135 Patient specific QA system: Experimental stress analysis by introducing intentional errors

Abstract Purpose QA procedures and their results are strictly connected to the measuring system. The purpose of this study is to investigate the sensitivity of Delta4 Phantom+ (Scandidos, 2015), by introducing intentional errors in some reference plans. It would be possible to evaluate the Delta4 ability to find errors related to the radiotherapy treatment. Methods Delta4 Phantom+ consists of two orthogonal detector arrays placed in a 3D cylindrical PMMA phantom. Three VMAT reference plans were created in treatment planning system (Monaco, Version 5.11) and measured with Delta4 Phantom+. Reference plans are characterized by a simple geometry and involve anatomic parts such as prostate, head and neck and brain (stereotactic). The same plans were measured introducing errors during delivery. Errors introduced are: Monitor Unit variation (3%), collimator rotation by 5° and 10°, gantry error angle of 2° and 4°. The measurements were analyzed using the inherent gamma passing rate %GP (or gamma failure rate) with 2% and 2 mm criteria and 3% and 3 mm criteria. Results The results show that the sensitivity of the system depends on the characteristics of irradiated plans. In particular, the stereotaxic plan shows a weakness due to the PTV volume considered and the reduced number of diodes directly interacting whit primary beam. When using 2% and 2 mm criteria and 95% as a pass rate the system detected 11 of 14 errors, while using 3% and 3 mm criteria only 6 of 14 errors were detected (Table 1). Conclusions Delta4 Phantom+ is an efficient system for QA verification of VMAT, it will be interesting to further analyze the stereotactic case to identify the most suitable procedure.