Answer to Question #10868 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q

How can I calculate the total-body dose from iodine-131 (131I) given as a hyperthyroidism treatment?

For example, a patient with a 35% 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?

A

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

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 11 December 2013. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.