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A Total Imaging Test Sequence to Determine the Isocenter Coordinates of Multi-Modality Imaging Systems On a Stereotactic Linac Relative to the Radiation Isocenter

F Hacker

F Hacker*, S Friesen, E Mannarino, M Wagar, J Lewis, Y Lyatskaya, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, MA

SU-E-T-135 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: A simple method is presented to quantify and track position of the imaging isocenters indicated by the ExacTrac kV (Brainlab) and OBI planar kV/CBCT on a Varian Novalis TX treatment unit with respect to the radiation isocenter.
Methods: A radio-opaque sphere is aligned to room lasers. Using a circular collimator and EPID a Winston-Lutz test is performed for 6 gantry/couch combinations. In Varian offline review the position of the sphere relative to the radiation isocenter for each port is determined to within 0.1 mm. Average position of the sphere for all ports determines laser isocenter relative to radiation isocenter. The sphere is imaged with the ExacTrac, orthogonal kV OBI and CBCT. The Detect Winston-Lutz Pointer utility in ExacTrac and manual match function in offline review determine the position of the sphere relative to the imaging isocenter for ExacTrac and kV/CBCT respectively. A coordinate transformation in Excel gives each imaging isocenter relative to radiation isocenter in RAS coordinates. Tracking coordinates over time identifies trends.
Results: 152 data collection days have been evaluated. A systematic shift was identified in the ExacTrac to laser isocenter of approximately 0.5, 0.3 and 0.2 mm in AP, LAT and VRT respectively. With knowledge of the laser to radiation isocenter the ExacTrac to radiation isocenters were adjusted to within 0.5 mm in all directions. For the kV and CBCT isocenters we are limited by the 1.0 mm precision of the coordinate readout on our Novalis TX. For a Truebeam STX this limitation would not exist and similar results to ExacTrac should be achievable.
Conclusion: This method very precisely gives coordinates of each imaging isocenter directly to the radiation isocenter. This eliminates the summation of errors from setting lasers to the radiation isocenter and imaging isocenters to the lasers allowing improved isocenter coincidence for high precision treatments.

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