Elemental Bio-imaging of Thorium, Uranium, and Plutonium in Tissues from Occupationally Exposed Former Nuclear Workers

Internal exposure from naturally occurring radionuclides (including the inhaled long-lived actinides Th-232 and U-238) is a component of the ubiquitous background radiation dose (National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States; NCRP Report No. 160; NCRP: Bethesda, MD, 2009). It is of interest to compare the concentration distribution of these natural a-emitters in the lungs and respiratory lymph nodes with those resulting from occupational exposure, including exposure to anthropogenic plutonium and depleted and enriched uranium. This study examines the application of laser ablation-inductively coupled plasma-mass spectrometry (IA-ICPMS) to quantifying and visualizing the mass distribution of uranium and thorium isotopes from both occupational and natural background exposure in human respiratory tissues and, for the first time, extends this application to the direct imaging of plutonium isotopes. Sections of lymphatic and lung tissues taken from deceased former nuclear workers with a known history of occupational exposure to specific actinide elements (uranium, plutonium, or americium) were analyzed by LA-ICPMS. Using a previously developed IA-ICPMS protocol for elemental bio-imaging of trace elements in human tissue and a new software tool, we generated images of thorium (Th-232), uranium (U-235 and U-238), and plutonium (Pu-239 and Pu-240) mass distributions in sections of tissue. We used a laboratory-produced matrix-matched standard to quantify the Th-232, U-235, and U-238 concentrations. The plutonium isotopes Pu-239 and Pu-240 were detected by IA-ICPMS in 65 mu m diameter localized regions of both a paratracheal lymph node and a sample of lung tissue from a person who was occupationally exposed to refractory plutonium (plutonium dioxide). The average (overall) Pu-239 concentration in the lymph node was 39.2 ng/g, measured by high purity germanium (HPGe) gamma-spectrometry (Lynch, T. P.; Tolmachev, S. Y.; James, A. C. Radiat. Prot. Dosim. 2009, 134, 94-101). Localized mass concentrations of thorium (Th-232) and uranium (U-238) in lymph node tissue from a person not occupationally exposed to these elements (chronic natural background inhalation expo-sure) ranged up to 400 and 375 ng/g, respectively. In lung samples of occupationally nonexposed to thorium and uranium workers, Th-232 and U-238 concentrations ranged up to 200 and 170 ng/g, respectively. In a person occupationally exposed to air-oxidized uranium metal (Adley, F. E.; Gill, W. E.; Scott, R. H. Study of atmospheric contaminiation in the melt plant buiding. HW-23352(Rev.); United States Atomic Energy Commission: Oakridge, TN, 1952, p 1-97), the maximum U-235 and U-238 isotopic mass concentrations in a lymph node, measured at higher resolution (with a 30 urn laser spot diameter), were 70 and 8500 ng/g, respectively. The ratio of these simultaneously measured mass concentrations signifies natural uranium. The current technique was not sufficiently sensitive, even with a 65 mu m laser spot diameter, to detect Am-241 (at an overall tissue concentration of 0.024 ng/g, i.e., 3 Bq/g).