Radium-226 and the Thyroid Gland: Maybe We Know Less than We Thought
S.L. Simon1; S.A. Ibrahim2; L. VanMiddlesworth3; R.E. Filipy4; and A. Bouville1 (1National Cancer Institute, National Institutes of Health; 2Colorado State University; 3University of Tennessee School of Medicine; 4US Transuranium and Uranium Registries, Washington State University)
Small amounts of Ra-226 and other radium isotopes routinely enter the human body through normal dietary intake and in some cases, through occupational exposure. Current radiation dosimetry models for Ra-226 in the human body assume a uniform distribution among soft tissues and a short retention time in those tissues These assumptions persist, despite publications in the mid-1980s indicating that radium concentrations in tissue are related to calcium levels in each organ and imply that the thyroid gland could accumulate greater concentrations of radium isotopes than any other tissue. Moreover, the natural intake or production of sulfate or barium compounds in the body could serve to precipitate radium in the thyroid gland, thereby immobilizing it, with the result that the tissue is continually irradiated for the remaining lifetime of the individual. Evidence of accumulation and immobilization of Ra-226 in thyroids of grazing animals has been documented since the 1960s by one of us (LVM). Little is known, however, about the concentration and retention of radium in the human thyroid. A collaboration to investigate these questions in man has recently begun. In preliminary work, thyroid tissues have been obtained from twenty deceased individuals to ascertain typical background concentrations of Ra-226 as well as the concentrations in two uranium miners, and in a single radium dial painter. Sensitive measurements have been made by the radon emanation technique. Our initial analysis indicates that concentrations of Ra-226 in the two uranium miners is one to three times the average background concentration, while the concentration in the dial painter thyroid may be up to fifty times the background concentration. Background concentrations vary widely, however, and continue to be investigated. The findings presented here have implications for improving dosimetry models of Ra-226 in soft tissues and for projecting health risks from natural radium intake.