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Gantry sag caused small misalignments between the superior (S) or inferior (I) borders of a test radiation field when the gantry was moved from 0° (G = 0) to 180° (G = 180) on linac C. (a) Collimator angle 0°; (b) collimator angle 90°; (c) collimator angle 90° with MLC leaf position compensation.
