Error detection capability of a novel transmission detector: a validation study for online VMAT monitoring

Pasler, Marlies; Michel, Kilian; Marrazzo, L.; Obenland, M.; Pallotta, S.; Björnsgard, M.; Lutterbach, Johannes

doi:10.1088/1361-6560/aa7dc7

Cited by 12 publications

(21 citation statements)

References 17 publications

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“…The IQM calculation function was soundly validated on the basis of 30 randomly chosen VMAT plans. The average deviation of −0.48% for 6 MV and 0.21% for 10 MV compares well with the value reported by Pasler et al., who obtained a value of 0.96% for a collection of 15 VMAT plans measured at an MLCi2 equipped Elekta linac. In our case, the calculation function is commissioned to model the more complex Agility MLC.…”

Section: Discussionsupporting

confidence: 90%

“…Error plans can generally be divided into two types: machine errors due to, for example, an MLC miscalibration, as studied by Pasler et al., and small, random plan variations, as we introduced by reoptimization and are the subject of the present study. The detection sensitivity for the former type critically depends on the exact error, that is, whether only selected or all leafs are affected.…”

Section: Discussionmentioning

confidence: 99%

“…While it is somehow arbitrary to use these values as a criterion, this approach is motivated by Das et al 18 and has been used in the same context of finding plan acceptance criteria, for example, by Basran et al 19 The chosen tolerance levels for this study (2% warning level, 3% action level) were validated, and we showed that they were small enough to detect the introduced errors with the IQM detector at least as good as with currently established QA tools. Error plans can generally be divided into two types: machine errors due to, for example, an MLC miscalibration, as studied by Pasler et al, 14 and small, random plan variations, as we introduced by reoptimization and are the subject of the present study. The detection sensitivity for the former type critically depends on the exact error, that is, whether only selected or all leafs are affected.…”

Section: Discussionmentioning

confidence: 99%

“…The clinical use of the IQM system for patient‐specific QA demands the detectability of relevant errors in delivery of treatment beams. The system's robustness, its stability in performance, basic error detection capabilities with regards to systematic errors in the form of a different number of MUs, wrong gantry angle or beam energy, as well as its high sensitivity for the detection of systematic errors in the leaf positions have been demonstrated . Furthermore, dosimetric changes in the delivered beam introduced by the system were subject of a previous study …”

Section: Introductionmentioning

confidence: 99%

“…The system's robustness, its stability in performance, basic error detection capabilities with regards to systematic errors in the form of a different number of MUs, wrong gantry angle or beam energy, as well as its high sensitivity for the detection of systematic errors in the leaf positions have been demonstrated. 13,14 Furthermore, dosimetric changes in the delivered beam introduced by the system were subject of a previous study. 15 This study aims to verify the calculation algorithm for a reliable dosimetric validation of VMAT plans and evaluate its sensitivity for potential clinically relevant errors.…”

Section: Introductionmentioning

confidence: 99%

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Sensitivity of the IQM transmission detector to errors of VMAT plans

et al. 2018

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Purpose The integral quality monitor (IQM) transmission detector is a wedge‐shaped large area ionization chamber that reports a position‐weighted dose area product for each control point of an IMRT or VMAT plan. In this study, the accuracy of the signal prediction is verified for the Synergy Agility MLC. Tolerance criteria for VMAT plan verification with the IQM were obtained from the observed sensitivity for the detection of incorrectly delivered plans. Methods The predicted IQM signal was compared to the measured signal recorded for a set of 30 VMAT plans for each beam quality of 6 and 10 MV. The system's capability to detect incorrectly delivered plans was tested by measuring altered plans containing small, random deviations. In addition, the observed deviations were related to measurements performed with a second QA phantom. Results The cumulative IQM signal per arc deviated from the respective calculation on average by −0.48% (6 MV) and +0.21% (10 MV) with a standard deviation of 1.08% in both cases, suggesting a 2% warning and 3% action threshold as plan acceptance criteria. This choice was confirmed by the optimum threshold of 2.5% obtained via receiver operating characteristic (ROC) analysis. Reproducibility of individual control points in multiply measured plans was low (on average 7% for 1SD) and thus, segment‐by‐segment comparison was impractical. A suitable criterion to resolve the angular distribution of the plan was identified by binning three to five control points as a running average. While the correlation between IQM signal deviations and gamma passing rates obtained with the ArcCHECK phantom was low for clinical plans, it was apparent for erroneous plans. Binning led to even higher sensitivity to errors. Conclusions The IQM was able to detect induced errors at least as reliable as the standard phantom and showed the potential to be used in pretreatment plan verification to ensure the correct plan transfer and delivery. However, there is no direct correlation between the IQM signal deviation and DVH metrics, so the IQM should be primarily used to screen for errors. Finer diagnostics should then be carried out using a different phantom.

