Use of an amorphous silicon EPID for measuring MLC calibration at varying gantry angle.

2.50
Hdl Handle:
http://hdl.handle.net/10541/66855
Title:
Use of an amorphous silicon EPID for measuring MLC calibration at varying gantry angle.
Authors:
Clarke, Mathew F; Budgell, Geoff J
Abstract:
Amorphous silicon electronic portal imaging devices (EPIDs) are used to perform routine quality control (QC) checks on the multileaf collimators (MLCs) at this centre. Presently, these checks are performed at gantry angle 0 degrees and are considered to be valid for all other angles. Since therapeutic procedures regularly require the delivery of MLC-defined fields to the patient at a wide range of gantry angles, the accuracy of the QC checks at other gantry angles has been investigated. When the gantry is rotated to angles other than 0 degrees it was found that the apparent pixel size measured using the EPID varies up to a maximum value of 0.0015 mm per pixel due to a sag in the EPID of up to 9.2 mm. A correction factor was determined using two independent methods at a range of gantry angles between 0 degrees and 360 degrees . The EPID was used to measure field sizes (defined by both x-jaws and MLC) at a range of gantry angles and, after this correction had been applied, any residual gravitational sag was studied. It was found that, when fields are defined by the x-jaws and y-back-up jaws, no errors of greater than 0.5 mm were measured and that these errors were no worse when the MLC was used. It was therefore concluded that, provided the correction is applied, measurements of the field size are, in practical terms, unaffected by gantry angle. Experiments were also performed to study how the reproducibility of individual leaves is affected by gantry angle. Measurements of the relative position of each individual leaf (minor offsets) were performed at a range of gantry angles and repeated three times. The position reproducibility was defined by the RMS error in the position of each leaf and this was found to be 0.24 mm and 0.21 mm for the two leaf banks at a gantry angle of 0 degrees . When measurements were performed at a range of gantry angles, these reproducibility values remained within 0.09 mm and 0.11 mm. It was therefore concluded that the calibration of the Elekta MLC is stable at all gantry angles.
Affiliation:
North Western Medical Physics, Christie Hospital NHS Foundation Trust, Withington, Manchester, M20 4BX, UK.
Citation:
Use of an amorphous silicon EPID for measuring MLC calibration at varying gantry angle. 2008, 53 (2):473-85 Phys Med Biol
Journal:
Physics in Medicine and Biology
Issue Date:
21-Jan-2008
URI:
http://hdl.handle.net/10541/66855
DOI:
10.1088/0031-9155/53/2/013
PubMed ID:
18185000
Type:
Article
Language:
en
ISSN:
0031-9155
Appears in Collections:
All Christie Publications ; Christie Medical Physics and Engineering Research

Full metadata record

DC FieldValue Language
dc.contributor.authorClarke, Mathew F-
dc.contributor.authorBudgell, Geoff J-
dc.date.accessioned2009-05-01T10:28:10Z-
dc.date.available2009-05-01T10:28:10Z-
dc.date.issued2008-01-21-
dc.identifier.citationUse of an amorphous silicon EPID for measuring MLC calibration at varying gantry angle. 2008, 53 (2):473-85 Phys Med Biolen
dc.identifier.issn0031-9155-
dc.identifier.pmid18185000-
dc.identifier.doi10.1088/0031-9155/53/2/013-
dc.identifier.urihttp://hdl.handle.net/10541/66855-
dc.description.abstractAmorphous silicon electronic portal imaging devices (EPIDs) are used to perform routine quality control (QC) checks on the multileaf collimators (MLCs) at this centre. Presently, these checks are performed at gantry angle 0 degrees and are considered to be valid for all other angles. Since therapeutic procedures regularly require the delivery of MLC-defined fields to the patient at a wide range of gantry angles, the accuracy of the QC checks at other gantry angles has been investigated. When the gantry is rotated to angles other than 0 degrees it was found that the apparent pixel size measured using the EPID varies up to a maximum value of 0.0015 mm per pixel due to a sag in the EPID of up to 9.2 mm. A correction factor was determined using two independent methods at a range of gantry angles between 0 degrees and 360 degrees . The EPID was used to measure field sizes (defined by both x-jaws and MLC) at a range of gantry angles and, after this correction had been applied, any residual gravitational sag was studied. It was found that, when fields are defined by the x-jaws and y-back-up jaws, no errors of greater than 0.5 mm were measured and that these errors were no worse when the MLC was used. It was therefore concluded that, provided the correction is applied, measurements of the field size are, in practical terms, unaffected by gantry angle. Experiments were also performed to study how the reproducibility of individual leaves is affected by gantry angle. Measurements of the relative position of each individual leaf (minor offsets) were performed at a range of gantry angles and repeated three times. The position reproducibility was defined by the RMS error in the position of each leaf and this was found to be 0.24 mm and 0.21 mm for the two leaf banks at a gantry angle of 0 degrees . When measurements were performed at a range of gantry angles, these reproducibility values remained within 0.09 mm and 0.11 mm. It was therefore concluded that the calibration of the Elekta MLC is stable at all gantry angles.en
dc.language.isoenen
dc.subjectElectronic Portal Imaging Devicesen
dc.subjectEPIDen
dc.subjectMultileaf Collimatoren
dc.subject.meshCalibration-
dc.subject.meshEquipment Design-
dc.subject.meshEquipment Failure Analysis-
dc.subject.meshGreat Britain-
dc.subject.meshRadiation Dosage-
dc.subject.meshRadiometry-
dc.subject.meshRadiotherapy, Conformal-
dc.subject.meshReproducibility of Results-
dc.subject.meshSensitivity and Specificity-
dc.subject.meshSilicon-
dc.titleUse of an amorphous silicon EPID for measuring MLC calibration at varying gantry angle.en
dc.typeArticleen
dc.contributor.departmentNorth Western Medical Physics, Christie Hospital NHS Foundation Trust, Withington, Manchester, M20 4BX, UK.en
dc.identifier.journalPhysics in Medicine and Biologyen

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