Evaluation of two methods of predicting MLC leaf positions using EPID measurements

Parent, L.; Seco, Joao; Evans, Philip; Dance, David R.; Fielding, Andrew

doi:10.1118/1.2335490

Cited by 50 publications

(47 citation statements)

References 6 publications

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“…Baker et al 19 and Parent et al 14 reported that a systematic tilt of the EPIDs was observed in their studies and indicated that it was likely to occur for all different EPIDs; Clarke and Budgell 20 have also demonstrated the effect of the gantry angle on the EPID sag. Therefore, it is expected that the imaging geometry for the EPID at different gantry angles might deviate from an ideal configuration that we use as the basis for the measurement of the leaf end position.…”

Section: Iib Geometric Status Of the Epidmentioning

confidence: 99%

“…Any systematic error introduced in the MLC calibration might lead to actual leaf positions different from the expected ones, resulting in dose errors. 14 In order to circumvent this dependence and provide a universal approach of probing the actual delivery of a fluence map, we propose using an amorphous silicon electronic portal imaging device ͑aSi-EPID͒ to capture every segment of the fluence map during the treatment. For each captured segment, the leaf positions for each pair of leaves are found by an edge detection algorithm; the fractional monitor unit ͑fMU͒ associated with this particular segment is also sampled.…”

Section: Introductionmentioning

confidence: 99%

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The use of EPID‐measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy

Lee

Mao

2008

Medical Physics

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For intensity modulated radiation treatment ͑IMRT͒ dose reconstruction, multileaf collimator ͑MLC͒ log files have been shown applicable for deriving delivered fluence maps. However, MLC log files are dependent on the accuracy of leaf calibration and only available from one linear accelerator manufacturer. This paper presents a proof of feasibility and principles in ͑1͒ using an amorphous silicon electronic portal imaging device ͑aSi-EPID͒ to capture the MLC segments during an IMRT delivery and ͑2͒ reconstituting a leaf sequence ͑LS͒ file based on the leaf end positions calculated from the MLC segments and their associated fractional monitor units. These EPIDmeasured LS files are then used to derive delivered fluence maps for dose reconstruction. The developed approach was tested on a pelvic phantom treated with a typical prostate IMRT plan. The delivered fluence maps, which were derived from the EPID-measured LS files, showed slight differences in the intensity levels compared with the corresponding planned ones. The dose distribution calculated with the delivered fluence maps showed a discernible difference in the high dose region when compared to that calculated with the planned fluence maps. The maximum dose in the former distribution was also 2.5% less than that in the latter one. The EPID-measured LS file can serve the same purpose as a MLC log files does for the derivation of the delivered fluence map and yet is independent of the leaf calibration. The approach also allows users who do not have access to MLC log files to probe the actual IMRT delivery and translate the information gained for dose reconstruction in adaptive radiation therapy.

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Section: Iib Geometric Status Of the Epidmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The use of EPID‐measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy

Lee

Mao

2008

Medical Physics

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“…Using the images of the square field, the collimator angle and the tilt of the EPID detector were corrected. 18 Gantry angle was 0°for all measurements.…”

Section: Iib Imager and Acceleratormentioning

confidence: 99%

“…The leaf positions for each pair were determined at the 50% of the intensity profile formed by the pair. 18 The leaf positions on the EPID images were determined using the same method as the one used for film. The leaf positions measured with the film and EPID were compared to determine the accuracy of the VORTE.…”

Section: Iic Measurement Of the Accuracy Of The Vortementioning

confidence: 99%

Verification of MLC based real‐time tumor tracking using an electronic portal imaging device

Han‐Oh

Lerma

et al. 2010

Medical Physics

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Purpose:The authors have developed a novel technique using an electronic portal imaging device ͑EPID͒ to verify the geometrical accuracy of delivery of dose-rate-regulated tracking ͑DRRT͒. This technique, called verification of real-time tracking with EPID ͑VORTE͒, can potentially be used for both on-line and off-line quality assurance ͑QA͒ of MLC-based dynamic tumor tracking. Methods: The shape and position of target as a function of time, which is assumed to be known, is projected onto the EPID plane. This projected sequence of apertures as a function of time ͑target motion͒ is then used as the reference. The accuracy of dynamic MLC tracking can then be assessed by how well the delivered beam follows this projected target motion without the use of a physical moving phantom. The beam apertures controlled by DRRT ͑aperture motion͒ is detected by the EPID as a function of time. The aperture motion is compared to the target motion to evaluate tracking error introduced by DRRT. The accuracy of VORTE was measured using film measurements of ten static fields. The VORTE for dynamic tumor tracking was tested with several target motions, including ͑1͒ rigid-body two-dimensional ͑2-D͒ cyclic motion in the superior-inferior direction with various period and amplitude; ͑2͒ the above 2-D cyclic motion plus cyclic deformation; and ͑3͒ 2-D cyclic motion with both deformation and rotation. For each target motion, the controlled aperture motion resulting from DRRT was acquired at ϳ8 Hz using EPID in the continuous-acquisition mode. Leaf positions in all captured frames were measured from the EPID and compared to their expected positions. The passing rate of 2 mm criteria for all leaves from all frames was calculated for each of the four patterns of tumor motion. Additionally, the root-meansquare ͑RMS͒ deviations of the centroid of the apertures between the designed and delivered beams were calculated for all three cases. Results: The accuracy of MLC-leaf position determination by VORTE is 0.5 mm ͑1 standard deviation͒ by comparison to film measurements. With DRRT, the passing rates using the 2 mm criteria for all acquired frames are 100% for the 2-D displacement, 99% for the 2-D displacement with deformation, and 88% for the 2-D displacement combined with both deformation and rotation. The RMS deviations are 0.6 mm for the 2-D displacement, 1.0 mm for the 2-D displacement with deformation, and 1.1 mm for the 2-D displacement combined with both deformation and rotation. Conclusions: The VORTE can measure the accuracy of MLC-based tumor tracking without the necessity of employing a moving phantom. Moreover, it can be used for complex target motion ͑i.e., 2-D displacement combined with deformation and rotation͒ that is difficult to create with physical moving phantoms. Therefore, the VORTE and the novel QA process illustrated by this study have a great potential for verifying real-time tumor tracking.

