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A Retrospective Analysis of Isocenter Coincidence Between ExacTrac and MV Delivery On a Varian 21iX

S Ellefson

S Ellefson*, E Clouser , Mayo Clinic Arizona, Phoenix, AZ


WE-RAM1-GePD-J(A)-3 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To ensure isocenter coincidence between radiation delivery, in-room lasers, and ExacTrac® images for linac-based stereotactic radiosurgery cases, a modified Winston Lutz test is performed weekly, the morning of and immediately pre-treatment for every case at our facility. The test measures the offset of the lasers relative to the radiation and ExacTrac® isocenters. The isocenters of the two systems are calibrated to be coincident within tolerance. The purposes of this work are (1) investigate whether the two systems give identical isocenter results within statistical significance, and (2) determine whether the fluctuations in each system’s results correlate with each other.

Methods: A retrospective analysis was performed on 60 modified Winston Lutz test results taken over a period of 7 months. The tests were all performed clinically by a team of 10 physicists. The differences between the X, Y, and Z coordinates for the two systems were analyzed independently. Statistical analysis was performed on the difference dataset to determine if the observed differences are significant for each coordinate and if the data from both systems correlates.

Results: No statistically significant difference is observed between the two systems for the Y and Z coordinates. The difference in the lateral (X) coordinate between the two systems is significant, with an average of 0.03±0.1 mm. Linear regression on the lateral coordinate of both systems showed there is significant linear correlation with a slope of 0.787±0.082. A statistical t-test showed the slope of the regression is significantly different from unity. The average absolute difference between the two isocenters is 0.17±0.14 mm.

Conclusion: The isocenters of the ExacTrac® and MV delivery systems are coincident within tight tolerances. However, the relationship between the two systems is not a simple systematic offset. This indicates a potential scaling discrepancy in one or both of the systems that warrants further investigation.

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