Systematic offset of kV and MV localization systems as a function of gantry angle

Mullins, John P.; Herman, Michael

doi:10.1120/jacmp.v12i1.3314

Cited by 3 publications

(2 citation statements)

References 8 publications

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“…Moreover, gravitational pull during gantry rotation affects on‐board imaging systems as well. Though flex maps are commonly used to account for the movement of the imager components, it may lead to an offset between the mechanical and the imaging isocenters 5,6 . Rigid quality control is therefore of utmost importance, and quality standards for stereotactic machines have been defined that exceed those required for conventional therapy according to TG‐142 7 …”

Section: Introductionmentioning

confidence: 99%

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets—Quantification and possible corrections

Wack

Exner

Wegener

et al. 2020

J Applied Clin Med Phys

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Purpose To assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D98, target volume coverage (TVC), conformity (CI), and gradient index (GI). Methods Winston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm3. Results ArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm3, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D98 (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D98 and TVC (7%) for targets below 2 cm3 treated on linac 1. Conclusion Mechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

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Section: Introductionmentioning

confidence: 99%

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets—Quantification and possible corrections

Wack

Exner

Wegener

et al. 2020

J Applied Clin Med Phys

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“…Each image was reviewed individually to ensure the correct location of the tin ball by the algorithm. The flex phenomenon (gravity-related detector panel sag with gantry rotation leading to systematic shifts in position image isocentre as a function of gantry angle [27,28]) was taken into account and quantified using a Ball Bearing phantom equipped with a 8 mm diameter steel ball placed at the imaging isocentre, as in 'figure 3(B)'. The fluctuations of the (u,v) steel ball CM coordinates on images during complete counterclockwise CBCT gantry rotation were automatically recorded thanks to our template based algorithm and then linked to their specific CBCT gantry angle.…”

Section: Experimental Studymentioning

confidence: 99%

Improvement of kilovoltage intrafraction monitoring accuracy through gantry angles selection

Veken

Dechambre

Michiels

et al. 2020

Biomed. Phys. Eng. Express

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Kilovoltage intrafraction monitoring (KIM) is a method allowing to precisely infer the tumour trajectory based on cone beam computed tomography (CBCT) 2D-projections. However, its accuracy is deteriorated in the case of highly mobile tumours involving hysteresis. A first adaptation of KIM consisting of a prior amplitude based binning step has been developed in order to minimize the errors of the original model (phase-KIM). In this work, we propose enhanced methods (KIMsub-arc optim and phase-KIMsub-arc optim) to improve the accuracy of KIM and phase-KIM which relies on the selection of the optimal starting CBCT gantry angle. Aiming at demonstrating the interest of our approach, we carried out a simulation study and an experimental study: we compared the accuracy of the conventional versus sub-arc optim methods on simulated realistic tumour motions with amplitudes ranging from 5 to 30 mm in 1 mm increments. The same approach was performed using a lung dynamic phantom generating a 30 mm amplitude sinusoidal motion. The results show that for in-silico simulated motions of 10, 20 and 30 mm amplitude, the three-dimensional root mean square error (3D-RMSE) can be reduced by 0.67 mm, 0.91 mm, 0.94 mm and 0.18 mm, 0.25 mm, 0.28 mm using KIMsub-arc optim and phase-KIMsub-arc optim respectively. Considering all in-silico simulated trajectories, the percentage of errors larger than 1 mm decreases from 21.9% down to 1.6% for KIM (p < 0.001) and from 6.6% down to 1.2% for phase-KIM (p < 0.001). Experimentally, the 3D-RMSE is lowered by 0.5732 mm for KIM and by 0.1 mm for phase-KIM. The percentage of errors larger than 1 mm falls from 39.7% down to 18.5% for KIM and from 23.2% down to 11.1% for phase-KIM. In conclusion, our method efficiently anticipates CBCT gantry angles associated with a significantly better accuracy by using KIM and phase-KIM.

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Implementation and evaluation of an end‐to‐end IGRT test

Kry

Jones²,

Childress³

2012

J Applied Clin Med Phys

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The goal of this work was to develop and evaluate an end‐to‐end test for determining and verifying image‐guided radiation therapy setup accuracy relative to the radiation isocenter. This was done by placing a cube phantom with a central tungsten sphere directly on the treatment table and offset from isocenter either by 5.0 mm in the longitudinal, lateral, and vertical dimensions or by a random amount. A high‐resolution cone‐beam CT image was acquired and aligned with the tungsten sphere in the reference CT image. The table was shifted per this alignment, and megavoltage anterior–posterior and lateral images were acquired with the electronic portal imaging device. Agreement between the radiation isocenter (based on the MV field) and the center of the sphere (i.e., the alignment point based on kV imaging) was determined for each image via Winston‐Lutz analysis. This procedure was repeated 10 times to determine short‐term reproducibility, and then repeated daily for 51 days in a clinical setting. The short‐term reproducibility test yielded a mean 3D vector displacement of 0.9±0.15mm between the imaging‐based isocenter and the radiation isocenter, with a maximum displacement of 1.1 mm. The clinical reproducibility test yielded a mean displacement of 1.1±0.4mm with a maximum of 2.0 mm when the cube was offset by 5.0 mm, and a mean displacement of 0.9±0.3mm with a maximum of 1.8 mm when the cube was offset by a random amount. These differences were observed in all directions and were independent of the magnitude of the couch shift. This test was quick and easy to implement clinically and highlighted setup inaccuracies in an image‐guided radiation therapy environment.PACS numbers: 87.55.km; 87.55.Qr; 87.56.Fc

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Systematic offset of kV and MV localization systems as a function of gantry angle

Cited by 3 publications

References 8 publications

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets—Quantification and possible corrections

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets—Quantification and possible corrections

Improvement of kilovoltage intrafraction monitoring accuracy through gantry angles selection

Implementation and evaluation of an end‐to‐end IGRT test

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