Assessment of Free Yttrium-90 Circulation During Radioembolization Procedures

1015 Introduction: Radioembolization requires the infusion of radioactive glass or resin spheres that are injected near the site of the selected lesion or liver region. During infusion free yttrium-90 may be in the solution from leaching or can leach off once in the body. This study examines preliminary results from a study monitoring the infusion of the radioembolization dose using external detectors, blood sampling, and for the first time using whole-body post-therapy PET/CT imaging to look for any unexpected aggregated off-target activity in the body. Methods: Two detectors were placed on the patient with one located laterally near the liver and the other located at the top of the foot to look for signal from possible free Y90 or off-target activity. The infusion delivery system also was fitted with two detectors with one monitoring the dose vial holder inside the delivery system box and the other located directly on the catheter tubing leading to the patient. Blood samples were collected at points prior to embolization, during the infusion, just after completion of the infusion, and just before post-therapy imaging. Blood samples were pipetted into matched volume tubes and counted in a gamma counter. Whole-body PET/CT imaging, covering a range from the top of the skull to the lower legs, was performed approximately 2 hours following completion of the radioembolization procedure. Patients were imaged using continuous bed motion imaging for approximately 45 minutes and data were reconstructed using resolution recovery and time of flight processing. Results: Blood sampling indicates a small, but detectable, amount of transient radiation in the blood during or immediately following infusion of the yttrium-90 spheres and suspension solution. This increase was not observed in all patients and was not detectable by the time of PET/CT imaging suggesting good clearance of circulating free Y-90. External detector monitoring of the infusion positively confirmed successful infusion of the spheres into the liver with increases in liver signal that correspond to infusion timing. Additionally, spikes in count rates were seen in the sensors on the top of the foot that occur within seconds of detection of the infusion. PET/CT imaging in 7 patients has not yet shown any detectable signs of aggregation of free Y-90 in the body of patients with either type of sphere, however, using whole-body techniques we can better confirm the distribution of spheres and potential presence of off-target activity. Conclusion: Free Y-90 may be present during infusion of Y-90 spheres. Although detectable in some patients, the free Y-90 measured in the blood appears to be fully cleared from the blood by the time the patient is discharged. External detectors can be used to assess the quality and estimate the success of the infusion. Spikes in count rates observed by the detector at the top of the foot, indicate a transient increase in radiation at that site which we believe to be beta interaction with the foot oriented detector as the dose passes out of the acrylic shielding through the catheter to the patient. Further work is underway to determine the cause of these count rate increases. We have not yet had any subjects with significant off-target activity, however, this methodology provides much greater insight and will better enable accurate dosimetry assessments of impacted regions when off-target activity is found.