(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To validate an EPID-based technique for patient specific IMRT QA and to compare it with the method using a 2D ion-chamber array device in terms of efficiency and accuracy.
Methods: A commercial EPID-based program was evaluated on two Varian Truebeam linacs equipped with the latest MV image panel (aS1200) and with available beam energies of 6 MV, 6 MV FFF, 10 MV, and 10 MV FFF. One machine is equipped with HD-120 MLC, and the other has Millennium-120 MLC. The EPID-based program was configured such that it can automatically transfer the EPID images and log files from the linac sequencer computer, calculate 2D/3D doses with its own superposition/convolution algorithm, compare doses with those generated by the treatment planning system, and generate reports including point dose and absolute 2D/3D gamma analysis. After standard EPID calibration in the dosimetry mode, the MV panel was further calibrated/verified based upon the vendor’s recommendation. During all calibrations, the source-imager-distance (SID) was set at 100 cm except that the SID was extended to 150 cm for FFF beams to reduce effective dose rates while enlarging the resolution. A total of 29 patient-specific IMRT QA (including 83 fields) were performed using 2D ion-chamber array devices and EPIDs for validation. We compared those two methods based on our clinical 2D gamma criterion (> 90% with 3mm/3%) for IMRT QA.
Results: All QAs passed our clinical 2D gamma criterion. The EPID-based method showed significantly better plan passing rates than chamber-based method, 99.4% ± 1.2% vs. 97.3% ± 2.7% (p = 0.0006). With the EPID-based method, the average QA time per plan was reduced due to the EPID software automation and the avoidance of setting up 2D ion-chamber array devices.
Conclusion: EPID-based patient specific QA is an attractive solution to improve efficiency and accuracy.
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