Optimization of the x‐ray monitoring angle for creating a correlation model between internal and external respiratory signals
Authors: Akimoto Mami , Nakamura Mitsuhiro , Mukumoto Nobutaka , Yamada Masahiro , Ueki Nami , Matsuo Yukinori , Sawada Akira , Mizowaki Takashi , Kokubo Masaki , Hiraoka Masahiro 
Published: September 2012

Purpose:To perform dynamic tumor tracking irradiation with the Vero4DRT (MHI‐TM2000), a correlation model [four dimensional (4D) model] between the displacement of infrared markers on the abdominal wall and the three‐dimensional position of a tumor indicated by a minimum of three implanted gold markers is required. However, the gold markers cannot be detected successfully on fluoroscopic images under the following situations: (1) overlapping of the gold markers; and (2) a low intensity ratio of the gold marker to its surroundings. In the present study, the authors proposed a method to readily determine the optimal x‐ray monitoring angle for creating a 4D model utilizing computed tomography (CT) images.Methods:The Vero4DRT mounting two orthogonal kV x‐ray imaging subsystems can separately rotate the gantry along an O‐shaped guide‐lane and the O‐ring along its vertical axis. The optimal x‐ray monitoring angle was determined on CT images by minimizing the root‐sum‐square of water equivalent path lengths (WEPLs) on the orthogonal lines passing all of the gold markers while rotating the O‐ring and the gantry. The x‐ray monitoring angles at which the distances between the gold markers were within 5 mm at the isocenter level were excluded to prevent false detection of the gold markers in consideration of respiratory motions. First, the relationship between the WEPLs (unit: mm) and the intensity ratios of the gold markers was examined to assess the validity of our proposed method. Second, our proposed method was applied to the 4D‐CT images at the end‐expiration phase for 11 lung cancer patients who had four to five gold markers. To prove the necessity of the x‐ray monitoring angle optimization, the intensity ratios of the least visible markers (minimum intensity ratios) that were estimated from the WEPLs were compared under the following conditions: the optimal x‐ray monitoring angle and the angles used for setup verification. Additionally, the intra‐ and interfractional variations in the intensity ratio were examined from the optimal x‐ray monitoring angle.Results:A negative strong correlation was observed between the WEPL (x) and the intensity ratio (y) (y = 6.57 exp[−0.0125x] + 1, R = −0.88 [95% confidence interval: −0.85 to −0.90], p < 0.01). Our proposed method effectively avoided having the x‐ray beam pass through high‐density structures, although there were large interpatient variations in the optimal x‐ray monitoring angle because of the geometric arrangement between the gold markers and the anatomical structures. The minimum intensity ratios that were estimated from the WEPLs at the optimal x‐ray monitoring angle ranged from 1.43 to 2.48, which was an average of 1.27 times (range, 1.02–1.66) higher than the angles used for setup verification. The maximum intra‐ and interfractional decreases in the intensity ratio were 0.23 and 0.17, respectively.Conclusions:The authors demonstrated that the optimal x‐ray monitoring angle for creating a 4D model can improve the visibility of gold markers.