Detection of the Linac Jaw face angle misalignment using high energy electron beam symmetry

To demonstrate the usage of a high energy electron beam symmetry measured at different collimator angles to detect jaws face angle misalignments on Varian TrueBeam linear accelerators (linac) during acceptance and commissioning processes.

During the linac acceptance, all beams were steered to a symmetric shape with the gantry and collimator at 0°. We noted that the symmetries of higher energies electrons (e.g., 16 MeV, 20 MeV) changed over 2% when measured with the collimator at 90° and/or 270° for two linacs commissioned within 5-months of each other. Misaligned applicators were ruled out as the cause of this issue. Eventually, this was traced to the X-jaw face angle not matching beam divergence. After the problem was identified and the jaws were realigned by manufacture's engineers and all beams were re-steered to a symmetric shape at collimator 0° and verified at collimator 90° and 270°.

Prior to the manufacturer adjusting the jaw face angles, the maximum observed electron beam symmetry changes with 90° collimator rotation were 2.0% and 2.6% for linac 1 and linac 2, respectively. After realigning, the jaw face angles the maximum observed electron beam symmetry changes with 90° collimator rotation were within 1.0% for all beams on both linacs.

We have demonstrated that changes in symmetry of high energy electron beam with 90° collimator rotation can signal that the jaw face angle is not properly aligned with beam divergence.