Answer to Question #846 Submitted to "Ask the Experts"

Q

How can you determine the total radiation dose that a person would get from ingesting a 100, 150, or 200 millicurie dose of radioactive ¹³¹I for a follow-up treatment for thyroid cancer?

A

The radiation dose associated with the quantities of ¹³¹I you mention, between 100 and 200 millicuries (mCi), varies from patient to patient. Variations in radiation dose are due to differences in uptake and bio-distribution of ¹³¹I in various organs and tissues from person to person, differences in body size, how quickly the radioiodine is cleared by the kidneys, and other factors. Dosages in the 100-200 mCi range are given for therapeutic purposes to patients with thyroid cancer and who have had most of their thyroid surgically removed or previously ablated with ¹³¹I.

In my experience, physicians may administer a much smaller dose of ¹³¹I, between 1 and 10 mCi, as an "uptake dose," to detect the presence of residual or recurrent thyroid cancer. Then, a therapuetic dose is chosen that maximizes therapeutic benefit and minimizes side effects. The actual radiation dose varies from tissue to tissue and is also dependent, to some degree, on where metastases are located. Metastatic tumor tissues may receive thousands of rads while normal tissues may receive tens to hundreds of rads. Useful information about ¹³¹I dosimetry appears in the International Commission on Radiological Protection (ICRP) Publication 53 (ICRP 1988), which was used to obtain the estimates of the radiation doses noted above. The reader is cautioned to view these estimates as just that—estimates. Iodine-131 was one of the first radionuclides used for diagnostic and therapeutic purposes. Its continued use for about one-half century is a testament to its efficacy; further, some patients receive several treatments over a period of years, to their obvious benefit. The use of ¹³¹I for therapeutic purposes is a good example of mixing the art of medicine with the science of dosimetry: the amount to be administered is based on past observations of efficacy, balancing risks against therapeutic benefit. Questions about the risks associated with administration of radioiodine should be discussed with your physician.

Roger W. Broseus, PhD
Certified Health Physicist

Reference
International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. New York: Pergamon Press; ICRP Publication 53; 1988.