SU-G-BRB-13: Precision QA Measurement of Gantry Angle and Speed in Tomotherapy Using ArcCheck.

PURPOSE: To develop a methodology using an ArcCheck for accurate QA measurements of the gantry angle and speed of a Tomotherapy unit.

METHODS: Two test plans recommended by TG148 were chosen for this study. One was the helical star shot test which had a total of 40 gantry rotations while the couch moved continuously and the multi-leaves opened for projections centered at 0°, 120° and 240°. The plan set a gantry start angle of -2.5° to ensure the projections were centered at the expected angles. The other test plan was the static star shot test which could be used to check the gantry positions at the expected static angles 0°, 45°, 90° and 135° without any couch movement. The ArcCheck was isocentrically set up and adjusted for couch sag. Movie files which took a snapshot exposure every 50ms were recorded. The virtual inclinometer tool of the ArcCheck was used to measure the gantry angle at each snapshot, it would record multiple angle values for each 5°projection with leaves opened. A mean value and the corresponding time for each projection were calculated. The gantry speed could then be calculated. The machine trajectory log data were also collected and analyzed for comparison.

RESULTS: The mean deviation of our measurement results from the log data for both plans was calculated to be less than 0.5°. The mean gantry speed was 18.02°/sec with an uncertainty ±0.07°/sec, which was very close to the plan setting of 18.00°/sec and the mean value of 18.02°/sec calculated from log data.

CONCLUSION: Compared with film-based measurement, a precise and efficient method for measuring the gantry angle and speed of Tomotherapy using ArcCheck has been developed and proved to be accurate compared with machine log data.