What's New?
.	April Newsletter
.	April Journal

Upcoming Events
.	2010 Summer Professional Development School Internal Dosimetry 23 - 26 June 2010 Idaho State University, Pocatello, Idaho
.	International Conference on Radiation Protection in Medicine 1-3 September 2010 Varna, Bulgaria
.	2010 Meeting of the Air Monitoring Users Group 4-7 May 2010 Las Vegas, NV

19 March 2010

Answer to Question #5780 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Doses from medical sources

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

In calculating fetal dose from multislice CT scanners I have used the methods and normalized fetal dose ratios (NFDR) data described by Felmlee et al. (1990). I have also looked at the CTDI (CT dose index) reference dose model described by Louis Wagner at the 2007 American Association for Physicists in Medicine (AAPM) meeting which is roughly the same as given in the 2006 HPS summer school publication Medical Health Physics (pp 15-17). These methods account for primary radiation as well as scattered radiation from adjacent slices. In examples of the calculations the center (primary dose point) only consists of one slice.

My question is this: What if the fetus is viewed within several slices (e.g., a CT scan 250 mm length and 5 mm slices where the fetus is seen from slices 20-35)? Should the primary dose (also called reference dose) be multiplied by the number of slices from which the fetus receives primary radiation before adding the contribution from scatter from offset slices? When I apply this thinking to either method I get some rather large fetal doses for a pelvic CT (9-10 cGy as opposed to 3-4 cGy). Does the calculation account for the added primary radiation when the fetus sits within more than 1 slice off center? Can you explain that?

Reference
Felmlee JP, Gray JE, Leetzow, Price JC. Estimated fetal radiation dose from multislice CT studies. Am J Roentgenol; 154:185-190; 1990.

The answer is that you do not multiply by the number of slices. The normalized fetal dose ratio is used to determine dose at a point in the abdomen. Using the method as you described (without the added multiplication of additional fetal slices) renders the correct dose at that point, in your case about 3-4 cGy. If you wanted to calculate the dose to the neighboring slice, you would have to do the entire calculation again to get the dose at that point. This would be a nice academic exercise, but it is not necessary because the dose would differ only slightly from the first calculation.

The important point to remember is that absorbed dose is the concentration of energy in tissue, measured as the energy deposited per unit mass. If we assume a perfect world where we perform five contiguous slices over a fetus and each slice deposits energy E with the mass of the fetus in each slice being M, then the dose to the fetal slice is E/M for one slice. For five separate contiguous slices the deposited energy is 5E and the fetal mass for five slices is 5M. The dose to the fetus is 5E/5M, or still just E/M. If you multiply by the number of slices that contain the fetus you would be effectively saying that the deposited energy per mass in each slice is 5E/M, which is most assuredly incorrect.

Now the world isn't perfect and the dose in each slice isn't exactly E/M because the adjacent slices introduce scattered radiation that boosts the dose a little. So this has to be taken into account. When you calculate the dose for a particular slice, you are calculating the concentration of energy for that slice in the fetus. The methodology takes into account the direct radiation for the specific slice (the zero term or primary reference dose in the calculation series) and all the contributions of scattered radiation from adjacent slices (the sum of the additional terms in the series).

I once had a concerned call from an obstetrician many years ago. His patient was seen at one of my hospitals where she had undergone a CT scan of the pelvis while pregnant. He asked me how much dose the baby received for one CT slice. I told him it was about 20 mGy (2 rad in that era's terminology). His voice sounded very alarmed and he said, "Oh no! You've killed her baby!" I quickly admonished him for this conclusion and asked him why he would say a thing like that. He explained that if the dose was 2 rad per slice, the baby would have received about 130 rad because we did 65 slices. I then had to explain to him that his logic was completely invalid for the same reasons that I explained above. The dose in reality was only about 3 rad with the extra 1 rad due to scatter from adjacent slices. The baby was not harmed and the mother benefitted from her medical care because she was an emergency-room patient.

Louis K. Wagner, PhD

Answer posted on 27 September 2006. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of 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 specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.