What Do We Mean When We Talk about the Linac Isocenter?

Zhang, Mutian; Zhou, Sumin; Qu, Tanxia

doi:10.4236/ijmpcero.2015.43028

Cited by 12 publications

(21 citation statements)

References 20 publications

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“…The gantry rotation isocenter was defined as the center of the smallest sphere through whose surface all the above straight lines passing. This intuitive definition is an approximation of what was previously proposed [16]. The actual implementation of such definition is shown in Figure 2.…”

Section: The Gantry Rotation Isocentermentioning

confidence: 81%

“…The estimation uncertainty introduced by the rough isocenter not being the real isocenter during this process is assumed minimal. Finally, the isocenter was determined by literally implementing the isocenter definition according to a previously published paper [16], i.e., to search a point within the field edge whose trajectory of rotation gives a minimum bounding circle given the relative shifts between all the rotations. This was achieved by a two-step brute-force searching algorithm [16].…”

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

“…Finally, the isocenter was determined by literally implementing the isocenter definition according to a previously published paper [16], i.e., to search a point within the field edge whose trajectory of rotation gives a minimum bounding circle given the relative shifts between all the rotations. This was achieved by a two-step brute-force searching algorithm [16]. The first step searched the approximate location of such point using a large search step size and then the second step continued to conduct the search near the approximate location with a much smaller search step size of 0.01 mm.…”

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

“…Because all the points on the rigid body have identical angular velocity relative to each other, the AOR has to be defined within the stationary reference system using the concept of the smallest enclosing sphere/circle, as suggested by the previous investigators [19] [20]. For detailed discussion on the isocenter definition of the collimator and table, please refer to the previously published work [16].…”

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

“…With high resolution EPID images, it is feasible to determine the isocenter diameter and spatial location based on our previously published theoretical framework of isocenter definition [16]. According to the proposed framework, the Linac gantry, collimator, and table systems are considered rigid bodies, and their isocenters are defined using the axis of rotation (AOR) observed in a stationary reference system.…”

Section: Introductionmentioning

confidence: 99%

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A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

Fan¹,

Zhou²,

Yu³

et al. 2018

IJMPCERO

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Purpose: With usually a millimeter-level PTV margin, stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) pose a stringent requirement on the isocentricity of the Linac. This requirement is partly fulfilled by routine isocenter quality assurance (QA) test to verify the size and location of the isocenter. The current common QA methods such as spoke shot were developed before SBRT/SRS became popular and when IGRT was largely absent and hence have their limitations. In this work, we describe an isocenter QA approach based on portal imaging to provide the community with a superior alternative. Methods: The proposed approach utilizes a BrainLab ball bearing (BB) phantom in conjunction with an electronic portal imaging devices (EPID) imager. The BB phantom was first aligned with a calibrated room laser system. Portal images were then acquired using 6 MV beam with a 2 × 2 cm 2 open field and a 15 mm cone on a Varian TrueBeam STx machine. The gantry, collimator, and table were rotated separately at selected angles to acquire a series of portal images in order to determine the isocenter of each rotating system. The location and diameter of these isocenters were determined by calculating the relative displacement of either BB or open field edge between the acquired EPID images. The demonstration of the reproducibility and robustness of this EPID-based approach was carried out by repeating measurements 10 times independently for each rotating system and simulating clinical scenarios of asymmetric jaws and misalignment of BB phantom, respectively. Results: For our TrueBeam STx machine, the isocenter diameter derived from open-field EPID images was roughly 0.15 mm, 0.18 mm, 0.49 mm for the collimator, table, and gantry, respectively. For the collimator and gantry, images taken with the cone gave considerably smaller isocenter diameter. Results remained almost unchanged despite the presence of How to cite this paper: Fan101 Int. J. Medical Physics, Clinical Engineering and Radiation Oncology simulated BB misalignment and asymmetric jaws error, and between independent measurements. Isocenter location and diameter derived from images obtained at a limited number of angles (≤11) were adequately accurate to represent those derived from images of densely sampled angles. Conclusions: An EPID-based isocenter QA approach is described and demonstrated to be accurate, robust, and reproducible. This approach provides a superior alternative to conventional isocenter QA methods with no additional cost. It can be implemented with convenience for any linear accelerator with an EPID imager.

