Sensitivity of array detector measurements in determining shifts of MLC leaf positions

Abstract

Using a MatriXX 2D ionization chamber array, we evaluated the detection sensitivity of systematically introduced MLC leaf positioning shifts to test whether the conventional IMRT QA method can be used for quality assurance of an MLC tracking algorithm. Because of finite special resolution, we first tested whether the detection sensitivity was dependent of the locations of leaf shifts and positions of ionization chambers. We then introduced the same systematic leaf shifts in two clinical intensity modulated radiotherapy plans (prostate and head and neck cancer). Our results reported differences between the measured planar doses with and without MLC shifts (errors). Independent of the locations of the leaf position shifts and positions of the detectors, for the simple rectangular fields, the MatriXX was able to detect ±2 mm MLC leaf positioning shifts with Gamma index of 3%/3 mm and ±1 mm MLC leaf position shifts with Gamma index of 2%/2 mm. For the clinical plans, measuring the fields individually, leaf positioning shifts of ±2 mm were detected using Gamma index of 3%/3 mm and a passing rate of 95%. When the fields were measured compositely, the Gamma index exhibited less sensitivity for the detection of leaf positioning shifts than when the fields were measured individually. In conclusion, if more than 2 mm MLC leaf shifts were required, the commercial detector array (MatriXX) is able to detect such MLC positioning shifts, otherwise a more sensitive quality assurance method should be used.

Effect of Multileaf Collimator Leaf Position Error Determined by Picket Fence Test on Gamma Index Value in Patient-Specific Quality Assurance of Volumetric-Modulated Arc Therapy Plans

“…To achieve better than 1 mm accuracy and precision of MLC leaf positioning, routine MLC QA testing is recommended by TG-142 [11] to be performed weekly using visual inspection of matched segments and monthly quantitative testing using a special test pattern such as a picket fence test described by Losasso [3]. Different methods have been described in the literature for MLC QA to deliver IMRT/VMAT plans accurately [18,[23][24][25][26]. One of the used methods for MLCQA for IMRT/VMAT delivery was the twodimensional (2D) evaluation of either film or scanned image by visual inspection, which are subjective, timeconsuming, sensitive to external conditions, and a less accurate way to define MLC positioning errors [25].…”

“…Different methods have been described in the literature for MLC QA to deliver IMRT/VMAT plans accurately [18,[23][24][25][26]. One of the used methods for MLCQA for IMRT/VMAT delivery was the twodimensional (2D) evaluation of either film or scanned image by visual inspection, which are subjective, timeconsuming, sensitive to external conditions, and a less accurate way to define MLC positioning errors [25]. Another method that does not require physical measurements for MLCQA, gantry angle, collimator angle, and stored cumulative dose per control point would be retrospectively analyzed via the MLC log files recorded by the machine end of each IMRT/VMAT delivery.…”

Sensitivity evaluation of two commercial quality assurance systems to organ-dose variations of patient-specific VMAT plans

“…It was concluded that the device is capable of detecting leaf bank errors at this field size. Shang, Godley, Huang, Qi, and Xia (2017) assessed the sensitivity of the MatriXX detector to ±1 and 2 mm alterations of MLC leaves of two IMRT plans of prostate and head-and-neck cancers. Their study reported that the MatriXX is sensitive to leaf positioning shifts of ±2 mm with a pass rate of 95% at gamma criterion of 3%/3 mm.…”

Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in Step-and-Shoot IMRT plans

“…This was followed by the I'MRT MatriXX (2D array) using a 3%/3 mm gamma criteria, then the single ion chamber measurement. Shang et al shifted the MLC leaves on one side of the bank by 1-4 mm and found that the MatriXX was able to detect a 2 mm shift with a 3%/3 mm gamma criteria and a 1 mm shift with a 2%/2 mm criteria [35].…”