1/1/2023 0 Comments Research site gamma control![]() ![]() This is because of increases in the dose calculation uncertainty and mechanical uncertainty. ![]() Although the photon beam modulation of VMAT enables to generate an optimal dose distribution, an excessive modulation of a VMAT plan results in discordance between the planned and actually delivered dose distributions to a patient, i.e., patients are not treated as intended with excessively modulated VMAT plans. The optimal dose distributions of VMAT are also attributed to photon beam modulations. Compared to IMRT, volumetric modulated arc therapy (VMAT) is possible to deliver equal to or better dose distributions than those of IMRT more rapidly by simultaneous mechanical modulations of the MLC positions, gantry rotation speeds, and dose rates. This is possible owing to photon beam intensity modulations by mechanical modulations of the multileaf collimator (MLC) positions. Intensity-modulated radiation therapy (IMRT) enables conformal delivery of prescription doses to target volumes while minimizing radiotherapy-induced complications by reducing the doses to organs at risk (OARs) located nearby the target volumes. 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. The GPRs varied according to the types of gamma analyses, dosimeters, and linacs. 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). Resultsįor 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 0.05), 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). 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. 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. 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, planar dose distributions were measured with both MapCHECK2 and ArcCHECK dosimeters. MethodsĪ total of 240 VMAT plans for various treatment sites, which were generated with Trilogy (140 plans) and TrueBeam STx (100 plans), were retrospectively selected. We investigate the gamma passing rate (GPR) consistency when applying different types of gamma analyses, linacs, and dosimeters for volumetric modulated arc therapy (VMAT). ![]()
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