The sensitivity of patient specific IMRT QC to systematic MLC leaf bank offset errors

Rangel, Alejandra; Palte, Gesa; Dunscombe, Peter

doi:10.1118/1.3453576

Cited by 33 publications

(32 citation statements)

References 9 publications

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“…Rangel et al 53 pointed out that patient-specific IMRT QA cannot replace routine MLC QA as none of their IMRT QA criteria tested were sufficiently sensitive to identify MLC offsets within a tolerance of 0.3 mm on a single field basis. MLC log files have been used previously to quantify leaf positional errors for IMRT 54,55 as well as RapidArc.…”

Section: Discussionmentioning

confidence: 99%

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

Betzel

Niu

et al. 2012

Medical Physics

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Purpose: Rotational IMRT has been adopted by many clinics for its promise to deliver treatments in a shorter amount of time than other conventional IMRT techniques. In this paper, the authors investigate whether RapidArc is more susceptible to delivery uncertainties than dynamic IMRT using fixed fields. Methods: Dosimetric effects of delivery uncertainties in dose rate, gantry angle, and MLC leaf positions were evaluated by incorporating these uncertainties into RapidArc and sliding window IMRT (SW IMRT) treatment plans for five head-and-neck and five prostate cases. Dose distributions and dose-volume histograms of original and modified plans were recalculated and compared using Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results of Gamma analyses using passing criteria ranging from 1%-1 mm up to 5%-3 mm were reported. Results: Systematic shifts in MLC leaf bank positions of SW-IMRT cases resulted in 2-4 times higher average percent differences than RapidArc cases. Uniformly distributed random variations of 2 mm for active MLC leaves had a negligible effect on all dose distributions. Sliding window cases were much more sensitive to systematic shifts in gantry angle. Dose rate variations during RapidArc must be much larger than typical machine tolerances to affect dose distributions significantly; dynamic IMRT is inherently not susceptible to such variations. Conclusions: RapidArc deliveries were found to be more tolerant to variations in gantry position and MLC leaf position than SW IMRT. This may be attributed to the fact that the average segmental field size or MLC leaf opening is much larger for RapidArc. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.

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Section: Discussionmentioning

confidence: 99%

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

Betzel

Niu

et al. 2012

Medical Physics

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“…The detection sensitivity of MLC leaf positioning shifts has been studied with different devices 18, 19, 20, 21, 22. Yan et al 18.…”

Section: Discussionmentioning

confidence: 99%

“…showed systematic outward MLC gap width shifts up to 2 mm could not be detected with ArcCHECK diode array using Gamma criterion 3%/2 mm for true composite dose analysis of both prostate and head and neck treatment, whereas an absolute dose measurement with the criterion of 2% at isocenter using an inserted micro‐ionization chamber was able to identify 1 mm MLC gap shifts. Rangel et al 20. reported the criteria most sensitive to detect the MLC leaf shifts were 3% absolute dose difference, 3 mm DTA for MapCHECK, the Gamma index with 2%/2 mm for the EPID.…”

Section: Discussionmentioning

confidence: 99%

Sensitivity of array detector measurements in determining shifts of MLC leaf positions

Shang

Godley

Huang

et al. 2017

J Applied Clin Med Phys

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Using a MatriXX 2D ionization chamber array, we evaluated the detection sensitivity of systematically introduced MLC leaf positioning shifts to test whether the conventional IMRT QA method can be used for quality assurance of an MLC tracking algorithm. Because of finite special resolution, we first tested whether the detection sensitivity was dependent of the locations of leaf shifts and positions of ionization chambers. We then introduced the same systematic leaf shifts in two clinical intensity modulated radiotherapy plans (prostate and head and neck cancer). Our results reported differences between the measured planar doses with and without MLC shifts (errors). Independent of the locations of the leaf position shifts and positions of the detectors, for the simple rectangular fields, the MatriXX was able to detect ±2 mm MLC leaf positioning shifts with Gamma index of 3%/3 mm and ±1 mm MLC leaf position shifts with Gamma index of 2%/2 mm. For the clinical plans, measuring the fields individually, leaf positioning shifts of ±2 mm were detected using Gamma index of 3%/3 mm and a passing rate of 95%. When the fields were measured compositely, the Gamma index exhibited less sensitivity for the detection of leaf positioning shifts than when the fields were measured individually. In conclusion, if more than 2 mm MLC leaf shifts were required, the commercial detector array (MatriXX) is able to detect such MLC positioning shifts, otherwise a more sensitive quality assurance method should be used.

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“…A number of groups have categorized the impact of these types of MLC errors in IMRT and VMAT for a range of treatment sites. (4,17–21) Rangel et al (18) found that an MLC gap error of 1 mm resulted in a systematic dose difference of 2.7% and 5.6% for prostate and head and neck IMRT respectively. Other groups have reported that a systematic MLC gap error of 1 mm can introduce dose errors of up to 10% in IMRT treatments (21) .…”

Section: Introductionmentioning

confidence: 99%

The dosimetric impact of control point spacing for sliding gap MLC fields

Zwan

Hindmarsh

Seymour

et al. 2016

J Applied Clin Med Phys

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Dynamic sliding gap multileaf collimator (MLC) fields are used to model MLC properties within the treatment planning system (TPS) for dynamic treatments. One of the key MLC properties in the Eclipse TPS is the dosimetric leaf gap (DLG) and precise determination of this parameter is paramount to ensuring accurate dose delivery. In this investigation, we report on how the spacing between control points (CPs) for sliding gap fields impacts the dose delivery, MLC positioning accuracy, and measurement of the DLG. The central axis dose was measured for sliding gap MLC fields with gap widths ranging from 2 to 40 mm. It was found that for deliveries containing two CPs, the central axis dose was underestimated by the TPS for all gap widths, with the maximum difference being 8% for a 2 mm gap field. For the same sliding gap fields containing 50 CPs, the measured dose was always within ±2% of the TPS dose. By directly measuring the MLC trajectories we show that this dose difference is due to a systematic MLC gap error for fields containing two CPs, and that the cause of this error is due to the leaf position offset table which is incorrectly applied when the spacing between CPs is too large. This MLC gap error resulted in an increase in the measured DLG of 0.5 mm for both 6 MV and 10 MV, when using fields with 2 CPs compared to 50 CPs. Furthermore, this change in DLG was shown to decrease the mean TPS‐calculated dose to the target volume by 2.6% for a clinical IMRT test plan. This work has shown that systematic MLC positioning errors occur for sliding gap MLC fields containing two CPs and that using these fields to model critical TPS parameters, such as the DLG, may result in clinically significant systematic dose calculation errors during subsequent dynamic MLC treatments.PACS number(s): 87.56.nk

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The sensitivity of patient specific IMRT QC to systematic MLC leaf bank offset errors

Abstract: None of the techniques or criteria tested is sufficiently sensitive, with the population of IMRT fields, to detect a systematic MLC offset at a clinically significant level on an individual field. Patient specific QC cannot, therefore, substitute for routine QC of the MLC itself.

Cited by 33 publications

References 9 publications

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

Sensitivity of array detector measurements in determining shifts of MLC leaf positions

The dosimetric impact of control point spacing for sliding gap MLC fields

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