INTERNAL DOSE FROM SHORT-LIVED RADIONUCLIDES By Karu Z. Monaan Oak Ridge National Laboratory, Oak Rulye, Tennessee INTERNAL DOSE FROM STFORT-LIVED RADIONUCLIDES The National Committce on Radiation Pro- tection (NCRP) and the International Commission on Radiological Protection (ICRP) set. the national and international standards for radiation protection. One of the important assignments of these organizations has been the establishment. of the maximumpermissible body burden, q, and the maximum permissible concentrations, MPC, of the radionuclides in air, water, and food. To the present time only q and MPC values for continuous exposure to the radionuclides have been published in the values for inhalation. The GJ. tract was not included as a critical body organ in Handbook 52 because at the time of the publication of Handbook 52 radiation damage per roentgen to the GI. tract was considered less significant thanthatto the other body organs and because insufficient data were available to estimate the absorbed dose and corresponding hazard to various portions of the GJ. tract. When Dr. E. E. Pochin® furnished to the ICRP Committee data on the mass and timedistribution of material in the G.I. tract, it became evident that the GI. tract receives the greatest absorbed dose from manyof the radionuclides. Values The lowerlarge intestine is the critical portion of the G.T. tract for all radionuclides considered will probably be several years before final agreement is reached on a set of values. Mn® and F*, in which cases the upper large intestine and stomach are the critical portions NCRP? and the ICRP? Handbooks. for single exposure are being considered but it The ICRP Handbook, which was published abouttwo years after the NCRP Handbook 52, differs in some respects from the earlier pub- lication. Perhaps the most important change was the incorporation of MPC values based on the G.I. tract as the critical body organ. The importance of this is emphasized by the fact that of the 355 MPC values listed in the ICRP Handbook, 71 percent for ingestion and 41 percent for inhalation refer to the G.T. tract as the critical body organ. The bone is the second most important body organ and is the critical body organ for 11 percent of the MPC values for ingestion and 28 percent of the MPC in the ICRP Handbook with the exception of of the G.I. tract, respectively. The lower large intestine is usually the critical portion of the G.I. tract for three reasons: 1, Only radionuclides with a radioactive half-life greater than 1 hour were considered in the preparation of the ICRP Handbook. 2. The contaminates} material remains in the lowerlarge intestine 18 of the 31 hours that it is in the GL tract. 3. The mass of material in which the radionuclide is diluted and to whichtheradiation dose is delivered is relatively small, i. e., 150 g in the lowerlarge intestine, 250 g in the stomach, 1100 g in the small intestine, and 135 g in the upper largeintestine. t Handbook 52, “Maximum Permissible Amounts of Radivisntopes in the Homan Body ond Permissible C i in Alr and Water," by Natlonal Cammittec on Radiation Protechon, National Bureau of Standards, Washington, 13, C. (1953) 2TCRP Handbook, “Recommendations of the International Com- i}rivate Communication from BE. BE. Paehin, Director, Department uf Clinical Research, University College Hoxpital, Medieal School, British Institute of Radiology, London (1955). dutedOctober o, 1B5% mission on Radiological Protection,” 4. Jr. of Radiotogy, Supplement 6, University Street, London, W. C. 1., Great Britain, to K. Z. Morgan, 149