Abstract

Image‐guided patient setup for respiratory‐gated radiotherapy often relies on a pair of respiratory‐gated orthogonal radiographs, acquired one after the other. This study quantifies the error due to changes in the internal/external correlation which may affect asynchronous (non‐simultaneous) imaging. The dataset from eight patients includes internal and external coordinates acquired at 30Hz during multi‐fraction SBRT treatments using the Mitsubishi RTRT system coupled with an external surrogate gating device. We performed a computational simulation of the position of an implanted fiducial marker in an asynchronous orthogonal image set. A comparison is made to the reference position, the actual 3D fiducial location at the initial time point, as would be obtainable by simultaneous orthogonal setup imaging at that time point. The time interval between the two simulated radiographic acquisitions was set to a minimum of 30, 60 or 90 seconds, based on our clinical experience. The setup position is derived from a combination of both the initial (AP) and the final (LR) simulated 2D images in the following way: LRsetup=LRinitial,SIsetup=SIinitial+(SIfinal−SIinitial)/2,APsetup=APfinal. The 3D error is then the magnitude of the vector from the initial (reference) position to the setup position. The calculation was done for every exhale phase in the data for which there was another one at least 30, 60 or 90 seconds later, at an amplitude within 0.5 mm from the first. A correlation between the time interval and the 3D error was also sought. The mean 3D error is found to be roughly equivalent for time intervals (tinterval) of 30, 60 and 90 seconds between the orthogonal simulated images (0.8 mm, 0.8 mm, 0.6 mm, respectively). The 3D error is less than 1, 2 and 3 mm for 77%, 89% and 98% of the data points, respectively. The actual time between simulated images turned out to be very close to tinterval, with 90% of the second simulated image acquisitions being completed within 38, 68 and 95 seconds of the first simulated image for tinterval of 30, 60 and 90 seconds, respectively. No correlation was found between the length of the time interval and the 3D error. When acquiring respiratory‐gated radiographs for patient setup, only small errors should be expected if those images are not taken simultaneously.PACS number: 87.55.ne

Highlights

  • 159 Berbeco et al.: Evaluation of gated setup imaging on some surrogate of tumor location

  • Simultaneous, stereoscopic imaging for patient setup has been achieved on several radiotherapy platforms

  • The Mitsubishi RTRT system (Mitsubishi­ Medical Systems, Inc., Japan), first introduced in Hokkaido, Japan, offers radiographic and fluoroscopic kV imaging from several oblique angles before and during radiotherapy.[3] this system employs fixed sources and imagers, they are positioned such that imaging may be performed when the linac is at any gantry angle

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Summary

Introduction

159 Berbeco et al.: Evaluation of gated setup imaging on some surrogate of tumor location. The Integrated Radiotherapy Imaging System (IRIS) at our institution is capable of acquiring simultaneous orthogonal kV images before and during treatment.[4,5] The imaging components of the IRIS are mounted to a commercial linac gantry, enabling orthogonal imaging at any gantry position. Any of these systems may be used for simultaneous stereoscopic imaging for patient setup. These are all specialized machines and not considered to be appropriate for routine clinical usage in many parts of the world

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