Answer to Question #10528 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

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

My 11-year-old daughter received a bone scan followed by a skeletal survey, CT (computerized tomography), and CT-guided biopsy. I was given the following information about the radiation doses she received:

Skeletal survey—0.8 mSv
CT lumbar sacral spine—0.7 mSv
CT biopsy—3.1 mSv
Bone scan—5.3 mSv

Total Effective Dose—9.9 mSv

I read a lot about risk from CT scans, but not bone scans. What can I understand from the above calculation? Is CT more dangerous because it is done to one part of the body and not the whole body? Is it right to say that her spine received a radiation dose of 0.7 + 3.1 + something else from skeletal survey and bone scan and it is around 4 mSv? And what does it mean? How much radiation is dangerous for an 11-year-old girl? 

Also, if she drank a lot of water in the first 24 hours after the bone scan, would it reduce her radiation dose? 
Effective dose takes into account that only a part of the body receives the radiation dose and that different parts of the body have different sensitivities to radiation. This allows an "apples to apples" comparison. So a 4 mSv dose from a bone scan and a 4 mSv dose from a CT scan carry the same risk. It also allows you to add the doses together. 

From the Health Physics Society’s position statement on radiation risk, for radiation exposures "below 50–100 mSv, risks of health effects are either too small to be observed or are nonexistent." The approximately 10 mSv received by your daughter is well below this. 

Regarding drinking lots of fluids during the first 24-hour period after the bone scan—it reduces the dose by helping to flush out the radioactive drug. Your daughter should be encouraged to drink lots of water and void her bladder frequently. Instructions typically also include drinking plenty of fluids in the several hours between the time the radioactive drug is administered and the time that the scan takes place. 

It is also important to remember that your daughter benefited from these procedures and the benefits no doubt significantly outweighed the risks.

Kent Lambert, CHP
Answer posted on 30 January 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.