Increasing deployment of radiation detectors designed to identify and interdict illicit radioactive sources has led to a phenomenon of patients being detected and investigated by emergency responders following administration of therapeutic and even diagnostic radiopharmaceuticals. We wish to assess the potential duration of this problem by analyzing sensitivity of detectors to medical radionuclides and applying this information to pharmacokinetic models of radiopharmaceutical excretion.
METHOD AND MATERIALS
Five personal radiation detectors, culled from operational units in the field, were studied: Pocket Pager (2 units), Pocket Pager-S and HRM, (all Sensor Technology Engineering, Inc., Santa Barbara, CA) and PM1703M (ThermoElectron Corporation, Minsk, Republic of Belarus). Calibrated amounts of 7 medically-relevant radionuclides (Tl-201, Tc-99m, Ga-67, I-123, In-111, I-131, F-18) were tested against the panel of detectors to determine levels at which they alarm. Approximately 100μCi of each radionuclide was translated towards the detectors at 0.5m increments and the distance at which each detector alarmed was noted. Using the gamma ray constant Γ, the triggering exposure rate was determined. This was compared to administered activity levels and pharmacokinetic models of excretion for diagnostic and therapeutic radiopharmaceuticals to assess potential alarm triggering at a 1m survey distance.
At standard operational settings, detectors exhibit similar sensitivity, alarming at rates between ~0.5 and 8 μR/hr in a photon energy-dependent manner. After 1 day F-18 FDG is below triggering levels. Patients administered Tc-99m radiopharmaceuticals or I-123 NaI may trigger these alarms for up to 3 days, In-111 WBC and platelets for 14-17 days, Ga-67 citrate and Tl-201 thallous chloride for up to ~30 days, and I-131 containing NaI and Bexxar for up to 95 days.
Personal radiation detectors used for Homeland Security are extremely sensitive and may detect low levels of radionuclides for prolonged periods of time. Patients should be appropriately counseled to carry information regarding administration of diagnostic and therapeutic radiopharmaceuticals for these extended periods.