doi:10.1016/j.ijrobp.2006.04.044 PHYSICS CONTRIBUTION DEVELOPMENT OF A FOUR-DIMENSIONAL IMAGE-GUIDED RADIOTHERAPY SYSTEM WITH A GIMBALED X-RAY HEAD YUICHIRO KAMINO, M.S.,* KENJI TAKAYAMA, M.D.,* † MASAKI KOKUBO, M.D., PH.D., ‡ YUICHIRO NARITA,PH.D., † ETSURO HIRAI,PH.D.,* NORIYUKI KAWAWDA, B.S.,* TAKASHI MIZOWAKI, M.D., PH.D., † YASUSHI NAGATA, M.D., PH.D., † TAKEHIRO NISHIDAI,PH.D., § AND MASAHIRO HIRAOKA, M.D., PH.D. † *Mitsubishi Heavy Industries, Ltd., Tokyo, Japan; † Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University, Kyoto, Japan; ‡ Institute of Biomedical Research and Innovation, Kobe, Japan; § Kyoto College of Medical Technology, Kyoto, Japan Purpose: To develop and evaluate a new four-dimensional image-guided radiotherapy system, which enables precise setup, real-time tumor tracking, and pursuit irradiation. Methods and Materials: The system has an innovative gimbaled X-ray head that enables small-angle (2.4°) rotations (pan and tilt) along the two orthogonal gimbals. This design provides for both accurate beam positioning at the isocenter by actively compensating for mechanical distortion and quick pursuit of the target. The X-ray head is composed of an ultralight C-band linear accelerator and a multileaf collimator. The gimbaled X-ray head is mounted on a rigid O-ring structure with an on-board imaging subsystem composed of two sets of kilovoltage X-ray tubes and flat panel detectors, which provides a pair of radiographs, cone beam computed tomography images useful for image guided setup, and real-time fluoroscopic monitoring for pursuit irradiation. Results: The root mean square accuracy of the static beam positioning was 0.1 mm for 360° of O-ring rotation. The dynamic beam response and positioning accuracy was 0.6 mm for a 0.75 Hz, 40-mm stroke and 0.4 mm for a 2.0 Hz, 8-mm stroke. The quality of the images was encouraging for using the tomography-based setup. Fluoroscopic images were sufficient for monitoring and tracking lung tumors. Conclusions: Key functions and capabilities of our new system are very promising for precise image-guided setup and for tracking and pursuit irradiation of a moving target. © 2006 Elsevier Inc. IGRT, 4D-IGRT, Organ motion compensation, Pursuit irradiation, CBCT. INTRODUCTION Image-guided radiotherapy (IGRT) is a key method for precise targeting for radiotherapy. There are two major functions of IGRT: one is for initial setup in each fraction (interfraction), and the other is for motion compensation during fraction (intrafraction). For precise initial setup, electronic portal imaging de- vices (EPID) were developed in the 1990s (1), which en- abled verification by means of bony structures or fiducial markers. Off-board or on-board kilovoltage (kV) radiograph and fluoroscopy devices were then introduced that provided better image quality than EPID. CyberKnife (2) and Exac- Trac (3) have an off-board stereoscopic kV-radiography system combined electronically with a linear accelerator (LINAC). More recently, Jaffray et al. (4) mounted a set of kV X-ray tubes and an image detector on the gantry of a LINAC. It is also useful to have a computed tomography (CT) scanner in the treatment room because of the advan- tage of volumetric imaging, which enables soft-tissue veri- fication as well as conventional verification using boney or fiducial markers (5). An on-board cone beam computed tomography (CBCT) is highly desirable because three-di- mensional (3D) imaging data can be acquired at one time for verification. Jaffray et al. (6) developed a CBCT func- tion using an on-board kV imaging subsystem. Similar systems have been developed and evaluated clinically (7, 8). Megavoltage (MV) CBCT systems (9, 10) and an MV CT system (11) have also been developed. Reprint requests to: Daisaku Horiuchi, MHI Medical Sys- tems, Inc., Mita-Nitto-Dai Building, 5th Floor, 11-36 Mita 3-Chome, Minato-Ku Tokyo, 108-0073 Japan. E-mail: daisaku_horiuchi@mhi.co.jp Supported by New Energy and Industrial Technology Develop- ment Organization (NEDO) of Japanese government as Key Tech- nology Research Promotion (Grant No. 05000942-0). Acknowledgments—We are thankful for the assistance of Mr. Ichiro Yamashita of Mitsubishi Heavy Industries, Ltd., with ac- celerator technology. We are also thankful for the assistance of Dr. Tokihiro Yamamoto of the Graduate School of Medicine, Osaka University, in the monitoring dose evaluation. Received Nov 26, 2005, and in revised form April 13, 2006. Accepted for publication April 14, 2006. Int. J. Radiation Oncology Biol. Phys., Vol. 66, No. 1, pp. 271–278, 2006 Copyright © 2006 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/06/$–see front matter 271