A method to enhance spatial resolution of a 2D ion chamber array for quality control of MLC

Abstract

This work introduces a new method for verifying MLC leaf positions with enough spatial resolution to replace film‐based methods in performing QA tests. It is implemented on a 2D ion chamber array, and it is based on the principle of varying signal response of a volumetric detector to partial irradiation. A PTW 2D‐ARRAY seven29 (PTW‐729 2D) array was used to assess a Siemens OPTIFOCUS MLC. Partial volume response curves for chambers in the array were obtained by irradiating them with the leaves of the MLC, progressively covering varying portions of the chambers correlated with the leaf positions. The readings from the array's chambers are processed with an in‐house program; it generates a reference response that translates readings into leaf positions. This principle allows discriminating errors in pairs of opposing leaves that could combine to cancel their detection with other tools.Patterns of leaf positions, similar to the Bayouth test but with different, purposefully introduced errors, were generated and used to test the effectiveness of the method. The same patterns were exposed on radiographic film and analyzed with the RIT software for validation. For four test patterns with a total of 100 errors of ±1 mm, ±2 mm and ±3 mm, all were correctly determined with the proposed method. The analysis of the same pattern with film using the Bayouth routine in the RIT software resulted in either somewhat low true positives combined with a large fraction of false positives, or a low true positive rate with a low false positive ratio, the results being significantly affected by the threshold selected for the analysis.This method provides an effective, easy to use tool for quantitative MLC QA assessment, with excellent spatial resolution. It can be easily applied to other 2D arrays, as long as they exhibit a partial volume detector response.PACS number: 87.55.Qr

EPID‐based verification of the MLC performance for dynamic IMRT and VMAT

“…2,12,15,27,29 EPIDs have been used for the QA of MLC performance in several studies. 7,9,15,[25][26][27][30][31][32][33] There have also been reports on the application of 2D array detectors such as MapCheck, MatriXX, and PTW-729 34,35 for this purpose. The major concern about these devices is their low spatial resolution.…”

An EPID‐based system for gantry‐resolved MLC quality assurance for VMAT

“…This type of error would occur if a single leaf requires recalibration and, even for these small magnitudes, could become significant for small field stereotactic treatments. Single‐leaf miscalibration errors on a small scale may not be detected by other QA methods, such as those which rely on DynaLog files (14) or low‐resolution devices 25 , 26 …”

“…Single-leaf miscalibration errors on a small scale may not be detected by other QA methods, such as those which rely on DynaLog files (14) or low-resolution devices. (25,26) Random leaf calibration errors may occur when a MLC has not been initialized frequently enough. These types of errors can easily be visualized in Figs.…”

“…To perform VMAT MLC positioning quality assurance, therefore, requires a detector with sufficient resolution both temporally and spatially. Groups have also investigated the use of 2D detector array devices such as MapCHECK, MatriXX, and PTW-729 (25,26) to assess dynamic MLC performance; however, the applicability of these devices for MLC QA is fundamentally limited by their low spatial resolution. Equipment setup and analysis time can also be significant.…”

Photon caliper to achieve submillimeter positioning accuracy

“…However, film requires careful handling, processing, and can become expensive over time. Alternatively, some researchers have proposed methods for increasing the spatial resolution of 2D detector arrays (Poppe et al 2007, Diaz Moreno et al 2011. Poppe et al (2007) successfully doubled the spatial resolution of a 2D detector array by performing two measurements; the first measurement with a standard detector array position and the second performed with a position offset of half the detector spacing.…”