Physics contribution
Speed and amplitude of lung tumor motion precisely detected in four-dimensional setup and in real-time tumor-tracking radiotherapy

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Background: To reduce the uncertainty of registration for lung tumors, we have developed a four-dimensional (4D) setup system using a real-time tumor-tracking radiotherapy system.

Methods and Materials: During treatment planning and daily setup in the treatment room, the trajectory of the internal fiducial marker was recorded for 1 to 2 min at the rate of 30 times per second by the real-time tumor-tracking radiotherapy system. To maximize gating efficiency, the patient’s position on the treatment couch was adjusted using the 4D setup system with fine on-line remote control of the treatment couch.

Results: The trajectory of the marker detected in the 4D setup system was well visualized and used for daily setup. Various degrees of interfractional and intrafractional changes in the absolute amplitude and speed of the internal marker were detected. Readjustments were necessary during each treatment session, prompted by baseline shifting of the tumor position.

Conclusion: The 4D setup system was shown to be useful for reducing the uncertainty of tumor motion and for increasing the efficiency of gated irradiation. Considering the interfractional and intrafractional changes in speed and amplitude detected in this study, intercepting radiotherapy is the safe and cost-effective method for 4D radiotherapy using real-time tracking technology.

Introduction

Four-dimensional (4D) radiotherapy, a concept to increase the accuracy of localization in space as well as in time by accounting for spatiotemporal changes in the anatomy during radiotherapy, has been receiving more and more attention (1). Four-dimensional treatment planning is increasingly used to reduce the uncertainty resulting from organ motion of computed tomography (CT) (1, 2, 3). Second, a therapeutic beam is delivered to the moving tumor by real-time tumor-tracking technology in which a gated beam intercepts the tumor trajectory (4) or in which the tumor position is pursued dynamically (5). Third, verification of the tumor position during irradiation is required for quality assurance (6). Each step in 4D radiotherapy (4DRT) is quite important. One of the most important steps is the accurate registration of the tumor position at the same phase of internal tumor motion as in the treatment planning. Without this, we will miss the tumor in 4DRT.

To our knowledge, no 4D setup method—that is, a method for setting up tumors in motion at the right time and with the right coordinates—has been reported. We have developed a 4D setup system using a fluoroscopic real-time tumor-tracking radiotherapy (RTRT) system (1, 4). In this paper, we describe our approach to performing 4D setup using the RTRT system. We describe also what we have found about interfractional and intrafractional changes in the amplitude and speed of internal tumor motion using the 4D setup system.

Section snippets

Four-dimensional setup

The RTRT system has been already described in detail (1, 4). In brief, three 1.5-mm fiducial markers are implanted near a lung tumor through bronchial fiberscopy. Two sets of fluoroscopy in the treatment room are used to detect the three-dimensional (3D) coordinates of each internal fiducial marker. The distances between markers are measured to rule out the possibility of the migration of the markers. The 3D coordinates of the marker that is closest to the tumor are automatically detected 30

Results

The absolute amplitudes of the trajectories in the x, y, and z directions in 21 patients are shown in Table 1. When the 4D setup data for multiple treatments were available, the absolute amplitude was averaged. In 60 absolute amplitudes along the RL, CC, and AP directions in 21 patients, the number of average absolute amplitudes longer than 10 mm—at which length we found, in a previous study, that gating allowed a meaningful reduction in safety margins (9)—was 20 (33%) (Table 1). The standard

Discussion

Engelsman et al. assessed the impact of both setup errors and respiration motion on the cumulative dose delivered to a clinical target volume in the lung and found that systematic setup errors have a dominant effect on the cumulative dose to the clinical target volume; random setup errors and breathing motion have smaller effects (9). Accordingly, they recommended minimizing systematic setup errors for lung tumors in motion. Bony landmarks have been used as surrogates for the setup of the lung

References (23)

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