Quantitative evaluation of a beam-matching procedure using one-dimensional gamma analysis

This article provides a quantitative evaluation of Varian Medical Systems' beam matching procedure. A one-dimensional y analysis is employed to investigate the level of agreement of matched beams. A customized concept of one-dimensional gamma evaluation was designed. Our algorithm first performs a "local" fit of the reference and the evaluated datasets. For a particular point on the fitted evaluated curve, the y is derived as the shortest distance between the point and the fitted reference curve. This approach removes variations of the obtained y value related to the discrete character and noise in the original datasets. Criteria of 1 mm distance-to-agreement and 1% dose difference were used to evaluate the level of agreement of according profiles. Relative point and profile measurements were performed for all photon and electron beams of two Varian Clinacs 2100C/D. Matched beams show a good level of agreement. 70% of profiles completely pass the chosen criteria. The analysis of remaining 30% of the profiles demonstrates that measurement error becomes a limiting factor in achieving a better score. The highest obtained y value was 1.70. The quality of beam matching allowed us to treat according beams of both treatment units as "identical" and to use the reference beam data for the new unit. Nevertheless, the vendor's acceptance criteria of beam matching are much more benevolent. It might happen that the acceptance criteria are met, however, resulting quality of beam matching does not allow full interchangeability of beams.