Answer to Question #10868 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
How can I calculate the total-body dose from iodine-131 (131I) given as a hyperthyroidism treatment?
For example, a patient with a 35 percent uptake was treated in my hospital with 17 mCi. With the data found on the RADAR website, I find 88.8 rem mCi-1 for the effective dose per unit activity. That gives a dose of 15 Sv. That’s more than the lethal dose so it can’t be right (the patient is still alive).
For the thyroid dose, I get 314,500 mSv. Assuming the gland is the only organ irradiated and multiplying that number by the weighted factor of 0.04 (ICRP 2007) I get 12.6 Sv for the effective dose. That is still over the lethal dose.
What am I doing wrong?
The problem is that effective dose and effective dose equivalent are not applicable to high-dose and high-dose rate situations such as nuclear medicine therapy. That is why the table in the spreadsheet that you reference above lists N/A for Not Applicable in the columns labeled Effective Dose or Effective Dose Equivalent.
As you state, 131I delivers most of its radiation dose in the thyroid. Low-dose therapeutic treatments for hyperthyroidism have not been shown to induce thyroid cancer, even with the high delivered doses. Also, the low-dose therapy is not low compared to diagnostic doses.
Marcia Hartman, MS