‐based QA of IMRT plans J. Calvo-Ortega 1 , P.B. Greer 2 , S. Moragues-Femenía 1 , C. Laosa-Bello 1 , J. Casals-Farran 1