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

Abstract

Multileaf collimator (MLC) positions should be precisely and independently measured as a function of gantry angle as part of a comprehensive quality assurance (QA) program for volumetric‐modulated arc therapy (VMAT). It is also ideal that such a QA program has the ability to relate MLC positional accuracy to patient‐specific dosimetry in order to determine the clinical significance of any detected MLC errors. In this work we propose a method to verify individual MLC trajectories during VMAT deliveries for use as a routine linear accelerator QA tool. We also extend this method to reconstruct the 3D patient dose in the treatment planning system based on the measured MLC trajectories and the original DICOM plan file. The method relies on extracting MLC positions from EPID images acquired at 8.41 fps during clinical VMAT deliveries. A gantry angle is automatically tagged to each image in order to obtain the MLC trajectories as a function of gantry angle. This analysis was performed for six clinical VMAT plans acquired at monthly intervals for three months. The measured trajectories for each delivery were compared to the MLC positions from the DICOM plan file. The maximum mean error detected was 0.07 mm and a maximum root‐mean‐square error was 0.8 mm for any leaf of any delivery. The sensitivity of this system was characterized by introducing random and systematic MLC errors into the test plans. It was demonstrated that the system is capable of detecting random and systematic errors on the range of 1–2 mm and single leaf calibration errors of 0.5 mm. The methodology developed in the work has potential to be used for efficient routine linear accelerator MLC QA and pretreatment patient‐specific QA and has the ability to relate measured MLC positional errors to 3D dosimetric errors within a patient volume.PACS number(s): 87.55.Qr

Quality assurance of geometric accuracy based on an electronic portal imaging device and log data analysis for Dynamic WaveArc irradiation

“…Zwan et al. presented a new detection method for collecting gantry sag using the EPID 34. However, these methods must incorporate preverification in order to validate the degree of sag.…”

Commissioning and quality assurance for VMAT delivery systems: An efficient time‐resolved system using real‐time EPID imaging

“…The gantry angle from the OBI gantry angle encoder was tagged to the header of each acquired image frame. 25,36 The gantry angle naming convention used by this system and subsequently throughout this work, is 0°= 0°I EC , 90°= 90°I EC , 179°= 179°I EC , À90°= 270°I EC and À179°= 181°I EC relative to IEC 61217 (2008) scale.…”

“…For each MLC test plan, the MLC positions were extracted from each EPID image frame using the methodology outlined in Zwan et al 25 In this process, collimator rotation is removed and correction are made for MLC, gantry and image panel sag using a measured sag map in order to precisely extract the profile through the center of each leaf pair. The radiation field edge can then be detected for each in-field MLC leaf and corrected for transmission through the curved leaf tip to give the light field edge position, i.e., the geometric leaf position 38 which is specified in the DICOM plan file.…”

“…Accessing the individual EPID image frames, rather than a single integrated EPID image, allows direct measurement of these time-varying components without compromising the efficiency of the procedure. A number of groups have investigated the use of timeresolved EPID imaging for linac QA [25][26][27][28][29][30][31] and plan-specific QA 25,[32][33][34][35][36] for IMRT and VMAT deliveries. Zwan et al 25 developed a system to automatically extract MLC trajectories from EPID image frames, which could be subsequently compared to the planned MLC positions in order to calculate MLC positioning errors for each individual leaf as a function of gantry angle.…”

“…There is widespread consensus in the medical physics community that the available tests for VMAT-specific linac QA are insufficient resulting in a dependence on patient specific QA. 5,6,[12][13][14]25,26,36 Many of the challenges associated with quality control of VMAT are addressed in the most recent report from the Netherlands Commission on Radiation Dosimetry (NCS Report 24), 11 who make recommendations for VMAT-specific linear accelerator QA and commissioning. Since the release of this report, no work has been done to develop a single system which systematically addresses all of these recommendations.…”