Answer to Question #10752 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine

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

Q


I am a 36-year-old female, weight 56 kg. I have recently had a nuclear stress test done with Myoview followed by a calcium score test. I was administered 500 MBq for the stress part of the test, and the rest part was cancelled as the stress part was normal. I was told that the effective dose I received during both the stress test and the calcium score was approximately 5 mSv (4+1). Two years ago I also had a thyroid scintigraphy using 19 MBq of 123I (an injection). This was followed by an abdominal CT (computerized tomography) on the same SPECT CT (SymbiaT2; Siemens) machine. I am not sure if the CT part was low dose or not. It was used to exclude struma ovarii, after the nuclear test showed homogeneous thyroid uptake and normal uptake by my kidneys/bladder. I was hyperthyroid during the test. My questions are:

  1. What dose did I receive during my thyroid test and abdominal CT?

  2. Do the tests I have had so far increase my chances of having thyroid cancer? I lived in Minsk, Belarus, during the Chernobyl accident and was 10 years old then, so thyroid cancer is something everyone in Belarus is very aware of. I have a thyroid ultrasound annually and no nodules have been found so far. However, I am really concerned that that extra dose of radioactive iodine was really dangerous for my thyroid.

  3. There is a breast cancer history in my family. Do the nuclear tests and the CT scans increase my chances of getting breast cancer? What is the uptake of Myoview and 123I by breast tissue? I have also had two mammograms and four or five chest x rays in my lifetime.

A

Following are my dose estimates for the various scans you have had:

Myoview Stress Test—effective dose 4.3 mSv (breast dose 1.1 mGy)
Cardiac CT Calcium Scoring—effective dose 3 mSv (breast dose 22 mGy)
123I Thyroid Scan—effective dose 2.3 mSv (thyroid dose 65 mGy) (breast dose 0.06 mGy)
SPECT/CT Abdomen (attenuation correction)—effective dose 2.4 mSv (breast dose negligible)
Mammogram (each view)—effective dose 0.13 mSv (breast dose 1.5 mGy)
Chest X Ray (PA and lateral combined)—effective dose (0.06 mSv) (breast dose 0.04 mGy)

Consequently, assuming you have had an additional two mammograms and five chest x rays, your dose due to these medical procedures would be a total effective dose of 12.5 mSv, a breast dose of 26.4 mGy and a thyroid dose of 65 mGy.

To put this into perspective, on average people receive an effective dose of 3 mSv per year from natural background sources. The thyroid and breast receive a dose of approximately 1 mGy per year from the natural background. Your cumulative dose from background sources of radiation after 36 years is an effective dose of 108 mSv and a thyroid and breast dose of 36 mGy. So your effective dose from these medical procedures is about 12 percent of your current natural dose, your breast dose is about 73 percent of your natural dose, and your thyroid dose is about 180 percent of your natural thyroid dose.

Although this might sound like a lot, these are relatively low doses and do not significantly alter your risk of cancer over your lifetime. In the atomic bomb survivor data, we begin to see a statistically significant excess cancer risk with effective doses greater than 1,000 mSv.

Although your family may have a history of breast cancer, the additional radiation doses received do not contribute significantly to your increased risk. Similarly, the isotope used in thyroid diagnosis is 123I, not 131I. Iodine-131 is the isotope of concern for thyroids as a result of the Chernobyl accident. Iodine-131 deposits much more dose to the thyroid due to the fact it is primarily a beta emitter and has a longer half-life of eight days, while 123I has a half-life of only 13.3 hours, with gamma rays as its primary emission. The thyroidal dose from the same activity of 131I as you received for your thyroid scan would have delivered 100 times more dose to your thyroid.

We need to keep the benefit-to-risk ratio in mind when considering medical radiation exposures. In medicine, imaging scans provide essential information in diagnosing medical issues in real time that provide immediate benefits, in exchange for a very small theoretical increased risk in the future. In the vast majority of cases, this benefit-to-risk ratio strongly points to the performance of the medical procedure.

Here are a number of references that may offer similar help to put your situation into perspective:

Benefits of Medical Radiation Exposures (Health Physics Society)
Radiation Risk in Perspective (Health Physics Society)
Medical Exposure from Medical Exams and Procedures (Health Physics Society)

Mike Bohan
Radiation Safety Officer

Answer posted on 16 July 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.