SU-E-T-88: Evaluating Gantry Sag on Linear Accelerators and Introducing an MLC-Based Compensation Strategy.
PURPOSE
Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. This study aims to quantify the gantry sag on multiple linacs and to investigate a multiple leaf collimator (MLC)-base strategy to compensate for gantry sag.


METHODS
We used the Winston-Lutz method to measure the gantry sag on three Varian linacs. A ball-bearing phantom was imaged with a square radiation field during gantry rotation. The images were analyzed to derive the radiation isocenter and subsequently the gantry sag, that is, the superior-inferior wobble of the radiation field center from the radiation isocenter as a function of gantry angle. Compensation for gantry sag was attempted by offsetting the MLC leaves at 90-degree collimator angle. The amount of offset was the opposite of measured gantry sag, which was gantry angle-specific.


RESULTS
Gantry sag was reproducible within a six-month period. On the three linacs, the maximum gantry sag was found to vary from 0.7 mm to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly, or away from the gantry, when the gantry was rotated from 0 to 180 degrees. Comparison of gantry sag at 0- and 90-degree collimator angles showed that the uncertainty in MLC leaf positions did not increase the gantry sag. Instead, gantry sag was caused primarily by nonideal gantry rotation. After the MLC compensation was applied, the maximum gantry sag was reduced to less than 0.2 mm.


CONCLUSIONS
The results indicated that gantry sag on a linac can be quantitatively measured with sub-millimeter precision, using a simple ball-bearing phantom and the electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90 degree collimator angle.