Physics contribution
Accuracy of a wireless localization system for radiotherapy

Presented at the 45th Annual ASTRO Meeting, Salt Lake City, UT, October 19–23, 2003.
https://doi.org/10.1016/j.ijrobp.2004.11.009Get rights and content

Purpose

A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials.

Methods and materials

We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds.

Results

Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (σx = 0.006 mm, σy = 0.01 mm, σz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (σx = 0.27 mm, σy = 0.36 mm, σz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s.

Conclusion

This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.

Introduction

Internal organ localization is a leading concern in the safe and effective delivery of precision radiotherapy. Numerous investigations have demonstrated that targets such as the prostate, focal liver tumors, and lung tumors move with respect to traditional positioning references of the skin and skeleton. Systems to localize and track the internal anatomy via implanted radiopaque fiducial markers have demonstrated effectiveness. These systems require the use of regional irradiation of the patient to permit visualization. Pretreatment localization via ultrasonography has been demonstrated, although current systems are subjective and unsuited for continuous monitoring of anatomy during treatment. Predictive systems that combine external references with periodic radiographic verification have also been described, although these systems require faith in the correlation between updates.

To help provide a system for internal fiducial localization and position monitoring, a wireless magnetic system has been developed. This system localizes one or more implantable transponders and monitors their position continuously during treatment.

A number of issues need to be addressed to ensure that this system is functional for internal localization within a typical linear accelerator environment. This study presents a brief description of the system, along with initial tests of accuracy.

A sketch of the complete localization system is shown in Fig. 1. One or more wireless transponders (Beacon) are implanted in the patient before acquisition of a treatment planning CT scan. These transponders are glass encapsulated and can be administered via a 14-gauge needle, in the same fashion as gold markers are inserted for prostate localization. The location of these transponders is determined relative to the isocenter from the treatment planning CT scan. When the patient is to be positioned for radiotherapy, a magnetic source and receiver coil array is used to determine the transponder positions. The array is tracked in real time relative to the isocenter through the use of an infrared optical tracking system. The combination of the array position in the room and the transponder position relative to the array yields the position of the transponder relative to the isocenter. This position is updated at a rate of 10 Hz, and thus provides continuous feedback for initial positioning of the prostate and subsequent monitoring of the prostate position during treatment.

A schematic drawing of the beacon localization system is shown in Fig. 2. The array contains 4 source coils and 32 receiver coils. The source coils generate an oscillating field, which induces a resonance in the transponder. When the field is switched off, the transponder signal during relaxation is used to establish its position and orientation (Fig. 3). This system has been designed to localize multiple transponders over a field of view spanning 14 × 14 cm wide, and up to 27 cm away from the source array.

A number of issues need to be addressed to ensure that this technology is safe and effective for clinical use. In preparation for the first expected application (pretreatment prostate localization), some initial investigations have been performed. This article deals specifically with the subsystem for localization of transponders with respect to the array, addressing the accuracy and reproducibility (precision) of the position readouts during short periods (for localization) and longer periods (for monitoring during the course of a treatment fraction). The ability of this system to track movement over a known trajectory is demonstrated.

Section snippets

Methods and materials

The transponder position is a combination of array-based localization of transponders, as well as camera-based tracking of the array. The camera system is similar in design to previously reported units in radiotherapy departments. Studies of these units have demonstrated submillimeter accuracy under well-controlled conditions; thus, this report focused on the accuracy of transponder localization relative to the array.

A test assembly was constructed using Ultem 10000 (GE Thermoplastics). This

Results

Figure 6 shows a plot of the measured position about all axes for 18-s periods for a transponder placed 80 mm away from the source array. At this distance, the random variation in position readout was extremely small, with the precision of the readout defined by a standard deviation of 0.006 mm, 0.01 mm, and 0.006 mm about the X, Y, and Z (offset) axes, respectively. Reproducibility decreased with the distance from the array. At 270 mm from the array, however, the precision was still

Discussion

The results above have demonstrated the inherent accuracy and precision of the array-transponder measurement configuration. Demonstration of the accuracy of this critical component is essential to the development of a complete integrated system for radiotherapy position management. The complete system requires the integration of multiple subsystems. The position of the transponders with respect to the isocenter is established through optical tracking of the array position in the treatment room.

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J. Balter is a member of the scientific advisory board of Calypso Medical Technologies.

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