Abstract
PurposeThe aim of this study was to investigate the correlations of the gamma passing rates (GPR) with the dose-volumetric parameter changes between the original volumetric modulated arc therapy (VMAT) plans and the actual deliveries of the VMAT plans (DV errors). We compared the correlations of the TrueBeam STx system to those of a C-series linac.MethodsA total of 20 patients with head and neck (H&N) cancer were retrospectively selected for this study. For each patient, two VMAT plans with the TrueBeam STx and Trilogy (C-series linac) systems were generated under similar modulation degrees. Both the global and local GPRs with various gamma criteria (3%/3 mm, 2%/2 mm, 2%/1 mm, 1%/2 mm, and 1%/1 mm) were acquired with the 2D dose distributions measured using the MapCHECK2 detector array. During VMAT deliveries, the linac log files of the multi-leaf collimator positions, gantry angles, and delivered monitor units were acquired. The DV errors were calculated with the 3D dose distributions reconstructed using the log files. Subsequently, Spearman’s rank correlation coefficients (rs) and the corresponding p values were calculated between the GPRs and the DV errors.ResultsFor the Trilogy system, the rs values with p < 0.05 showed weak correlations between the GPRs and the DV errors (rs<0.4) whereas for the TrueBeam STx system, moderate or strong correlations were observed (rs≥0.4). The DV errors in the V20Gy of the left parotid gland and those in the mean dose of the right parotid gland showed strong correlations (always with rs > 0.6) with the GPRs with gamma criteria except 3%/3 mm. As the GPRs increased, the DV errors decreased.ConclusionThe GPRs showed strong correlations with some of the DV errors for the VMAT plans for H&N cancer with the TrueBeam STx system.
Highlights
The most popular method of pre-treatment patient-specific quality assurance (QA) for intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) in the clinic is the gamma index method proposed by Low et al [1]
Nelms et al demonstrated that there is a lack of correlation between the gamma passing rates (GPR) and clinically relevant dose-volumetric parameter changes between plans and deliveries (DV errors) by utilising a total of 24 IMRT plans generated with a C-series linac [4]
Several studies suggested log-file-based pretreatment QA or calculation of the modulation indices as a pre-treatment patient-specific QA method for IMRT or VMAT [6,7,8,9,10]. These methods have a limitation in that they are not based on independent dose measurements; the gamma evaluation is still widely adopted in the clinic as a verification method of IMRT and VMAT plans
Summary
The most popular method of pre-treatment patient-specific quality assurance (QA) for intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) in the clinic is the gamma index method proposed by Low et al [1]. Stasi et al showed that there were weak correlations between the GPRs and the DV errors of clinically relevant DV endpoints by utilising 27 prostate and 15 head and neck (H&N) IMRT plans [5] They showed cases where high GPRs did not necessarily indicate good consistency in anatomy dose metrics (i.e., false negatives) [5]. In this respect, several studies suggested log-file-based pretreatment QA or calculation of the modulation indices as a pre-treatment patient-specific QA method for IMRT or VMAT [6,7,8,9,10]. These methods have a limitation in that they are not based on independent dose measurements; the gamma evaluation is still widely adopted in the clinic as a verification method of IMRT and VMAT plans
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