ABSTRACT
AIM: The aim of this study was to investigate the sensitivity of the trajectory log file based quality assurance to detect potential errors such as MLC positioning and gantry positioning by comparing it with EPID measurement using the most commonly used criteria of 3%/3 mm. MATERIALS AND METHODS: An in-house program was used to modified plans using information from log files, which can then be used to recalculate a new dose distribution. The recalculated dose volume histograms (DVH) were compared with the originals to assess differences in target and critical organ dose. The dose according to the differences in DVH was also compared with dosimetry from an electronic portal imaging device. RESULTS: In all organs at risk (OARs) and planning target volumes (PTVs), there was a strong positive linear relationship between MLC positioning and dose error, in both IMRT and VMAT plans. However, gantry positioning errors exhibited little impact in VMAT delivery. For the ten clinical cases, no significant correlations were found between gamma passing rates under the criteria of 3%/3 mm for the composite dose and the mean dose error in DVH (r < 0.3, P > 0.05); however, a significant positive correlation was found between the gamma passing rate of 3%/3 mm (%) averaged over all fields and the mean dose error in the DVH of the VMAT plans (r = 0.59, P < 0.001). CONCLUSIONS: This study has successfully shown the sensitivity of the trajectory log file to detect the impact of systematic MLC errors and random errors in dose delivery and analyzed the correlation of gamma passing rates with DVH.
ABSTRACT
The delivery consistency of a Varian Edge linear accelerator over the entire course of treatment for nasopharynx carcinoma (NPC) and prostate cancer intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) treatment plans was investigated using four different approaches. Three NPCs and three prostate plans were delivered in 34 and 29 consecutive days, respectively, using a Varian Edge equipped with a 120 high-definition (HD) multileaf collimator (MLC). All deliveries were measured with an electronic portal imaging device (EPID), and MapCheck2 and ArcCheck commercial systems with gamma analysis used to compare the results of all daily measurements against the pretreatment patient-specific quality assurance. The daily log files generated were also assessed for differences between the actual and planned doses using an in-house program to replace the original values in the DICOM plan files with the delivered parameter values from the log file, and then exporting the plans back to the treatment planning system for reconstruction of the actual dose delivered. The trajectory log file and EPID methods showed very good agreement, with minimal deviations between the daily delivered and reference doses. However, comparisons of the MapCheck2 and ArcCheck with the EPID revealed statistically significant differences (P < 0.001, one-tailed) with greater daily fluctuations, raising concerns over the performance, and reliability of the MapCheck2 and ArcCheck systems when being used to identify IMRT and VMAT plans with poor dosimetric accuracy. We conclude that the Varian Edge linear accelerator equipped with a 120 HD MLC can consistently deliver IMRT and VMAT plans over the entire treatment course.
ABSTRACT
The main objectives of this study are to (1) analyze the sensitivity of various gamma index passing rates using different types of detectors having different resolutions and (2) investigate the sensitivity of various gamma criteria in intensity-modulated radiation therapy (IMRT) and volumetrically modulated arc therapy (VMAT) quality assurance (QA) for the detection of systematic multileaf collimator (MLC) errors using an electronic portal imaging device (EPID) and planar (MapCheck2) and cylindrical (ArcCheck) diode arrays. We also evaluated whether the correlation between the gamma passing rate (%GP) and the percentage dose error (%DE) of the dose-volume histogram (DVH) metrics was affected by the finite spatial resolution of the array detectors. We deliberately simulated systematic MLC errors of 0.25 mm, 0.50 mm, 0.75 mm, and 1 mm in five clinical nasopharyngeal carcinoma cases, thus creating 40 plans with systematic MLC errors. All measurements were analyzed field by field using gamma criteria of 3%/3 mm, 3%/2 mm, 3%/1 mm, and 2%/2 mm, with a passing rate of 90% applied as the action level. Our results showed that 3%/1 mm is the most sensitive criterion for the detection of systematic MLC errors when using EPID, with the steepest slope from the best-fit line and an area under the receiver operating characteristic (ROC) curve >0.95. With respect to the 3%/1 mm criterion, a strong correlation between %GP and %DE of the DVH metrics was observed only when using the EPID. However, with respect to the same criteria, a 0.75 mm systematic MLC error can go undetected when using MapCheck2 and ArcCheck, with an area under the ROC curve <0.75. Furthermore, a lack of correlation between %GP and %DE of the DVH metrics was observed in MapCheck2 and ArcCheck. In conclusion, low-spatial resolution detectors can affect the results of a per-field gamma analysis and render the analysis unable to accurately separate erroneous and non-erroneous plans. Meeting these new sensitive criteria is expected to ensure clinically acceptable dose errors.
