The influence of swing motion on the dosimetric parameters in real-time tumor tracking using the gimbaled X-ray head

We assessed the changes in dosimetric parameters of the VERO-4DRT system when a gimbaled X-ray head swings from the central axis for real-time tumor tracking (RTTT). The dosimetric parameters, such as beam quality, symmetry, and flatness, were obtained from the measured percent depth doses (PDDs) and dose profiles when a gimbaled X-ray head was tilted from 0.6 to 2.4 degrees for swings. The dose distributions of the intensity-modulated radiotherapy (IMRT) beams were measured according to the RTTT procedures and were acquired by varying the breaths per minute (BPM) from 10 to 30. The symmetry deviations increased with the expansion of the swing angle for all square field sides and depths. The maximal deviation exceeded 1% when the depth and swing angle exceeded 150 mm and 1.8 degrees, respectively. When the acceptance criterion was 1 mm/1%, the gamma passing rate was lowered to less than 95% at the swing speed corresponding to a respiration rate of 20 BPM or higher. It is necessary to ensure the correct angle and speed of the swing during RTTT when using the gimbaled X-ray head.