Gamma index comparison of three VMAT QA systems and evaluation of their sensitivity to delivery errors

The purpose of this study was to compare the MLC error sensitivity of various measurement devices for VMAT pre‐treatment quality assurance (QA). This study used four QA devices (Scandidos Delta4, PTW 2D‐array, iRT systems IQM, and PTW Farmer chamber). Nine retrospective VMAT plans were used and nine MLC error plans were generated for all nine original VMAT plans. The IQM and Farmer chamber were evaluated using the cumulative signal difference between the baseline and error‐induced measurements. In addition, to investigate the sensitivity of the Delta4 device and the 2D‐array, global gamma analysis (1%/1, 2%/2, and 3%/3 mm), dose difference (1%, 2%, and 3%) were used between the baseline and error‐induced measurements. Some deviations of the MLC error sensitivity for the evaluation metrics and MLC error ranges were observed. For the two ionization devices, the sensitivity of the IQM was significantly better than that of the Farmer chamber (P < 0.01) while both devices had good linearly correlation between the cumulative signal difference and the magnitude of MLC errors. The pass rates decreased as the magnitude of the MLC error increased for both Delta4 and 2D‐array. However, the small MLC error for small aperture sizes, such as for lung SBRT, could not be detected using the loosest gamma criteria (3%/3 mm). Our results indicate that DD could be more useful than gamma analysis for daily MLC QA, and that a large‐area ionization chamber has a greater advantage for detecting systematic MLC error because of the large sensitive volume, while the other devices could not detect this error for some cases with a small range of MLC error.

Section: Discussionmentioning

confidence: 99%

Comparison of MLC error sensitivity of various commercial devices for VMAT pre‐treatment quality assurance

Saito

Sano

Shibata

et al. 2018

“…On the one hand, using the popular method of gamma‐evaluation and subsequent pass rate analysis,26 one can argue about the correct distance‐to‐agreement and dose criteria or pass rate, that has to be used 2, 9, 10, 12, 13, 15, 18. On the other hand, one can question this method in general,27, 28 as there might be a lack of correlation between the results of the gamma‐analysis and the clinical implication 9, 11, 13, 14, 27, 29, 30, 31, 32. Furthermore, the optimal criteria might depend on the measurement device,11, 16, 17, 18 which is supported by our results.…”

Section: Discussionmentioning

confidence: 99%

“…In addition, improvements in tests for the correct beam model parameters were proposed 5, 6, 7, 8. Nevertheless, the evaluation of treatment plan QA is still cumbersome as shown by ongoing discussions about the widely used gamma‐evaluation method itself, the optimal evaluation parameters and the comparison between different measurement devices 9, 10, 11, 12, 13, 14, 15, 16, 17, 18…”

Section: Introductionmentioning

confidence: 99%

Practical implications for the quality assurance of modulated radiation therapy techniques using point detector arrays

Kantz

McDermott

Söhn

et al. 2017

PurposeLinac parameters potentially influencing the delivery quality of IMRT and VMAT plans are investigated with respect to threshold ranges, consequently to be considered in a linac based quality assurance procedure. Three commercially available 2D arrays are used to further investigate the influence of the measurement device.MethodsUsing three commercially available 2D arrays (Mx: MatriXXevolution, Oc: Octavius1500, Mc: MapCHECK2), simple static measurements, measurements for MLC characterization and dynamic interplay of gantry movement, MLC movement and variable dose rate were performed. The results were evaluated with respect to each single array as well as among each other.ResultsSimple static measurements showed different array responses to dose, dose rate and profile homogeneity and revealed instabilities in dose delivery and profile shape during linac ramp up. Using the sweeping gap test, all arrays were able to detect small leaf misalignments down to ±0.1 mm, but this test also demonstrated up to 15% dose deviation due to profile instabilities and fast accelerating leaves during linac ramp up. Tests including gantry rotation showed different stability of gantry mounts for each array. Including gantry movement and dose rate variability, differences compared to static delivery were smaller compared to dose differences when simultaneously controling interplay of gantry movement, leaf movement and dose rate variability.ConclusionLinac based QA is feasible with the tested commercially available 2D arrays. Limitations of each array and the linac ramp up characteristics should be carefully considered during individual plan generation and regularly checked in linac QA. Especially the dose and dose profile during linac ramp up should be checked regularly, as well as MLC positioning accuracy using a sweeping gap test. Additionally, dynamic interplay tests including various gantry rotation speeds and angles, various leaf speeds and various dose rates should be included.

