Migration of Calypso beacon transponders for hepatic stereotactic body radiotherapy: a report of two cases

BACKGROUND: The Calypso 4-dimensional Localization System allows the delivery of high-dose of radiation to a target guided by the implanted transponders. Calypso beacons are used for prostate and liver tumors treated with stereotactic body radiation therapy (SBRT). Several risks associated with this procedure have been previously observed. Here, we report on two cases where Calypso soft tissue transponders migrated to the lung shortly after implantation in liver. CASE DESCRIPTION: Two male patients with hepatocellular carcinoma (HCC) underwent insertion of Calypso beacons in liver under image-guidance in preparation for SBRT. Post-procedure images confirmed the presence of the transponders within the liver. However, few days after implant, further imaging revealed a missing marker, in each patient, that had migrated to the right lung. Patients were asymptomatic and SBRT was delivered uneventfully. CONCLUSIONS: This is the first report of migration of Calypso beacons from liver to lung. In order to reduce the risk of migration, a Doppler ultrasound (US) prior to insertion could be performed to ensure that the transponders are at a safe distance from blood vessels. Anchored Calypso beacons, currently approved for insertion in the lung, could be tested as a suitable alternative to soft tissue beacons with a lower risk of migration.