International Journal of Radiation Oncology*Biology*Physics
Physics contributionPhysical aspects of a real-time tumor-tracking system for gated radiotherapy☆
Introduction
Organ motion and setup error are important uncertainties in external radiotherapy for extracranial diseases. The planning target volume (PTV) must have a larger margin for moving tumors compared to static tumors with the same clinical target volume (CTV).
One approach to improve the treatment accuracy for targets in or near the lung, is to gate the accelerator to the respiratory motion 1, 2, 3, 4, 5. We have previously reported that lung tumors follow complicated three-dimensional (3D) tracks 6, 7. By detecting the internal movement of a tumor in real time during treatment and by using this information to gate the accelerator, the accuracy of irradiation can be improved (8).
We have developed a system which is capable of tracking the 3D position of a metallic marker in the body in real time by means of two sets of diagnostic X-ray imaging equipment. This system is in clinical use for patients with extracranial diseases (9). The linear accelerator is gated by the real-time tumor-tracking system to irradiate the target volume only when the internal marker is near its planned position. About 30 patients with lung tumors, liver tumors, and other diseases have been treated using this system. The aims of this study are to determine the accuracy of the system and the additional dose due to the diagnostic X-ray, and to determine its performance during clinical use.
Section snippets
Hardware
The real-time tumor-tracking system was constructed by Mitsubishi Electronics Co., Ltd., Tokyo, for this study. It consists of four sets of diagnostic X-ray TV systems that are mounted in the treatment room (Fig. 1). The X-ray images are fed to an image processor unit that consists of two image acquisition units, two image recognition units, and a central processor unit (CPU). The output of the real-time tumor-tracking system gates a dual-energy linear accelerator (EXL-20DP; Mitsubishi
Absorbed dose
Initial clinical experience with the system for about 30 patients showed that about 2 min of diagnostic X-ray exposure was required, on average, for patient setup and tumor tracking during a daily irradiation of 2 Gy. The additional dose due to real-time tracking radiotherapy, was, therefore, estimated in this study, assuming 2 min of exposure from two sets of diagnostic X-ray tubes for a daily irradiation of 2 Gy giving a total of 60 Gy in 30 fractions. The dose rate was estimated to be from
Discussion
Stereotactic radiotherapy in which a high dose of irradiation is given to a small target volume using a precise localization and immobilization system has proven successful in treatment of various brain diseases (14). Similar localization and immobilization accuracy are desired for the treatment of extracranial diseases in which the goal is to administer a high dose to a small target volume. Other new techniques, such as conformal 3D radiotherapy and intensity-modulated radiotherapy, also
Author contributions
Guarantor of integrity of entire study. Shirato H. Study concepts. Shirato H, Kunieda T. Study design. Shirato H., Miyasaka M Definition of intellectual content. Shirato H. Literature research. Shirato H, Shimizu S, Fujita K, Aoyama H. Clinical studies. Shirato H, Kagei, K, Shimizu S, Kitamura K. Experimental studies. Shirato H, Shimizu S, Aoyama H, Kunieda T, Nishioka T, Fujita K, Tsuchiya K. Data acquisition. Shirato H, Shimizu S, Tsuchiya K, Hashimoto S, Kudo K. Data analysis. Shimizu S,
Appendix
The fluoroscopic transformation matrix M required in the central processing unit is calculated as follows. A spatial coordinate calibrator is used which consists of acrylic cube with sides of 58 mm in which 8 markers with a diameter of 2 mm which are precisely centered on the vertexes of the cube. The isocenter is indicated with precise markers in the center of the faces of the cube. After aligning the calibrator with the room lasers, fluoroscopic images are taken and the operator gives
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This work was supported by a grant from the Ministriy of Educational, Science, Sports, and Culture, Japan (No. 12470182) and the Japanese Association for Nuclear Technology in Medicine.