Longitudinal Assessment Of Anchored Radio-Frequency Transponder Displacement Following Lung Stereotactic Body Radiation Therapy

Radio-frequency Beacon transponders provide continuous, high fidelity target localization during radiation therapy when implanted into soft tissue. In order to extend this technology to lung tumor localization, a 5-legged nitinol stabilization system has been developed to anchor the transponder within the lung. The short-term stability and accuracy of anchored Beacon transponders has previously been shown. The primary purpose of this study was to assess the long-term stability of the anchored transponders and compare displacement rates with other commercially available lung fiducial markers. We also sought to describe late toxicity attributable to fiducial implantation or migration. The transponder cohort was comprised of the seventeen patients at our institution who enrolled in a multi-site prospective clinical trial (NCT01396551) and underwent bronchoscopic implantation of 3 anchored transponders into small (2-2.5mm) airways. Each patient had a pathologic diagnosis of malignancy and was planned to undergo stereotactic body radiotherapy (SBRT) of the lung. We generated a comparison cohort of 34 patients by selecting patients from our institutional lung SBRT database and matching 2:1 based on the lobe of the lung containing tumor and proximity to the bronchial tree (e.g. peripheral v. central). Assessment of migration was performed by rigidly registering the most recent follow-up CT scan, up to 2.5 years post-SBRT, to the simulation scan and assessing whether the relative geometry of the fiducial markers had changed by more than 5mm. Toxicity outcomes of interest were hemoptysis and pneumothorax. Between group comparison of frequencies was assessed using the χ2 test. The median follow-up of patients in the transponder cohort was 25.3 months and the median follow-up in the comparison cohort was 21.7 months. Fiducials placed in the comparison cohort were gold seeds with nitinol coils in 18 (53%) patients, fibered platinum vascular occlusion coils in 12 (35%) patients, and were of unspecified type in 4 (12%) patients. When assessing the most recent CT, all fiducial markers were within 5mm of their position at CT simulation in 11 (65%) patients in the transponder group as compared to 23 (68%) in the comparison group (p=0.28). Across both groups, no fiducial marker noted at the time of CT simulation was unaccounted for at the most recent CT. One case of hemoptysis was identified in the transponder cohort, and bronchoscopy confirmed bleeding from recurrent tumor; no cases of hemoptysis were noted in the comparison cohort. No case of pneumothorax was noted in either group. No significant difference in the rates of fiducial marker retention and migration were noted when comparing patients who had anchored transponders placed into small airways and a 2:1 matched cohort of patients who had other commercially available lung fiducial markers placed. In both groups, no late or chronic toxicity appeared to be related to the implanted fiducial markers.