Radiation protection practice requires the knowledge of estimated absorbed radiation doses to aid in the understanding of the potential detriment of various exposures. In nuclear medicine, the radiation doses to the internal organs of the subject are commonly calculated using the MIRD methods and equations. The absorbed dose to the embryo or fetus has long been an area of concern. The recent release of the pregnant female phantom series, and its incorporation into the MIRDOSE 3 computer software, has made possible the estimation of absorbed doses from radionuclides in the body to the fetus in early pregnancy and at 3, 6, and 9 mo gestation. A survey of several major medical institutions was made to determine the radiopharmaceuticals which might be given, whether intentionally or not, to women of childbearing years. Biokinetic data for these radiopharmaceuticals were gathered from various documents and other resources, and the absorbed doses to the embryo and fetus at these different stages of gestation from radiations originating within the mother's organs were estimated. In addition, information about activity distributed within the placenta and fetus was included where quantitative data were available. These absorbed dose estimates can be used to evaluate the risk associated with the use of different radiopharmaceuticals so that a more informed evaluation of the risks and benefits of the different procedures may be made. Further research is needed into the mechanisms and quantitative aspects of the placental transfer of many radiopharmaceuticals.
A geometric model that takes into account the changes in size, shape and position of the uterus during pregnancy has been designed for calculating the radiation dose to the uterus and the fetus from a radionuclide in the mother's bladder. The bladder is assumed to fill at a constant rate to 300 ml before voiding. The average dose in rads per photon emitted from a source in the bladder during one bladder filling has been calculated for monoenergetic photons of 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1 .O, 1.5, 2.0 and 4.0 MeV. Since the dose to specific organs of the fetus could not be calculated, the fetal space was divided into 12 compartments. The dose was calculated for each compartment, for the average of the entire fetal space, and for the uterus. The average dose per photon to each compartment decreases as the fetus increases in size. The highest dose rate to the uterine wall occurs at the point where it touches the bladder. Because the uterus enlarges, the point on the uterus receiving the highest dose rate changes during pregnancy.
The MIRD scheme is not restricted to calculating mean absorbed doses in organs but can be extended to any tissue for which distribution and retention data can be obtained and for which a reasonably accurate mathematical description of the source and target tissues can be determined. The development of more accurate absorbed dose estimates and the correlation of these estimates with radiation effects will lead to a better understanding of the results from radiotherapeutic agents such as radiolabeled monoclonal antibodies. Therefore, radiobiologists and internal dosimetrists need to combine their efforts and work toward the common goal of improving the treatment of malignant diseases.
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