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29 August 2008

Answer to Question #6437 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

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

With reference to your answer to question #4362, you discuss the role of scatter in the effectiveness of shielding in the area of the gonads. What proportion of the initial radiation exposure, or of the combined exposure, does scatter represent? In this particular case, the shielding was provided by a lead-lined glove placed on top of a light blanket over the area of the gonads while the lower intestine was x rayed in a seven year old boy. Would scatter have represented a significant part of the total radiation exposure in this case? What effect could the scatter radiation have had on the testes or penis in this case?

The radiation exposure from scattered radiation depends upon several factors, including:

scatter angle,
kVp,
anode material,
filtration,
field size impinging upon the patient,
size and shape of the patient,
distance from the tube focal spot, and
distance from the exposed area on the patient to the point of interest.

It is, therefore, impossible to accurately determine the gonadal dose to this child. Having said that, the radiation exposure due to scatter, Ks, can be approximated with the following equation:

equation

where

K_p   =   the exposure from the primary beam at 1 meter from the focal spot

a₁   =   the scatter fraction which ranges from 3.7 x 10^-6 to 7.6 x 10^-6 from a typical radiographic unit (e.g., tungsten anode, aluminum filtration) depending upon the tube potential (50 - 150 kVp) and on the scatter angles (20-135 degrees).

F   =   field size in cm2 at the image receptor

d_f   =   source to image receptor distance in meters

d_s   =   scattered radiation distance (from midpoint in the patient) in meters

For an example calculation, let

a₁   =   4.75 x 10-6 (70 kVp, 90° scatter)

F   =   400 cm²

d_f   =   1 meter

d_s   =   0.2 meter (~8 inches)

Further, there is a matter of geometry. In the simple model shown here, the lead shield does not intercept the scattered radiation. (One can, of course draw models where the shield intercepts the scattered radiation.) This shielding will, however, intercept leakage radiation (from the x-ray tube protective housing). The Food and Drug Administration limits on leakage radiation are 100 millirem per hour at 1 meter with the x-ray unit operating at its maximum kVp and maximum continuous tube current (mA). Assuming a technique of 70 kVp, 20 mAs, the dose from leakage at 1 meter would be less than 0.04 millirem per film.

There would be no effects to the testes and penis of a seven-year-old child from scattered radiation in the case you describe. The exposure would not cause sterility, affect hormonal development, or have any genetic effects on the eventual offspring from this child.

Kent N. Lambert, CHP

Reference:
National Council on Radiation Protection and Measurements. Structural shielding design for medical x-ray imaging facilities. Bethesda, MD: NCRP; NCRP Report 147; 2004.

Answer posted on 26 June 2007. 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.