An area of particular concern in nuclear medicine is the pregnant or potentially pregnant patient. A 1997 document in the Health Physics Journal Russell JR and Stabin MG, Sparks RB and Watson EE. Radiation Absorbed Dose to the Embryo/Fetus from Radiopharmaceuticals. Health Phys 73(5):756-769, 1997) gave estimates of fetal dose from over 80 radiopharmaceuticals, based on some standardized kinetic models (not necessarily identical to those shown on this web site) and, in some cases, including knowledge of the amount of radiopharmaceutical crossover, as measured in animal or human studies. To see a copy of two summary tables from that document, including a few sample calculations, click here.

There are some special cases to be considered for the pregnant patient:

1) Fetal thyroid dose - if radioiodine is administered to a woman who has passed about 10-13 weeks of gestation, the fetal thyroid will have been formed, and this tiny organ concentrates the iodine which crosses the placenta. Evelyn Watson calculated doses to the fetal thyroid per unit activity adminstered to the mother (Watson EE. Radiation Absorbed Dose to the Human Fetal Thyroid. In: Fifth International Radiopharmaceutical Dosimetry Symposium. Oak Ridge, Tennessee: Oak Ridge Associated Universities, pp 179-187, 1992). Her results are presented here (the doses are in mGy to the fetal thyroid per MBq administred to the mother):

2) The hyperthyroid patient - fetal dose has not been well established for patients whose iodine kinetics differ from the standard model for I-131 NaI. In early pregnancy (when most of these exposures should occur, as the therapy will be clearly contraindicated in patients known to be pregnant), values from a 1991 Journal of Nuclear Medicine article (Stabin MG, Watson EE, Marcus CS and Salk RD, Radiation dosimetry for the adult female and fetus from iodine-131 administration in hyperthyroidism, J Nucl Med 32:808-813, 1991) should serve well.  Their estimates were (doses are in mGy/MBq administered):

"Fast" thryoid uptake meant an uptake half-time of 2.9 hours, "normal" meant a half-time of 6.1 hours.

3) The athyroid patient -  In thyroid cancer patients, I-131 NaI is often given to patients whose thyroids have been mostly removed surgically. There may be a remnant of thyroid tissue, and/or some thyroid cancer metastases around the body, but usually a large amount of activity is given (enough to destroy all remaining thyroid tissue and the mets). In a study involving a few athyroidic subjects (Rodriguez M. Development of a kinetic model and calculation of radiation dose estimates forsodium-iodide-131I in athyroid individuals. Master’s Project, Colorado State University, 1996.) it was found that the kinetics could be well characterized by treating the iodine not taken up by the thyroid by the normal kinetics of urinary bladder excretion (6.1 hour half-time). Using these assumptions, and assuming that the other normal soft tissue uptakes occur, and using Russell’s results for fetal residence times (here it seems reasonable to assume that the standard kinetic model for maternal-fetal exchange of iodine would be similar to the euthyroid case), we obtain the following dose estimates:

Again, the dose estimates in later pregnancy are not likely to be of interest very often, as this kind of therapy should not be carried out on a pregnant woman.

4) Post administration conception - An unusual kinetic picture sometimes arises when conception occurs after the iodine has been administered. In this case, the iodine has already started to wash out of the body, and whatever iodine is left will irradiate the embryo. This problem was studied recently (Sparks and Stabin. Fetal radiation dose estimates for I-131 sodium iodide in cases where accidental conception occurs after administration. Presented at the Sixth International Radiopharmaceutical Dosimetry Symposium held May 7-10, 1996 in Gatlinburg, TN; Oak Ridge Associated Unversities, 1999, A. Stelson, M. Stabin, R. Sparks eds, pp 500-512); the results can be viewed by clicking here.

The Breastfeeding Patient

Therapy Applications

In therapeutic situations, even though there are standardized models for radiopharmaceutical kinetics available, it is far more important that individual-specific kinetics and dose conversion factors be applied to as great an extent as possible. This is a very complex procedure, involving calibration of medical imaging equipment, design of biokinetic studies, gathering and reduction of data, selection of models from which to extract DCFs, modification of the DCFs for individual patients, and other considerations. The RADAR site has some general documents for download to help in this area, and RADAR members are also available for consulting on this subject.