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Determination of the Optimal X-Ray Monitoring Angle for Creating a Correlation Model in Dynamic Tumor Tracking Irradiation with Vero4DRT(MHI-TM2000)

M Akimoto¹*, M Nakamura¹, N Mukumoto¹, M Yamada², N Ueki¹, Y Matsuo¹, T Mizowaki¹, A Sawada^{1, 3}, M Kokubo⁴, M Hiraoka¹, (1) Kyoto University Graduate School of Medicine, Kyoto, Japan, (2) Kyoto University Hospital, Kyoto, Japan, (3) Kyoto College of Medical Science, Nantan, Japan, (4) Institute of Biomedical Research and Innovation, Kobe, Japan

SU-C-BRA-4 Sunday 1:30:00 PM - 2:15:00 PM Room: Ballroom A

Purpose: The Vero4DRT(MHI-Tm²000) can perform dynamic tumor tracking (DTT) irradiation using two special features; orthogonal kV x-ray imaging subsystems and a gimbaled x-ray head. Prior to DTT irradiation a correlation model between the tumor position estimated from implanted fiducials and IR marker position on the abdomen is required. The purpose of this study was to estimate an optimal x-ray monitoring angle for creating the correlation model utilizing CT images.

Methods: A water equivalent path length (WEPL) on CT images was employed as an indicator to estimate the optimal angles. First, the relationship between the WEPL on 4D-CT images at the end-exhalation phase (4D-CT EE) and an intensity ratio of the fiducial to its surroundings on fluoroscopic images was examined to assess the validity of our proposed methods. Subsequently, the optimal angles were calculated on 4D-CT EE images for 11 patients with 3 to 5 implanted fiducials as follows; (1) detection of fiducials on CT images, (2) exclusion of angles at which a fiducial was overlapped with other fiducials, (3) calculation of WEPLs on the orthogonal lines passing the fiducials, and (4) determination of the monitoring angles by minimizing the sum of the WEPLs. The determined optimal angles were also applied to 4D-CT images at the end-inhalation phase and other 4D-CT sets.

Results: The WEPL was highly correlated with the logarithm of the intensity ratio (R=-0.89 [95%CI: -0.86--0.91], p<0.01). The optimal gantry and ring angles depended on the geometric arrangement between fiducials and anatomical structures. The inter- and intrafractional variations in the intensity ratios estimated from WEPLs were observed; however, the intensity ratios exceeded a threshold level to detect fiducials.

Conclusions: We demonstrated that the optimal x-ray monitoring angles for the correlation model were determined by minimizing the WEPLs on CT images.

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