AbstractID: 7847 Title: Beam Symmetry at the Beginning of Exposure and its Importance for IMRT Accurate delivery of intensity-modulated beams requires adequate beam stability. We studied beam symmetry at the beginning of exposure for a Varian 2100C accelerator. A 51-channel Profiler (linear diode array) was inserted in the blocking tray slot and exposed by a 25x25cm2 open field. We found significant asymmetry in cross-plane direction at the beginning of exposure eventually corrected by the servo. Thus, 15 MV beam had 5.9% asymmetry after the first 5 MU, dropping to 4.6% after the next 5 MU, then to 1.9%, and finally to 1.4% after the total of 20 MU. For a 6 MV beam, the asymmetry was smaller but still quite noticeable: 3.3% after the first 5 MU, 2.9% after next 5 MU, and about 2% after 15 MU total. These results are not important for higher MU settings typical for conventional teletherapy. A small asymmetry at the beginning is “diluted” by the subsequent symmetrical portion of the delivery occurring after the servo corrects the beam. One should be more careful with IMRT fields where the moving leaf opening can be located over some parts of the field only at the beginning of exposure. For certain patterns of the leaf motion, the beam asymmetry found could lead to a local dose error of a few percent at off-axis positions. As a practical recommendation, we suggest annual check of the beam symmetry at small MU settings (about 5 MU) using a profiler or a ready-pack “TL” film at a depth of dmax in a solid phantom.