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Section: The Gantry Rotation Isocentermentioning

confidence: 81%

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

Section: Collimator and Table Rotation Isocentermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

Fan¹,

Zhou²,

Yu³

et al. 2018

IJMPCERO

Self Cite

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A novel method to determine linac mechanical isocenter position and size and examples of specific QA applications

Chojnowski

Sykes

Thwaites

2021

J Applied Clin Med Phys

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The most important geometric characteristic of stereotactic treatment is the accuracy of positioning the target at the treatment isocenter and the accuracy of directing the radiation beam at the treatment isocenter. Commonly, the radiation isocenter is used as the reference for the treatment isocenter, but its method of localization is not strictly defined, and it depends on the linac-specific beam steering parameters. A novel method is presented for determining the linac mechanical isocenter position and size based on the localization of the collimator axis of rotation at arbitrary gantry angle. The collimator axis of rotation position is determined from the radiation beam center position corrected for the focal spot offset. The focal spot offset is determined using the image center shift method with a customdesign rigid phantom with two sets of ball-bearings. Three specific quality assurance (QA) applications and assessment methods are also presented to demonstrate the functionality of linac mechanical isocenter position and size determination in clinical practice. The first is a mechanical and radiation isocenters coincidence test suitable for quick congruence assessment of these two isocenters for a selected energy, usually required after a nonroutine linac repair and/or energy adjustment. The second is a stereotactic beam isocentricity assessment suitable for pretreatment stereotactic QA. The third is a comprehensive linac geometrical performance test suitable for routine linac QA. The uncertainties of the method for determining mechanical isocenter position and size were measured to be 0.05 mm and 0.04 mm, respectively, using four available photon energies, and were significantly smaller than those of determining the radiation isocenter position and size, which were 0.36 mm and 0.12 mm respectively. It is therefore recommended that the mechanical isocenter position and size be used as the reference linac treatment isocenter and a linac mechanical characteristic parameter respectively.

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AAPM Medical Physics Practice Guideline 8.b: Linear accelerator performance tests

Krauss,

Balik,

Cirino

et al. 2023

J Applied Clin Med Phys

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The purpose of this guideline is to provide a list of critical performance tests to assist the Qualified Medical Physicist (QMP) in establishing and maintaining a safe and effective quality assurance (QA) program. The performance tests on a linear accelerator (linac) should be selected to fit the clinical patterns of use of the accelerator and care should be given to perform tests which are relevant to detecting errors related to the specific use of the accelerator. Current recommendations for linac QA were reviewed to determine any changes required to those tests highlighted by the original report as well as considering new components of the treatment process that have become common since its publication. Recommendations are made on the acquisition of reference data, routine establishment of machine isocenter, basing performance tests on clinical use of the linac, working with vendors to establish QA tests and performing tests after maintenance and upgrades. The recommended tests proposed in this guideline were chosen based on consensus of the guideline's committee after assessing necessary changes from the previous report. The tests are grouped together by class of test (e.g., dosimetry, mechanical, etc.) and clinical parameter tested. Implementation notes are included for each test so that the QMP can understand the overall goal of each test. This guideline will assist the QMP in developing a comprehensive QA program for linacs in the external beam radiation therapy setting. The committee sought to prioritize tests by their implication on quality and patient safety. The QMP is ultimately responsible for implementing appropriate tests. In the spirit of the report from American Association of Physicists in Medicine Task Group 100, individual institutions are encouraged to analyze the risks involved in their own clinical practice and determine which performance tests are relevant in their own radiotherapy clinics.

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What Do We Mean When We Talk about the Linac Isocenter?

Cited by 12 publications

References 20 publications

A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

A novel method to determine linac mechanical isocenter position and size and examples of specific QA applications

AAPM Medical Physics Practice Guideline 8.b: Linear accelerator performance tests

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