Stability, Visibility, and Histologic Analysis of a New Implanted Fiducial for Use as a Kilovoltage Radiographic or Radioactive Marker for Patient Positioning and Monitoring in Radiotherapy

Purpose: To analyze the stability, visibility, and histology of a novel implantable soft-tissue marker (nonradioactive and radioactive) implanted in dog prostate and rabbit liver. Methods and Materials: A total of 34 nonradioactive and 35 radioactive markers were implanted in 1 dog and 16 rabbits. Stability was assessed by measuring intermarker distance (IMD) variation relative to IMDs at implantation. The IMDs were measured weekly for 4 months in the dog and biweekly for 2-4 weeks in the rabbits. Ultrasound and X-ray imaging were performed on all subjects. Computed tomography and MRI were performed on the dog. Histologic analysis was performed on the rabbits after 2 or 4 months. Results: A total of 139 measurements had a mean (± SD) absolute IMD variation of 1.1 ± 1.1 mm. These IMD variations are consistent with those reported in the literature as due to random organ deformation. The markers were visible, identifiable, and induced minimal or no image artifacts in all tested imaging modalities. Histologic analysis revealed that all pathologic changes were highly localized and not expected to be clinically significant. Conclusions: The markers were stable from the time of implantation. The markers were found to be compatible with all common medical imaging modalities. The markers caused no significant histologic effects. With respect to marker stability, visibility, and histologic analysis these implanted fiducials are appropriate for soft-tissue target positioning in radiotherapy.