Correlation of phantom‐based and log file patient‐specific QA with complexity scores for VMAT

Abstract

The motivation for this study was to reduce physics workload relating to patient‐specific quality assurance (QA). VMAT plan delivery accuracy was determined from analysis of pre‐ and on‐treatment trajectory log files and phantom‐based ionization chamber array measurements. The correlation in this combination of measurements for patient‐specific QA was investigated. The relationship between delivery errors and plan complexity was investigated as a potential method to further reduce patient‐specific QA workload. Thirty VMAT plans from three treatment sites — prostate only, prostate and pelvic node (PPN), and head and neck (H&N) — were retrospectively analyzed in this work. The 2D fluence delivery reconstructed from pretreatment and on‐treatment trajectory log files was compared with the planned fluence using gamma analysis. Pretreatment dose delivery verification was also carried out using gamma analysis of ionization chamber array measurements compared with calculated doses. Pearson correlations were used to explore any relationship between trajectory log file (pretreatment and on‐treatment) and ionization chamber array gamma results (pretreatment). Plan complexity was assessed using the MU/ arc and the modulation complexity score (MCS), with Pearson correlations used to examine any relationships between complexity metrics and plan delivery accuracy. Trajectory log files were also used to further explore the accuracy of MLC and gantry positions. Pretreatment 1%/1 mm gamma passing rates for trajectory log file analysis were 99.1% (98.7%–99.2%), 99.3% (99.1%–99.5%), and 98.4% (97.3%–98.8%) (median (IQR)) for prostate, PPN, and H&N, respectively, and were significantly correlated to on‐treatment trajectory log file gamma results (normalR=0.989,p<0.001). Pretreatment ionization chamber array (2%/2 mm) gamma results were also significantly correlated with on‐treatment trajectory log file gamma results (normalR=0.623,p<0.001). Furthermore, all gamma results displayed a significant correlation with MCS (normalR>0.57,p<0.001), but not with MU/arc. Average MLC position and gantry angle errors were 0.001±0.002.15emmm and 0.025°±0.008° over all treatment sites and were not found to affect delivery accuracy. However, variability in MLC speed was found to be directly related to MLC position accuracy. The accuracy of VMAT plan delivery assessed using pretreatment trajectory log file fluence delivery and ionization chamber array measurements were strongly correlated with on‐treatment trajectory log file fluence delivery. The strong correlation between trajectory log file and phantom‐based gamma results demonstrates potential to reduce our current patient‐specific QA. Additionally, insight into MLC and gantry position accuracy through trajectory log file analysis and the strong correlation between gamma analysis results and the MCS could also provide further methodologies to both optimize the VMAT planning and QA process.PACS number: 87.53.Bn, 87.55.Kh, 87.55.Qr

Feasibility of Estimating Patient-Specific Dose Verification Results Directly from Linear Accelerator Log Files in Volumetric Modulated Arc Therapy

“…Table 2 also supported this observation by the calculated correlation coefficients. However, these results do not agree with Agnew's data [9], reporting that the average and standard deviation of gantry angle errors were not related to the gamma index pass rates. This discrepancy may be explained by the fact that Agnew's data was based on Varian RapidArc having a different VMAT control algorithm, where the gantry speed was maintained constant or slowly varied under continuously varying dose rate [10].…”

“…For example, Agnew et al reported that the correlations between the gantry angle errors and the dose accuracy were not observed [9]. In this paper, VMAT delivery accuracy has been more extensively investigated by calculating the linac dynamic parameters consisting of control errors, speeds and accelerations of gantry angles and MLC leaf positions using log files.…”

The use of log file analysis within VMAT audits

“…This software creates a report of MLC error histograms and the average RMS of MLC positioning errors. It has been previously validated for step-and-shoot IMRT, 3,8 VMAT in TrueBeams 19 and also for VMAT using Clinacs with this data set. All data were presented as median values with associated ranges (minimum and maximum).…”

“…These log files were analyzed using the in-house code. 19 This software has the ability to analyze the MLC leaf pairs in DynaLog files created using Varian Clinacs (.DLG files) and trajectory log files created by Varian TrueBeams (.INI files) compared with the planned positions defined in the plan files (digital imaging and communications in medicine). This software creates a report of MLC error histograms and the average RMS of MLC positioning errors.…”

A software tool to automatically assure and report daily treatment deliveries by a cobalt‐60 radiation therapy device

“…Methods for analysis and presentation of machine log files have been previously reported for linear accelerator‐(linac‐) based external beam radiation therapy (EBRT) treatments 9 , 10 , 11 , 12 , 13 . The MLC dynamic log files are available on Varian linac treatment machines (14) and have been utilized at the authors' institution and by other groups to validate the accuracy and completeness of the treatment deliveries 15 , 16 , 17 , 18 , 19 , 20 , 21 .…”