Purpose: Using single isocenter for treatment of multiple brain lesions with Intensity Modulated Radiosurgery (IMRS) have gained acceptance in recent years. One of the challenges of this technique is conducting a patient-specific quality assurance (QA), involving accurate gamma passing rate (GPR) calculations for small and wide spread-out targets. We evaluated effects of parameters such as dose grid and energy on GPR using our clinical IMRS plans.
Methods: 10 patients with total of 40 volumetric modulated arc therapy (VMAT) plans were created in Raystation(V.8A) treatment planning system (TPS) for the Varian Edge Linac using 6 and 10 Flattening Filter-Free (FFF) beams and planned dose grids of 1mm and 2mm resulting in 4 plans with 6 to 10 targets per patient. All parameters and objectives except dose grid and energy were kept the same in all plans. Next, patient specific QA’s were measured evaluating GPR with10% threshold, 3%/3mm objective, and an acceptance criterion of 95%. Modulation factors (MF) and confidence intervals were calculated. Two modes of measurements, standard density (SD) and high density (HD) were used.
Results: 1mm planned dose grid has higher GPR’s than 2mm dose grid. The GPR’s of 6FFF plans were higher than those of 10FFF. GPR showed no noticeable difference between HD and SD measurements. No correlation between MF and GPR was observed. The SD pass rates fall within the confidence interval of HD.
Conclusion: Calculated dose grid should be less than or equal to 1/3 of distance to agreement, thus 1mm planned dose grid is recommended to reduce artifacts in gamma calculation. GPR of SD and HD measurement modes is almost the same which indicates SD mode is clinically preferable for performing patient specific QAs. According to our results using 6FFF with 1mm planned dose grid is more accurate and reliable for dose calculation of IMRS plans.