Physics contribution
Target localization and real-time tracking using the Calypso 4D localization system in patients with localized prostate cancer

https://doi.org/10.1016/j.ijrobp.2006.01.050Get rights and content

Purpose: The Calypso 4D Localization System is being developed to provide accurate, precise, objective, and continuous target localization during radiotherapy. This study involves the first human use of the system, to evaluate the localization accuracy of this technique compared with radiographic localization and to assess its ability to obtain real-time prostate-motion information.

Methods and Materials: Three transponders were implanted in each of 20 patients. Eleven eligible patients of the 20 patients participated in a study arm that compared radiographic triangulated transponder locations to electromagnetically recorded transponder locations. Transponders were tracked for 8-min periods.

Results: The implantations were all successful, with no major complications. Intertransponder distances were largely stable. Comparison of the patient localization on the basis of transponder locations as per the Calypso system with the radiographic transponder localization showed an average (±SD) 3D difference of 1.5 ± 0.9 mm. Upon tracking during 8 min, 2 of the 11 patients showed significant organ motion (>1 cm), with some motion lasting longer that 1 min.

Conclusion: Calypso transponders can be used as magnetic intraprostatic fiducials. Clinical evaluation of this novel 4D nonionizing electromagnetic localization system with transponders indicates a comparable localization accuracy to isocenter, (within 2 mm) compared with X-ray localization.

Introduction

Knowledge of the location of a tumor is critical during external-beam radiation therapy (EBRT) to optimize therapy and minimize complications. This requirement applies especially to emerging radiation-therapy techniques that involve dose escalation, hypofractionation, and gated therapy. Multiple factors affect the location of a radiation target, including daily internal variations caused by changes that occur between treatment days (interfraction variations) and changes that occur while the patient is on the table (intrafractional changes). In the treatment of localized prostate cancers, many studies have been done to determine the daily variations in target location throughout the course of treatment. These studies have been done with different techniques, including ultrasound, infrared cameras, X-rays, portal imaging, multiple diagnostic, and megavoltage computerized tomography (CT) (1, 2, 3, 4). Daily prostate position variations are caused by variations in bladder and rectal filling. These variations can also be caused by changes in the patient’s surface and by surface marks that are used to traditionally align patients.

In addition to the daily prostate variations, recent studies have been done to determine the intrafraction motion that can occur while a patient is undergoing a single fraction of treatment. Motion caused by breathing and other biologic factors, such a gradual rectal distention that occurs over a few minutes, can cause the prostate to change location while the radiation is being delivered. These studies have been done by application of cine magnetic resonance imaging, by CT, or by evaluating markers in the prostate under fluoroscopy. These studies indicate that whereas breathing can induce significant prostate movement under extreme conditions (prone patients breathing deeply), the prostate does not move much with breathing when the patient is comfortable and in the supine position (5, 6). All such studies indicate that prostate motion can occur during a treatment course. However, these motions are not as great as the daily variations and are mostly secondary to rectal peristalsis (7).

Implanted markers are frequently used to establish the position of the prostate gland at treatment during EBRT. This technique has been reported in the literature and applied by institutions that utilize kilovoltage (KV) X-rays, portal images, and either megavoltage (MV) CT or cone beam CT (1, 3, 8, 9, 10). These markers serve as proxies for prostate-gland position. With intraprostatic marker implantation becoming an increasingly routine procedure, the use of an implantable radiofrequency emitting device is being proposed as an alternative to radiopaque fiducial markers and radiographic localization to provide an objective, accurate, real-time method of localizing and monitoring target position. A prototype for electromagnetic localization has been developed and is under investigation for use in radiotherapy. This report is the first on the clinical use of this system in vivo, with special emphasis on the validation of the independent-signaling Calypso system and the radiographic localization of the implanted devices.

Section snippets

System overview

The Calypso 4D Localization System (Calypso Medical Technologies, Seattle, WA) is designed to provide accurate, precise, objective, and continuous target localization during external-beam radiotherapy. The Calypso system consists of “wireless,” implanted Beacon transponders and a noncontact AC magnetic array. Figure 1 shows an actual transponder implant. The system is designed such that the magnetic array is localized to the room coordinate system via fixed infrared cameras that are calibrated

Results

The implantation procedures were successful, with no major complications. Of the 60 implanted transponders, 1 was lost (presumably implanted in the urethra and subsequently voided, with the patient remaining asymptomatic), 1 transponder migrated approximately 3 cm from the apical area to the level of the seminal vesicles around the 4th day after the implant (presumably traveling within the venous plexus, with the patient remaining asymptomatic and the transponder subsequently remaining stable),

Discussion

The Calypso 4D Localization System is a novel method of target localization during the course of external-beam radiation therapy. The current study constitutes the first attempt at characterizing the system in the clinical setting, specifically in patients with localized prostate cancer to be treated with external-beam radiotherapy. Compared with the two different KV X-ray localization methods used for the study, the accuracy of the Calypso system for localization was less than 1.5 mm. The

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