SU-E-T-78: Evaluation of IMRT QA Methods Using a Configurational Cylindrical Phantom

Abstract

Purpose: To develop and use a common phantom system to characterize and compare 2D and 3D data from different intensity modulated radiation therapy (IMRT) quality assurance (QA) measurement tools. Method and Materials:The IMRT QA tools under consideration in this work are: a) Single ion chamber, b) 2D matrix ion chamber, c) Gafchromic film, d) EPID-based fluence measurement and dose prediction algorithm, and e) Fricke-based gel dosimeter with optical CT readout. A common phantom with different internal configurations was designed for accommodating the different QA tools. The exterior geometry of all phantoms was dictated by the EPID-based fluence-to-dose prediction method, (Ansbacher et al. 2006), which calculates on a surrogate/virtual patient that is a 20.4 cm diameter uniform density cylinder. The cylindrical phantoms were made from plexiglass, 30 cm in length, with a mounting bracket for positioning over the end of the treatment couch. The phantoms were scanned on a CT simulator and the data imported into a treatment planning system (TPS). Non-IMRT and IMRT test plans in addition to selected patient plans were generated in the TPS and delivered to the different phantom configurations on the same linac. Dicom dose data were analyzed and compared, where possible geometrically, using commercial and in-house software. Results: Dose comparisons ranging from single point dose differences to full 3D gamma analysis show significant differences between the QA tools. Depending on the plan, some show a pass while others show a fail. The more complex the plan, the greater the differences between the QA tools. Conclusions: Patient specific IMRT QA is an important task in clinical practise. Many different test tools are available for this work, but the systems are not always well benchmarked against each other. A common platform than can incorporate various test tools is necessary to establish limitations of various tools.