Comparisons of volumetric modulated arc therapy (VMAT) quality assurance (QA) systems: sensitivity analysis to machine errors.
BACKGROUND
In volumetric modulated arc therapy (VMAT), gantry angles, dose rate and the MLC positions vary with the radiation delivery. The quality assurance (QA) system should be able to catch the planning and machine errors. The aim of this study was to investigate the sensitivity of three VMAT QA systems to machine errors.


METHODS
Several types of potential linac machine errors unique to VMAT delivery were simulated in sinusoidal function of gantry angle, including gantry angle itself, MLC position and linac output. Two commercial QA systems, ArcCheck and Delta4, and an in-house developed EPID technique were compared in this study. Fifteen full arcs from head and neck plans were selected and modified to include five magnitudes of each type of error, resulting in measurements and γ analyses of 240 arcs on each system. Both qualitative and quantitative comparisons were performed using receiver operating characteristic (ROC), γ pass rate gradient, and overlap histogram methods.


RESULTS
In ROC analysis, the area under curve (AUC) represents the sensitivity and increases with the error magnitude. Using the criteria of 2 %/2 mm/2° (angle to agreement, ATA, only for EPID) and keeping AUC > 0.95, the minimum error detectable of ArcCheck, Delta4and EPID are (2, 3, 3)° in gantry angle and (4, 2, 3) mm in MLC positions for the head and neck plans. No system is sensitive to the simulated output error, the AUC values were all below 0.70 even with 5 % output error. The γ gradient for gantry angle, MLC position and output errors are (-5.1, -2.6, -3.6)%/°, (-2.6, -7.1, -3.3)%/mm and (-0.2, -0.2, -0.3)%/% for ArcCheck, Delta4and EPID, respectively. Therefore, these two analyses are consistent and support the same conclusion. The ATA parameter in EPID technique can be adjusted to tune its sensitivity.


CONCLUSIONS
We found that ArcCheck is more sensitive to gantry angle error and Delta4is more sensitive to MLC position error. All three systems are not sensitive to the simulated output error. With additional analysis parameter, the EPID technique can be tuned to have optimal sensitivity and is able to perform QA for full field size with highest resolution. In addition, ROC analysis avoids the choice of γ pass rate threshold and is more robust compared with other analysis methods.
Image Processing Center, Beihang University, Beijing, 100191, People's Republic of China.
Bin
B
Liang
Image Processing Center, Beihang University, Beijing, 100191, People's Republic of China. bo.liu@buaa.edu.cn.
Bo
B
Liu
Image Processing Center, Beihang University, Beijing, 100191, People's Republic of China.
Fugen
F
Zhou
Department of Radiation Oncology, Duke University Medical Center, PO Box 3295, Durham, NC, 27710, USA.
Fang-Fang
FF
Yin
Department of Radiation Oncology, Duke University Medical Center, PO Box 3295, Durham, NC, 27710, USA. qiuwen.wu@duke.edu.
Qiuwen
Q
Wu
D006801:Humans; D011785:Quality Assurance, Health Care; D012372:ROC Curve; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000592:standards*
Gamma analysis; Quality assurance; Receiver operator characteristic; Volumetric modulated arc therapy