Multileaf collimator performance monitoring and improvement using semiautomated quality control testing and statistical process control

Abstract

A MLC performance monitoring system has been developed and implemented to acquire high-quality QC data at high frequency. This is enabled by the relatively short acquisition time for the images and automatic image analysis. The monitoring system was also used to record and track the rate of MLC-related interlocks and servicing events. MLC performances for two commercially available MLC models have been assessed and the results support monthly test frequency for widely accepted ± 1 mm specifications. Higher QC test frequency is however required to maintain tighter specification and in-control behavior.

Evaluation of Elekta Agility multi‐leaf collimator performance using statistical process control tools

“…The increase in measured leaf position error with the distance to the radiation isocenter and the leaf bank motion in the same direction may be explained by a problem with the leaf gain calibration parameter and/or a difference in image pixel scaling factor between the daily MLC QC tests and the image‐based procedure used by the manufacturer for the MLC calibration. Both the daily MLC QC test and the manufacturer MLC calibration method used an object of known dimensions to determine the image pixel scaling factor. However, it is not possible to compare these two factors, as the manufacturer method does not easily output this result.…”

“…Leaf positions were collected daily for 13 Elekta units over 11–37 (average 22) months using the automated QC test, which analyzes 23 MV images to determine the location of MLC leaves relative to the radiation isocenter . First, the location of the radiation isocenter is detected using 9 MV images of a 4 × 4 cm 2 field acquired at various collimator angles.…”

“…In total, extending the imaging panel, running the beam, collecting the images, and performing the analysis takes about 7–8 minutes and requires no user intervention after starting the first beam. More details about the leaf measurement procedure can be found in the work of Létourneau et al…”

“…A previous study introduced an automated QC test for MLC leaf positioning accuracy, which was performed three to four times per week on two units to assess the performance of the Elekta MLCi and MLCi2 Elekta MLC models. The purpose of this work was to apply this test daily, along with statistical process control tools, to evaluate the long‐term performance and stability of the Elekta Agility MLC model.…”

What can statistical process control show us about ionization chamber stability?

“…In recent years several groups have published work which illustrates how SPC can be applied to the different systems controlled by medical physicists in the various radiotherapy processes related to linear particle accelerator (linac) monitoring (Pawlicki et al, 2005;Able et al, 2011;de la Vega et al, 2012;Sanghangthum et al, 2013b;Létourneau et al, 2014;López-Tarjuelo et al, 2015), image guided radiation therapy (IGRT; Ung and Wee, 2011), monitor-unit verifications (Nordström et al, 2012), high-dose rate (HDR) brachytherapy treatments (Able et al, 2013), proton therapy beams (Rah et al, 2014), and different aspects of intensity-modulated treatment quality assurance (Breen et al, 2008;Pawlicki et al, 2008a;Pawlicki et al, 2008b;Gérard et al, 2009;Sanghangthum et al, 2013c;Sanghangthum et al, 2013a;Gagneur and Ezzell, 2014).…”