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“…The electronic portal imaging device (EPID) is a dependable system when corrections are made for systematic tilts and shifts [7,8] and when image sagging due to gantry angle [9] has been taken into account. A significant number of researchers have investigated MLC QA by film or EPID [7][8][9][10][11][12][13] to measure the accuracy of the MLC controller independently and ensure that the MLC edge positions agree with the radiation field edges to within 0.3 mm [14]. EPID measurements are highly reproducible, with a standard deviation of ,0.1 mm for individual leaf/collimator positions and ,0.05 mm for a 10610 cm 2 field [7].…”

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confidence: 99%

Quality assurance of dynamic parameters in volumetric modulated arc therapy

Manikandan

Sarkar²,

Holla³

et al. 2012

BJR

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Objectives: The purpose of this study was to demonstrate quality assurance checks for accuracy of gantry speed and position, dose rate and multileaf collimator (MLC) speed and position for a volumetric modulated arc treatment (VMAT) modality (Synergy S; Elekta, Stockholm, Sweden), and to check that all the necessary variables and parameters were synchronous. Methods: Three tests (for gantry position-dose delivery synchronisation, gantry speed-dose delivery synchronisation and MLC leaf speed and positions) were performed. Results: The average error in gantry position was 0.5 u and the average difference was 3 MU for a linear and a parabolic relationship between gantry position and delivered dose. In the third part of this test (sawtooth variation), the maximum difference was 9.3 MU, with a gantry position difference of 1.2 u . In the sweeping field method test, a linear relationship was observed between recorded doses and distance from the central axis, as expected. In the open field method, errors were encountered at the beginning and at the end of the delivery arc, termed the ''beginning'' and ''end'' errors. For MLC position verification, the maximum error was 22.46 mm and the mean error was 0.0153 ¡0.4668 mm, and 3.4% of leaves analysed showed errors of .¡1 mm. Conclusion: This experiment demonstrates that the variables and parameters of the Synergy S are synchronous and that the system is suitable for delivering VMAT using a dynamic MLC. The concept of volumetric modulated arc therapy (VMAT) has been described in many studies [1][2][3][4][5]. VMAT is a system for intensity-modulated radiotherapy treatment (IMRT) delivery that achieves high dose conformity by optimising the dose rate, gantry speed and leaf positions of the dynamic multileaf collimator (MLC) [6]. One study [5] demonstrated quality assurance (QA) checks using dynamic MLC controller log files (Dynalog) for VMAT systems such as RapidArcH (Varian Medical Systems Inc., Palo Alto, CA). It is assumed that the actual delivery process is truly represented in the log files [6]. The major disadvantage of this method is that Dynalog files need to be validated against an independent system. The electronic portal imaging device (EPID) is a dependable system when corrections are made for systematic tilts and shifts [7,8] and when image sagging due to gantry angle [9] has been taken into account. A significant number of researchers have investigated MLC QA by film or EPID [7][8][9][10][11][12][13] to measure the accuracy of the MLC controller independently and ensure that the MLC edge positions agree with the radiation field edges to within 0.3 mm [14]. EPID measurements are highly reproducible, with a standard deviation of ,0.1 mm for individual leaf/collimator positions and ,0.05 mm for a 10610 cm 2 field [7]. Few studies [15][16][17] have demonstrated commissioning, QA and patient-specific QA for VMAT using both the RapidArc and the SynergyH S (Elekta, Stockholm, Sweden) systems. The purpose of this study was to demonstrate QA checks for accuracy of gantry...

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Evaluation of two methods of predicting MLC leaf positions using EPID measurements

Abstract: In intensity modulated radiation treatments (IMRT),

Cited by 50 publications

References 6 publications

The use of EPID‐measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy

The use of EPID‐measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy

Verification of MLC based real‐time tumor tracking using an electronic portal imaging device

Quality assurance of dynamic parameters in volumetric modulated arc therapy

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