AbstractID: 7529 Title: A Comparison of Open and Wedged Photon Beam Data on Five Varian Linacs
The dosimetric characteristics of five Varian linear accelerators have been investigated for both 6 MV (all 5 linacs) and 18
MV (3 linacs) photons to determine the cross-agreement between linacs. The goal of this work was to determine whether a patient could
be transferred to another linac in an emergency situation without the patient’s dose distribution having to be re-calculated. Since wedges
are routinely used in photon beam radiotherapy to improve the distribution, and due to their different physical construction as well as
their different relative position from the x-ray source, open, statically-wedged (for both upper and lower wedges), and dynamicallywedged beams were measured. In order to maintain an overall dosimetric uncertainty of ±5%, AAPM TG-40 report recommends that
annually-measured dosimetric constancy checks be within ±2% of the nominal value. For this reason, we have allowed a tolerance of
±2% on the percent difference between the beam data for each linac compared to the average of the linacs. Open and wedged beam data
for the Varian 6EXs and 21EXs agree within our stated tolerance; however, the CL2300 C/D does not remain within tolerance for certain
static wedge angles because of differences in wedge materials. Furthermore, despite the relative dose factor agreement between linacs to
within ±2%, wedge factors varied by up to ±8% between the CL2300 and the EX machines. We therefore conclude that most dose
distributions can be interchanged; however, it is important that MU calculations be redone using the appropriate relative dose factor,
wedge factor, and tray factor.