VMAT Commissioning and Quality Assurance (QA) of an Elekta Synergy-STM Linac Using ICOM Test HarnessTM
A. Nguyen,1,3 P. Rajaguru,1 D. Kaurin,2 T. Paul,3 R. He,2,3 A. Plowman,4 C. Yang1
1University of Mississippi Medical Center, Jackson, MS; 2Northwest Medical Physics Center, Lynnwood, WA; 3Ironwood CRC, Phoeniz, AZ; 4Remote Dosimetry Services, Lucedale, MS
AAPM Annual Scientific Meeting, Austin, TX (2014).
Purpose: To establish a set of tests based on the iCOM software that can be used to commission and perform periodic QA of VMAT delivery on the Elekta Synergy-S, commonly known as the Beam Modulator (BM).
Methods: iCOM is used to create and deliver customized treatment fields to characterize the system in terms of 1) MLC positioning accuracy under static and dynamic delivery with full gantry rotation, 2) MLC positioning with known errors, 3) Maximum dose rate, 4) Maximum MLC speed, 5) Maximum gantry speed, 6) Synchronization: gantry speed versus dose rate, and 7) Synchronization: MLC speed versus dose rate.
The resulting images were captured on the iView GT and exported in DICOM format to Dosimetry CheckTMsystem for visual and quantitative analysis. For the initial commissioning phase, the system tests described should be supplemented with extensive patient QAs covering all clinically relevant treatment sites.
Results: The system performance test suite showed that on our Synergy-S, MLC positioning was accurate under both static and dynamic deliveries. Intentional errors of 1 mm were also easily identified on both static & dynamic picket fence tests. Maximum dose rate was verified with stop watch to be consistently between 475-480 MU/min. Maximum gantry speed and MLC speed were 5.5 degree/s and 2.5 cm/s respectively. After accounting for beam flatness, both synchronization tests, gantry versus dose rate and MLC speed versus dose rate, were successful as the fields were uniform across the strips and there were no obvious cold/hot spots.
Conclusion: VMAT commissioning and quality assurance should include machine characterization tests in addition to patient QAs. Elekta iCOM is a valuable tool for the design of customized VMAT field with specific MU, MLC leaf positions, dose rate, and indirect control of MLC and gantry speed at each of its control points.