Prog Med Phys.  2018 Jun;29(2):66-72. 10.14316/pmp.2018.29.2.66.

Verification of Mechanical Leaf Gap Error and VMAT Dose Distribution on Varian VitalBeamâ„¢ Linear Accelerator

• Kim MS
• Choi CH
• An HJ
• Son JM
• Park SY

• KMID: 2415905
• DOI: http://doi.org/10.14316/pmp.2018.29.2.66

Abstract

The proper position of a multi-leaf collimator (MLC) is essential for the quality of intensity-modulated radiation therapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) dose delivery. Task Group (TG) 142 provides a quality assurance (QA) procedure for MLC position. Our study investigated the QA validation of the mechanical leaf gap measurement and the maintenance procedure. Two VitalBeamâ„¢ systems were evaluated to validate the acceptance of an MLC position. The dosimetric leaf gaps (DLGs) were measured for 6 MV, 6 MVFFF, 10 MV, and 15 MV photon beams. A solid water phantom was irradiated using 10×10 cm² f ield s ize a t s ource-to-surface distance (SSD) of 90 cm and depth of 10 cm. The portal dose image prediction (PDIP) calculation was implemented on a treatment planning system (TPS) called Eclipseâ„¢. A total of 20 VMAT plans were used to confirm the accuracy of dose distribution measured by an electronic portal imaging device (EPID) and those predicted by VMAT plans. The measured leaf gaps were 0.30 mm and 0.35 mm for VitalBeam 1 and 2, respectively. The DLG values decreased by an average of 6.9% and 5.9% after mechanical MLC adjustment. Although the passing rates increased slightly, by 1.5% (relative) and 1.2% (absolute) in arc 1, the average passing rates were still within the good dose delivery level (>95%). Our study shows the existence of a mechanical leaf gap error caused by a degenerated MLC motor. This can be recovered by reinitialization of MLC position on the machine control panel. Consequently, the QA procedure should be performed regularly to protect the MLC system.

Keyword

Optimization of mechanical leaf gap; Dosimetric leaf Gap; PDIP calculation

MeSH Terms

Particle Accelerators*
Radiotherapy
Water
Water

Figure

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  Fig. 1. Mechanical multi-leaf collimator (MLC) gap measurement. (a) Millennium 120 MLCs of VitalBeamTM accelerator in a 180° rotated position for measuring actual leaf gap width (b) a filler gauge used for leaf gap width assessment.

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  Fig. 2. The dosimetic leaf gap (DLG) measurement using ionization chamber and solid water phantom: (a) schematic design and (b) solid water phantom (30 cm×30 cm), and famer ion-chamber on a couch in VitalBeamTM.

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  Fig. 3. Example of predicted linear dependence for 6 MV photon beam and equation for obtaining the absolute value of b in order to seek the dosimetric leaf gap (DLG).

Reference

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