Room: Exhibit Hall
Purpose: To evaluate the effect of gantry angle and â€˜Auto Alignâ€™ (automatic offset correction) on gamma analysis of the Varian Portal Dosimetry.
Methods: Mechanical detector shifts were investigated using 10Ã—10 cmÂ² open fields at each gantry angle separated by 45Â° starting at 180Â° (8 gantry angles) with clockwise and counter-clockwise rotation for two Varian LINACs (Trilogy (aS1000) and 21EX (aS500)). Portal Dosimetry QA (PDQA) was performed for 10 IMRT plans independently on two LINACs. For each plan, each field was measured at the planned gantry angle as well as gantry angle 0Â°. Gamma passing rates (3%/3 mm) of the individual fields were computed using the Varian Portal Dosimetry software and the MU-weighted mean gamma passing rate was computed for each plan with and without â€˜Auto Alignâ€™. Composite gamma analysis for both measurements was also performed with inherent alignment correction.
Results: The detector offset of Trilogy was less than 1.5 mm for all angles, while that for 21EX ranged from 1.6 mm (gantry angle=0Â°) to 3.1 mm (gantry angle=180Â°). For Trilogy, mean gamma passing rates were 97.4Â±1.5/97.6Â±1.5/99.4Â±1.0% (MU-weighted mean/MU-weighted mean with auto alignment/composite) measured at the planned gantry angle and 97.6Â±1.2/97.9Â±1.2/99.4Â±1.4% with gantry angle 0Â°. For 21EX, rates were 95.5Â±2.9/98.3Â±1.7/98.1Â±2.0% and 98.3Â±1.7/98.6Â±1.5/99.4Â±1.0% for planned gantry angle and 0Â°, respectively. In general, the gamma test with gantry angle 0Â° showed better passing rates than that with planned angle. The auto alignment significantly improved the passing rates for 21EX but marginally for Trilogy. The composite analysis showed better gamma passing rates than individual field comparison.
Conclusion: PDQA with gantry angle 0Â° and auto alignment is preferred to avoid the effect of differential mechanical detector offset for IMRT with fixed gantry angles. The composite PDQA and auto alignment should be performed with great caution by investigating the mechanical panel offset at different gantry angles.