On the use of the MLC dosimetric leaf gap as a quality control tool for accurate dynamic IMRT delivery

Mei, Xiangyang; Nygren, Ian; Villarreal-Barajas, J. Eduardo

doi:10.1118/1.3567148

Cited by 35 publications

(56 citation statements)

References 11 publications

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“…Different approaches to measuring the DLG have been proposed by many authors, and similar results have been obtained 20 , 21 . Due to the complexity of the MLC system, using the measured DLG cannot guarantee good agreement of planned and measured doses.…”

Section: Discussionmentioning

confidence: 72%

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

Yao

Farr

2015

J Applied Clin Med Phys

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Individual QA for IMRT/VMAT plans is required by protocols. Sometimes plans cannot pass the institute's QA criteria. For the Eclipse treatment planning system (TPS) with rounded leaf‐end multileaf collimator (MLC), one practical way to improve the agreement of planned and delivered doses is to tune the value of dosimetric leaf gap (DLG) in the TPS from the measured DLG. We propose that this step may be necessary due to the complexity of the MLC system, including dosimetry of small fields and the tongue‐and‐groove (T&G) effects, and report our use of test fields to obtain linac‐specific optimal DLGs in TPSs. More than 20 original patient plans were reoptimized with the linac‐specific optimal DLG value. We examined the distribution of gaps and T&G extensions in typical patient plans and the effect of using the optimal DLG on the distribution. The QA pass rate of patient plans using the optimal DLG was investigated. The dose‐volume histograms (DVHs) of targets and organs at risk were checked. We tested three MLC systems (Varian millennium 120 MLC, high‐definition 120 MLC, and Siemens 160 MLC) installed in four Varian linear accelerators (linacs) (TrueBEAM STx, Trilogy, Clinac 2300 iX, and Clinac 21 EX) and 1 Siemens linac (Artiste). With an optimal DLG, the individual QA for all those patient plans passed the institute's criteria (95% in DTA test or gamma test with 3%/3 mm/10%), even though most of these plans had failed to pass QA when using original DLGs optimized from typical patient plans or from the optimization process (automodeler) of Pinnacle TPS. Using either our optimal DLG or one optimized from typical patient plans or from the Pinnacle optimization process yielded similar DVHs.PACS number: 87.55Qr

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Section: Discussionmentioning

confidence: 72%

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

Yao

Farr

2015

J Applied Clin Med Phys

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“…Five equally spaced MLC control points were programmed instead of only two required to define the sliding window movement. The choice of five control points was made following work of Mei et al, (7) where the authors observed a considerable discrepancy between the measurement and the calculation when using only two control points. The leaf trajectory and the jaw openings were chosen similarly to Bhagwat et al (15) (i.e., the X jaws were set to (symmetric) 12 cm and the center of the MLC window coincided with the edge of one of the X jaws at the beginning and the end of the movement of the leafs).…”

Section: Methodsmentioning

confidence: 99%

“…Mei et al (7) employed a variety of methods to find the optimum value of the DLG for use in dynamic IMRT plans for 6 MV photon beams. They inferred the value of the DLG from time‐integrated reading of an ionization chamber placed in the field of a MLC window sliding across the chamber for decreasing values of the width of the window.…”

Section: Introductionmentioning

confidence: 99%

On using the dosimetric leaf gap to model the rounded leaf ends in VMAT/RapidArc plans

Szpala

Cao

Kohli

2014

J Applied Clin Med Phys

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Partial transmission through rounded leaf ends of Varian multileaf collimators (MLC) is accounted for with a parameter called the dosimetric leaf gap (DLG). Verification of the value of the DLG is needed when the dose delivery is accompanied by gantry rotation in VMAT plans. We compared the doses measured with GAFCHROMIC film and an ionization chamber to treatment planning system (TPS) calculations to identify the optimum values of the DLG in clinical plans of the whole brain with metastases transferred to a phantom. We noticed the absence of a single value of the DLG that properly models all VMAT plans in our cohort (the optimum DLG varied between 0.93±0.15 mm and 2.2±0.2 mm). The former value is considerably different from the optimum DLG in sliding window plans (about 2.0 mm) that approximate IMRT plans. We further found that a single‐value DLG model cannot accurately reproduce the measured dose profile even of a uniform static slit at a fixed gantry, which is the simplest MLC‐delimited field. The calculation overestimates the measurement in the proximal penumbra, while it underestimates in the distal penumbra. This prompted us to expand the DLG parameter from a plan‐specific number to a mathematical concept of the DLG being a function of the distance in the beam's eye view (BEV) between the dose point and the leaf ends. Such function compensates for the difference between the penumbras in a beam delimited with a rounded leaf MLC and delimited with solid jaws. Utilization of this concept allowed us generating a pair of step‐and‐shoot MLC plans for which we could qualitatively predict the value of the DLG providing best match to ionization chamber measurements. The plan for which the leafs stayed predominantly at positions requiring low values of the DLG (as seen in the profiles of 1D slits) yielded the combined DLG of 1.1±0.2 mm, while the plan with leafs staying at positions requiring larger values of the DLG yielded the DLG 2.4±0.2 mm. Considering the DLG to be a function of the distance (in BEV) between the dose point and the leaf ends allowed us to provide an explanation as to why conventional single‐number DLG is plan‐specific in VMAT plans.PACS numbers: 87.56.jf, 87.56.nk

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“…6 MLC characteristics defined in the TPS, such as leaf transmission and dosimetric leaf gap (DLG) are crucial to the final dose delivered.…”

Section: Introductionmentioning

confidence: 99%

Spatial variation of dosimetric leaf gap and its impact on dose delivery

et al. 2014

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The spatial variation in DLG is caused by the variation of intraleaf transmission through MLC leaves. Fluences centered on the CAX would not be affected since DLG does not vary; but any fluences residing significantly off axis with narrow sweeping leaves may exhibit significant dose differences. This is due to the fact that there are differences in DLG between the true DLG exhibited by the 1.0 cm width outer leaves and the constant DLG value utilized by the TPS for dose calculation. Since there are large differences in DLG between the 0.5 cm width leaf pairs and 1.0 cm width leaf pairs, there is a need to correct the TPS plans, especially those with high modulation (narrow dynamic MLC gap), with 2D variation of DLG.

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On the use of the MLC dosimetric leaf gap as a quality control tool for accurate dynamic IMRT delivery

Cited by 35 publications

References 11 publications

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

On using the dosimetric leaf gap to model the rounded leaf ends in VMAT/RapidArc plans

Spatial variation of dosimetric leaf gap and its impact on dose delivery

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