“…Vieillevigne at al (17) . evaluated the performances of three QA systems (i.e., ArcCHECK, 2D Array 729, and EPID) in the detection of opening and closing MLC errors by gamma index comparison.…”

Section: Discussionmentioning

confidence: 99%

Evaluation of MLC leaf positioning accuracy for static and dynamic IMRT treatments using DAVID in vivo dosimetric system*

Karagoz

Zorlu

Yeğiner

et al. 2016

Accuracy and precision of leaf positioning in multileaf collimators (MLCs) are significant factors for the accuracy of IMRT treatments. This study aimed to investigate the accuracy and repeatability of the MLC leaf positioning via the DAVID in vivo dosimetric system for dynamic and static MLC systems. The DAVID system was designed as multiwire transmission ionization chamber which is placed in accessory holder of linear accelerators. Each wire of DAVID system corresponds to a MLC leaf‐pair to verify the leaf positioning accuracy during IMRT treatment and QA. In this study, verifications of IMRT plans of five head and neck (H&N) and five prostate patients treated in a Varian DHX linear accelerator with 80‐leaf MLC were performed using DAVID system. Before DAVID‐based dosimetry, Electronics Portal Imaging Device (EPID) and PTW 2D ARRAY dosimetry system were used for 2D verification of each plan. The measurements taken by DAVID system in the first day of the treatments were used as reference for the following measurements taken over the next four weeks. The deviations in leaf positioning were evaluated by “Total Deviation (TD)” parameter calculated by DAVID software. The delivered IMRT plans were originally prepared using dynamic MLC method. The same plans were subsequently calculated based on static MLC method with three different intensity levels of five (IL5), 10 (IL10) and 20 (IL20) in order to compare the performances of MLC leaf positioning repeatability for dynamic and static IMRT plans. The leaf positioning accuracy is also evaluated by analyzing DynaLog files based on error histograms and root mean square (RMS) errors of leaf pairs’ positions. Moreover, a correlation analysis between simultaneously taken DAVID and EPID measurements and DynaLog file recordings was subsequently performed. In the analysis of DAVID outputs, the overall deviations of dynamic MLC‐based IMRT calculated from the deviations of the four weeks were found as 0.55%±0.57% and 1.48%±0.57% for prostate and H&N patients, respectively. The prostate IMRT plans based on static MLC method had the overall deviations of 1.23%±0.69%, 3.07%±1.07%, and 3.13%±1.29% for intensity levels of IL5, IL10, and IL20, respectively. Moreover, the overall deviations for H&N patients were found as 1.87%±0.86%, 3.11%±1.24%, and 2.78%±1.31% for the static MLC‐based IMRT plans with intensity levels of IL5, IL10 and IL20, respectively. Similar with the DAVID results, the error rates in DynaLog files showed upward movement comparing the dynamic IMRT with static IMRT with high intensity levels. In respect to positioning errors higher than 0.005 cm, static prostate IMRT plans with intensity levels of IL10 and IL20 had 1.5 and 2.6 times higher error ratios than dynamic prostate IMRT plans, respectively, while these values stepped up to 8.4 and 12.0 for H&N cases. On the other hand, according to the leaf pair readings, reconstructed dose values from DynaLog files had significant correlation false(r=0.80false) with DAVID and EPID readings while a stronger relationship fal...