The in vivo assessment of thorium body burden by gamma ray sepctrometry.

Thorium is used in many different industrial technologies and is widely found in nature. Internal contamination with thorium is considered as highly hazardous because of its radiological and chemical toxicities, which depend on the chemical form in which thorium appears. The assessment of the thorium body burden is then of primary importance in detecting the risks of personal contamination, and for appropriate counteractions when contamination is detected. The in vivo assessment of the thorium lung burden is commonly achieved by gamma ray spectrometry of its progeny. Three methods for the assessment of thorium in the lungs are compared. In the first method, the radionuclide examined is 208Tl, measured with a Nal(TI) detector. This simple method can be affected by systematic errors due to 220Rn exhalation and because of the assumption of equilibrium between 232Th and 228Th. The second method, based on the measurement of the gamma rays emitted by 228Ac, requires the use of high-resolution gamma spectrometry (HPGe detectors). The accuracy of the thorium quantification is better with this technique. The third method is based on the measurement of exhaled 220Rn.