Virtual Micro MLC Commissioning

Drever, Laura; Dickof, Peter

doi:10.1120/jacmp.2024.25332

Cited by 1 publication

(1 citation statement)

References 7 publications

(10 reference statements)

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“…Wong (2004) describes a device to measure MLC fields at varying gantry angles; the results of his tests with a microMLC suggest 'that gravitational acceleration influences leaf position error, which subsequently leads to inaccurate dosimetry. ' Drever & Dickof (2005) report variation of 1.3 mm with varying gantry angle measured using films for a double-focused MLC. Parent et al (2006) recently described a calibration method using the same optically controlled MLC and amorphous silicon EPID used in this work and report no significant effects of gravity at gantry angles 90 • and 270 • .…”

Section: Introductionmentioning

confidence: 94%

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

Clarke¹,

Budgell²

2007

Phys. Med. Biol.

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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.

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Section: Introductionmentioning

confidence: 94%