Reliability of the gamma index analysis as a verification method of volumetric modulated arc therapy plans

Abstract

BackgroundWe investigate the gamma passing rate (GPR) consistency when applying different types of gamma analyses, linacs, and dosimeters for volumetric modulated arc therapy (VMAT).MethodsA total of 240 VMAT plans for various treatment sites, which were generated with Trilogy (140 plans) and TrueBeam STx (100 plans), were retrospectively selected. For each VMAT plan, planar dose distributions were measured with both MapCHECK2 and ArcCHECK dosimeters. During the planar dose distribution measurements, the actual multileaf collimator (MLC) positions, gantry angles, and delivered monitor units were recorded and compared to the values in the original VMAT plans to calculate mechanical errors. For each VMAT plan, both the global and local gamma analyses were performed with 3%/3 mm, 2%/2 mm, 2%/1 mm, 1%/2 mm, and 1%/1 mm. The Pearson correlation coefficients (r) were calculated 1) between the global and the local GPRs, 2) between GPRs with the MapCHECK2 and the ArcCHECK dosimeters, 3) and between GPRs and the mechanical errors during the VMAT delivery.ResultsFor the MapCHECK2 measurements, strong correlations between the global and local GPRs were observed only with 1%/2 mm and 1%/1 mm (r > 0.8 with p < 0.001), while weak or no correlations were observed for the ArcCHECK measurement. Between the MapCHECK2 and ArcCHECK measurements, the global GPRs showed no correlations (all with p > 0.05), while the local GPRs showed moderate correlations only with 2%/1 mm and 1%/1 mm for TrueBeam STx (r > 0.5 with p < 0.001). Both the global and local GPRs always showed weak or no correlations with the MLC positional errors except for the GPRs of MapCHECK2 with 1%/2 mm and 1%/1 mm for TrueBeam STx and the GPR of ArcCHECK with 1%/2 mm for Trilogy (r < − 0.5 with p < 0.001).ConclusionsThe GPRs varied according to the types of gamma analyses, dosimeters, and linacs. Therefore, each institution should carefully establish their own gamma analysis protocol by determining the type of gamma index analysis and the gamma criterion with their own linac and their own dosimeter.

Three-dimensional dose reconstruction-based pretreatment dosimetric verification in volumetric modulated arc therapy for prostate cancer

“…However, previous studies have mainly focused on the correlation analysis between gamma results and DVH deviations. Similar to our findings, previous studies reported weak or absence of correlation between GFR and DVH deviations in VMAT [17][18][19][20] and IMRT [21][22][23][24] plans for prostate cancers [17,18,20,23], as well as for other cancers [19][20][21][22][23][24]. Fundamentally, clinically relevant DVH parameters are extracted from the statistical analyses for the entire dose distribution, despite gamma results evaluating dose differences point by point.…”

Analysis of the applicability of two‐dimensional detector arrays in terms of sampling rate and detector size to verify scanned intensity‐modulated proton therapy plans

“…Furthermore, gamma index analysis was performed by comparing the raw data of the array measurements to TPS calculated dose distributions using gamma index criteria of 3 mm/3% and 2 mm/2% 37 . The evaluation was differentiated between global gamma index analysis (dose difference is calculated from the maximum dose of the investigated field) and local gamma index analysis (dose difference is calculated from the local dose at the point of evaluation) 38 . All dose values below 10% of the maximum dose were suppressed.…”

“…37 The evaluation was differentiated between global gamma index analysis (dose difference is calculated from the maximum dose of the investigated field) and local gamma index analysis (dose difference is calculated from the local dose at the point of evaluation). 38 All dose values below 10% of the maximum dose were suppressed.…”

Impact of delivery characteristics on dose delivery accuracy of volumetric modulated arc therapy for different treatment sites

“…The evaluation was based on a retrospective setting with a relatively large sample size ( n = 344), and all treatment plans and QA data were generated from the same TPS and delivered by the same Linac with the same 2D detector array in our department. Previous reports showed that GPR for the same plan varied with the types of dosimeters and QA software [40, 41]. Although not in true 3D with high spatial resolution, dose distributions measured with nonplanar detector arrays can give a more realistic picture of VMAT delivery compared with planar dose distributions using 2D arrays [42–44].…”