Isocenter verification for linac‐based stereotactic radiation therapy: review of principles and techniques

Rowshanfarzad, Pejman; Sabet, Mahsheed; O’Connor, D.J.; Greer, Peter B.

doi:10.1120/jacmp.v12i4.3645

Cited by 71 publications

(82 citation statements)

References 72 publications

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“…The test was originally developed to use very complicated film-based measurements that were dependent on the observer's measurement skill [1], but it later progressed to electronic portal imaging device (EPID)-based measurements that were easy to obtain using commercial software. This had become easy implemented after Rowshanfarzad et al wrote a broad review of the test [10] and Childress et al delineated a practical method to perform an EPID-based Winston-Lutz test in linear accelerator systems [11]. However, the standard Winston-Lutz test focuses only on the machine isocenter; it cannot accurately determine whether the center of the mechanical field (which is represented by the light field) and the center of the radiation field coincide when both are off from the isocenter (i.e., an asymmetric field).…”

Section: Introductionmentioning

confidence: 99%

Off-Isocenter Winston-Lutz Test for Stereotactic Radiosurgery/Stereotactic Body Radiotherapy

Gao¹,

Liu²

2016

IJMPCERO

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The single-isocenter technique in linear accelerator-based stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) has been broadly used to treat multiple lesions. However, quantitative study to verify that the mechanical field center coincides with the radiation field center when both are off from the isocenter has never been performed. We developed an innovative method to measure this accuracy, called the off-isocenter Winston-Lutz test, and here we provided a practical clinical guideline to implement this technique. We used ImagePro V.6 to analyze images of a Winston-Lutz phantom obtained using a Varian 21EX linear accelerator with an electronic portal imaging device, set up as for single-isocenter SRS/SBRT for multiple lesions. We investigated asymmetry field centers that were 3 cm and 5 cm away from the isocenter, as well as performing the standard Winston-Lutz test. We used a special beam configuration to acquire images while avoiding collision, and we investigated both jaw and multileaf collimation. For the jaw collimator setting, at 3 cm off-isocenter, the mechanical field deviated from the radiation field by about 2.5 mm; at 5 cm, the deviation was above 3 mm, up to 4.27 mm. For the multileaf collimator setting, at 3 cm offisocenter, the deviation was below 1 mm; at 5 cm, the deviation was above 1 mm, up to 1.72 mm, which was 72% higher than the tolerance threshold. These results indicated that the further the asymmetry field center is from the machine isocenter, the larger the deviation of the mechanical field from the radiation field, and the distance between the center of the asymmetry field and the isocenter should not exceed 3 cm in our clinic. We recommend that every clinic that uses linear accelerator, multileaf collimator-based SRS/SBRT perform the off-isocenter Winston-Lutz test in addition to the standard Winston-Lutz test and use their own deviation data to create planning guideline. KeywordsOff-Isocenter Winston-Lutz Test, SRS, SBRT

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Section: Introductionmentioning

confidence: 99%

Off-Isocenter Winston-Lutz Test for Stereotactic Radiosurgery/Stereotactic Body Radiotherapy

Gao¹,

Liu²

2016

IJMPCERO

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“…The rising interest in frame‐less stereotactic radiotherapy to body sites with image guidance has presented new challenges for quality assurance of linear accelerators. The requirement of placing high‐dose gradients in extreme proximity to critical structures, such as stereotactic radiotherapy of spine malignancies and the spinal cord, 20 , 21 , 22 , 23 , 24 , 25 has resulted in more stringent AAPM guidelines for treatment accuracy and the development of precision methods to characterize performance (26) …”

Section: Discussionmentioning

confidence: 99%

Improved accuracy for noncoplanar radiotherapy: an EPID‐based method for submillimeter alignment of linear accelerator table rotation with MV isocenter

Nyflot

Cao

Meyer

et al. 2014

J Applied Clin Med Phys

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Accurate alignment of linear accelerator table rotational axis with radiation isocenter is critical for noncoplanar radiotherapy applications. The purpose of the present study is to develop a method to align the table rotation axis and the MV isocenter to submillimeter accuracy. We developed a computerized method using electronic portal imaging device (EPID) and measured alignment stability over time. Mechanical and radiation isocenter coincidence was measured by placing a steel ball bearing at radiation isocenter using existing EPID techniques. Then, EPID images were acquired over the range of table rotation. A MATLAB script was developed to calculate the center of rotation, as well as the necessary adjustment to move the table rotational axis to MV isocenter. Adjustment was applied via torque to screws at the base of the linac table. Stability of rotational alignment was measured with 49 measurements over 363 days on four linacs. Initial rotational misalignment from radiation isocenter ranged from 0.91−2.11 mm on the four tested linacs. Linac‐A had greatest error false(>2 mmfalse) and was adjusted with the described method. After adjustment, the error was significantly decreased to 0.40±0.12 mm. The adjustment was stable over the course of 15 measurements over 231 days. Linac‐B was not adjusted, but tracked from time of commissioning with 27 measurements over 363 days. No discernible shift in couch characteristics was observed (mean error 1.40±0.22 mm). The greater variability for Linac‐B may relate to the interchangeable two‐piece couch, which allows more lateral movement than the one‐piece Linac‐A couch. Submillimeter isocenter alignment was achieved by applying a precision correction to the linac table base. Table rotational characteristics were shown to be stable over the course of twelve months. The accuracy and efficiency of this method may make it suitable for acceptance testing, annual quality assurance, or commissioning of highly‐conformal noncoplanar radiotherapy programs.PACS number: 87

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“…Then, projection images of the phantom are acquired and discrepancy between radiation and mechanical isocenter is reported whenever the center of the ball -as projected onto detector plane -deviates from the center of the radiation field. Measurements are repeated for a few angular positions of a gantry of a therapeutic device (Rowshanfarzad et al, 2011).…”

Section: Niedźwiecki M Et Al: Quality Assurance In Radiotherapymentioning

confidence: 99%

Application of Image Analysis Methods for Isocenter Quality Assurance in Radiotherapy

et al. 2017

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One of the major procedures for testing the geometrical accuracy of devices used in radiotherapy treatments is the test of the geometrical position of the radiation isocenter. The importance of the test reflects the fact that geometrical position of the radiation isocenter generally affects the tumor targeting. At present the geometric accuracy is assessed with the Winston-Lutz test which checks the position of an image of a ball marker with the respect to the center of the radiation field as projected on a detector plane. Obviously, determination of coordinates of a single marker is not sufficient to fully account for a complicated geometry of a therapeutic device. The purpose of the study was to design a new image analysis tool to better determine the isocenter. The proposed automated procedure for determining isocenter position uses projection data acquired for a special cube phantom. The projection images of a phantom are acquired for various angles of rotation of the gantry. A procedure is proposed to extract some geometric characteristics of a therapeutic device from the projection images.

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Isocenter verification for linac‐based stereotactic radiation therapy: review of principles and techniques

Cited by 71 publications

References 72 publications

Off-Isocenter Winston-Lutz Test for Stereotactic Radiosurgery/Stereotactic Body Radiotherapy

Off-Isocenter Winston-Lutz Test for Stereotactic Radiosurgery/Stereotactic Body Radiotherapy

Improved accuracy for noncoplanar radiotherapy: an EPID‐based method for submillimeter alignment of linear accelerator table rotation with MV isocenter

Application of Image Analysis Methods for Isocenter Quality Assurance in Radiotherapy

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