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Section: Discussionsupporting

confidence: 90%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Sensitivity of the IQM transmission detector to errors of VMAT plans

et al. 2018

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A systematic evaluation of the error detection abilities of a new diode transmission detector

Sarkar

Paxton

Kunz

et al. 2019

J Applied Clin Med Phys

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Transmission detectors meant to measure every beam delivered on a linear accelerator are now becoming available for monitoring the quality of the dose distribution delivered to the patient daily. The purpose of this work is to present results from a systematic evaluation of the error detection capabilities of one such detector, the Delta4 Discover. Existing patient treatment plans were modified through in‐house‐developed software to mimic various delivery errors that have been observed in the past. Errors included shifts in multileaf collimator leaf positions, changing the beam energy from what was planned, and a simulation of what would happen if the secondary collimator jaws did not track with the leaves as they moved. The study was done for simple 3D plans, static gantry intensity modulated radiation therapy plans as well as dynamic arc and volumetric modulated arc therapy (VMAT) plans. Baseline plans were delivered with both the Discover device and the Delta4 Phantom+ to establish baseline gamma pass rates. Modified plans were then delivered using the Discover only and the predicted change in gamma pass rate, as well as the detected leaf positions were evaluated. Leaf deviations as small as 0.5 mm for a static three‐dimensional field were detected, with this detection limit growing to 1 mm with more complex delivery modalities such as VMAT. The gamma pass rates dropped noticeably once the intentional leaf error introduced was greater than the distance‐to‐agreement criterion. The unit also demonstrated the desired drop in gamma pass rates of at least 20% when jaw tracking was intentionally disabled and when an incorrect energy was used for the delivery. With its ability to find errors intentionally introduced into delivered plans, the Discover shows promise of being a valuable, independent error detection tool that should serve to detect delivery errors that can occur during radiotherapy treatment.

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Comparison of pretreatment VMAT quality assurance with the integral quality monitor (IQM) and electronic portal imaging device (EPID)

Ghafarian

Price

Morales-Paliza

2021

J Applied Clin Med Phys

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The purpose of this study was to compare pretreatment volumetric modulated arc therapy (VMAT) quality assurance (QA) measurements and evaluate the multileaf collimator (MLC) error sensitivity of two detectors: the integral quality monitor (IQM) system (iRT systems IQM) and the electronic portal imaging device (EPID) (Varian PortalVision aS1200). Pretreatment QA measurements were performed for 20 retrospective VMAT plans (53 arcs). A subset of ten plans (23 arcs) was used to investigate MLC error sensitivity of each device. Eight MLC error plans were created for each VMAT plan. The errors included systematic opening/closing (AE0.25, AE0.50, AE0.75 mm) of the MLC and random positional errors (1 mm) for individual/groups of leaves. The IQM was evaluated using the percent error of the measured cumulative signal relative to the calculated signal. The EPID was evaluated using two methods: a novel percent error of the measured relative to the predicted cumulative signals, and gamma (γ) analysis (1%/1 mm, 2%/2 mm, 3%/3 mm and 3%/1 mm for Stereotactic Body Radiation Therapy plans). The average change in maximum dose obtained from dose-volume histogram (DVH) data and change in detector signals for different systematic MLC shifts was also compared. Cumulative signal differences showed similar levels of agreement between measured and expected detector signals (IQM: 1.00 AE 0.55%; EPID: 1.22 AE 0.92%). Results from γ analysis lacked specificity. Only the 1%/1 mm criteria produced data with remarkable differences. A strong linear correlation was observed between IQM and EPID cumulative signal differences with MLC error magnitude (R = 0.99). Likewise, results indicate a strong correlation between the cumulative signal for both detectors and DVH dose (R IQM = 0.99; R EPID = 0.97). In conclusion, use of cumulative signal differences could be more useful for detecting errors in treatment delivery in EPID than γ analysis.

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Error detection capability of a novel transmission detector: a validation study for online VMAT monitoring

Cited by 12 publications

References 17 publications

Sensitivity of the IQM transmission detector to errors of VMAT plans

Sensitivity of the IQM transmission detector to errors of VMAT plans

A systematic evaluation of the error detection abilities of a new diode transmission detector

Comparison of pretreatment VMAT quality assurance with the integral quality monitor (IQM) and electronic portal imaging device (EPID)

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