Methodology for intrafraction organ motion tracking using ultrasound for respiratory-gated radiotherapy.

Respiratory-gated radiotherapy techniques have been introduced into the stereotactic body radiotherapy (SBRT) to manage intrafraction organ motions, as follows: respiratory gating using optical tracking, respiratory gating using X-ray fluoroscopy, and breath hold or force shallow breathing. Particularly in respiratory-gated radiotherapy for lung cancer, optical tracking techniques have been performed by tracking the up-and-down movement of an infrared reflective marker placed on the abdomen or around xiphoid tip of the patient to obtain respiratory waveforms (e.g., real-time position management [RPM] system, Varian Medical Systems, Palo Alto, CA). However, the up-and-down movement of the abdomen is highly variable during breathing, and there are limitations to using respiratory waveforms obtained by this method to accurately surrogate the respiratory displacement of lung cancer. To address several limitations, in this study, we demonstrated a methodology for respiratory-gated radiotherapy using ultrasound. The proposed system enables non-invasive, real-time observation of internal organs. It directly tracks liver edges using the Shi-Tomasi method for edge detection and the Lucas-Kanade method with Gaussian pyramid techniques for motion tracking. Irradiation timing is determined using Euclidean distance and histogram matching with predefined thresholds for "BEAM ON" output. Phantom and volunteer studies-performed to evaluate feasibility. As a result, the usefulness in the human body was confirmed. However, further investigation is required to resolve the issue of fixing the ultrasound probe on the abdomen under natural breathing conditions before the proposed method can be applied to achieve non-invasive and highly accurate respiratory-gated radiation therapy.