Abstract
The purpose of this study was to quantify postimplantation migration of percutaneously implanted cylindrical gold seeds (“seeds”) and platinum endovascular embolization coils (“coils”) for tumor tracking in pulmonary stereotactic ablative radiotherapy (SABR). We retrospectively analyzed the migration of markers in 32 consecutive patients with computed tomography scans postimplantation and at simulation. We implanted 147 markers (59 seeds, 88 coils) in or around 34 pulmonary tumors over 32 procedures, with one lesion implanted twice. Marker coordinates were rigidly aligned by minimizing fiducial registration error (FRE), the root mean square of the differences in marker locations for each tumor between scans. To also evaluate whether single markers were responsible for most migration, we aligned with and without the outlier causing the largest FRE increase per tumor. We applied the resultant transformation to all markers. We evaluated migration of individual markers and FRE of each group. Median scan interval was 8 days. Median individual marker migration was 1.28 mm (interquartile range [IQR] 0.78−2.63 mm). Median lesion FRE was 1.56 mm (IQR 0.92−2.95 mm). Outlier identification yielded 1.03 mm median migration (IQR 0.52−2.21 mm) and 1.97 mm median FRE (IQR 1.44−4.32 mm). Outliers caused a mean and median shift in the centroid of 1.22 and 0.80 mm (95th percentile 2.52 mm). Seeds and coils had no statistically significant difference. Univariate analysis suggested no correlation of migration with the number of markers, contact with the chest wall, or time elapsed. Marker migration between implantation and simulation is limited and unlikely to cause geometric miss during tracking.PACS number: 87.57.N‐; 87.57.nm; 87.53.Ly
Highlights
Stereotactic ablative radiotherapy (SABR), known as stereotactic body radiation therapy (SBRT), has been an important development in radiation therapy for small malignant lung tumors.[1,2] SABR involves the precise delivery of high doses of conformal radiation over fewer fractions to ablate lesions while sparing surrounding healthy tissue
The use of surrogates was originally pioneered for intracranial radiation therapy in lieu of a much more invasive rigid skull fixation.[3]. In a continuing effort to reduce invasiveness, interventional radiological implantation of round gold markers facilitating the use of real-time stereoscopic fluoroscopy in automated image-guided respiratory-gated radiation therapy was later developed in Japan.[4,5] These principles were soon after applied to a commercial system in the form of dynamic tumor tracking using automated detection and trajectory modeling of implanted gold markers (CyberKnife, Accuray, Inc., Sunnyvale, CA)
Other previous studies of marker migration have focused primarily on similar problems in the prostate and other sites subject to lesser degrees of respiratory motion. These primarily used Euclidian distances[12,13] and comparison to the center of mass of the target organ.[14]. While the distance to the center of mass as a measure for marker migration is suitable where the target moves in conjunction with a well-defined larger organ, this approach may be limited for targets that move within the larger organ — like pulmonary tumors — as the center of mass will change its location as a consequence of respiratory motion
Summary
Stereotactic ablative radiotherapy (SABR), known as stereotactic body radiation therapy (SBRT), has been an important development in radiation therapy for small malignant lung tumors.[1,2] SABR involves the precise delivery of high doses of conformal radiation over fewer fractions to ablate lesions while sparing surrounding healthy tissue. Because pulmonary lesions in soft tissue lack high radiopaque contrast with planar imaging, IGRT of the lung remains challenging. The use of surrogates was originally pioneered for intracranial radiation therapy in lieu of a much more invasive rigid skull fixation.[3] In a continuing effort to reduce invasiveness, interventional radiological implantation of round gold markers facilitating the use of real-time stereoscopic fluoroscopy in automated image-guided respiratory-gated radiation therapy was later developed in Japan.[4,5] These principles were soon after applied to a commercial system in the form of dynamic tumor tracking using automated detection and trajectory modeling of implanted gold markers (CyberKnife, Accuray, Inc., Sunnyvale, CA). The CyberKnife uses periodic stereoscopic X-ray imaging in combination with continuous optical imaging of light-emitting diode markers placed on the external body surface to facilitate tracking of lesions.[6,7]
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