Room: ePoster Forums
Purpose: AAPMâ€™s Task Group 142 report stresses the importance of radiation-imaging coincidence, especially when treating SRS/SBRT. However, a machine may only allow a single imaging isocenter for multiple beam energies used for SRS/SBRT. A procedure was developed to simultaneously optimize radiation-imaging isocenter coincidence for all VersaHD photon energies.
Methods: Prior to starting, the collimator crosshair was adjusted to minimize runout. Next, a Half Beam Block (HBB) test was performed by taping a farmer chamber to a block tray at the crosshair central axis (CA) and closing one MLC bank to the CA. Two measurements, with collimator 180 degrees apart, were taken so that the MLC blocked alternating sides of the beam. Difference between measurements was calculated to quantify beam misalignment with zero being ideal. A BB was placed near isocenter, and AP Winston-Lutz images were acquired for all energies. Gun-Target (GT) deviations between BB and radiation isocenter were correlated to HBB results using a linear regression and the corresponding GT location for ideal HBB results was calculated. Bending fine for each energy was adjusted, while acquiring AP WL images until GT deviation matched ideal. Next, Sun Nuclearâ€™s Hancock WL test consisting of gantry, couch, and collimator rotations were used to adjust the couch isocenter, thereby minimizing the isocentricty of the machine. Finally, lasers and kV imaging isocenter were calibrated to match radiation isocenter.
Results: Using this method, GT radiation isocenter was adjusted within 0.05mm for all energies. After bending fine and couch adjustment, average radiation-imaging isocentricity for all energies was 0.79mm [0.66mm â€“ 0.91mm] for a single imaging isocenter.
Conclusion: Correlating HBB and WL results provides a quantitative evaluation of isocenter coincidence across photon energies on the Versa HD. The correlations can be used to guide bending fine adjustments to achieve a radiation-imaging coincidence <1.0mm for all energies.