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Assessing the Accuracy of IsoCal-Calibrated Image Centers On Varian TrueBeam Machines

W Du

W Du1*, S Gao1 , W Jiang2 , (1) University of Texas MD Anderson Cancer Center, Houston, TX, (2) Yantai Yuhuangding Hospital, Qingdao University School of Medicine, Yantai, Shandong, China


SU-I-GPD-T-452 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: IsoCal is a vendor-provided calibration procedure that aims at correcting the mechanical inaccuracies of the imaging components on Varian linacs. In this study, we developed an independent, Winston-Lutz (WL) test based method to evaluate the effectiveness of IsoCal in improving the image center accuracies on Varian TrueBeam machines.

Methods: We performed digital WL tests on two Varian TrueBeam machines with IsoCal installed. The digital WL tests used a simple ball-bearing phantom and in-house software for image analysis. The images of the phantom were obtained using the 2D kilovoltage (kV) x-ray, the 2D megavoltage (MV) x-ray, and the 3D kV cone-beam CT (CBCT) imaging systems. The radiation isocenter was determined with a multi-leaf collimator (MLC)-shaped, square radiation beam incident at the phantom from eight gantry angles. At each gantry angle, two opposing collimator angles were used to reduce the effect of MLC mechanical inaccuracies. The offsets of the kV, MV, and CBCT image centers from the radiation isocenter were computed.

Results: On the first TrueBeam linac, the maximum offsets from the radiation isocenter were 0.15 mm, 0.34 mm, 0.22 mm for the MV, kV, and CBCT image centers respectively. On the second TrueBeam linac, the maximum offsets were 0.21 mm, 0.33 mm, 0.27 mm for the MV, kV, and CBCT image centers respectively. Repeated measurement showed that these offsets were reproducible with variations < 0.1 mm.

Conclusion: The MV, kV, and CBCT image centers were calibrated clearly within 0.5 mm from the radiation isocenter on the studied TrueBeam machines. The effectiveness of the IsoCal can be checked with a simple yet highly precise quality assurance method that is independent of the linac vendor.

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