Author: Peter Balter, Andres V Cibrian, Song Gao 👨🔬
Affiliation: The University of Texas MD Anderson Cancer Center 🌍
Purpose: The purpose of this study is to demonstrate a means of detecting misaligned jaw face angles on the Varian TrueBeam based on a beam symmetry metric for electron beam profiles.
Methods: During the acceptance of two new linacs, all beams were steered to achieve a point-to-point symmetry better than manufacturer’s specifications (symmetry < 0.7%), this was confirmed with independent measurements using an ionization chamber array (IC Profiler). This was done at gantry and collimator angles of 0o. However, the symmetries of higher energy electrons (12-20 MeV) degraded to worse than 2.0% when the collimator was rotated to 90o and/or 270o. In particular, the 20 MeV beam with collimator at 90o; case 1, Y-symmetry was -2.6%, case 2, X-symmetry was -2.7%. Working we the vendor this was determined to be caused by a misalignment the jaw face angle. After the problem was identified, the jaws were realigned to the manufacture’s specification and all beams were re-steered to a symmetric shape. After adjustment all beam profiles were re-measured with different collimator angles and the results of the symmetry of all beams were compared.
Results: Both X1 and X2 jaw face angles were adjusted, and all beams were re-steered to achieve a symmetry for both X- and Y- axes of better than 0.5% with the collimator at 0o. The symmetry was then measured with collimator at 90o and 270o and was within 1.0% for all beams.
Conclusion:
We have demonstrated that comparing the symmetry of high energy electron beams with the collimator at different angles can be used to identify jaw face angle misalignment during linac acceptance and/or QA processes for Varian TrueBeam.