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Answer to Question #6241 Submitted to "Ask the Experts"Category: Medical and Dental Equipment/Shielding — Equipment The following question was answered by an expert in the appropriate field: Q
I read the recently published National Council on Radiation Protection and Measurements (NCRP) Report No. 145
on "Radiation Protection in Dentistry" and found that it provides a
formula to calculate scattered radiation and scatter fraction from a
patient. In the formula for calculating the scatter fraction, the
scattered angle "theta" is required. However, it didn't tell the unit
of it. Is it in degrees or radian? Moreover, if I put it as zero
degrees, then only the first term dominates and if the operating
potential is less than 125 kVp, then the scatter fraction becomes a
negative value. It seems odd to me. Did I perform something wrong?
Besides, as a rule of thumb, the scattered radiation from patient shall
be 0.1 percent of the patient dose. Where does this come from?
A
I would like to refer you to an article in Health Physics, "Secondary
Shielding Barriers for Diagnostic X-Ray Facilities: Scatter and Leakage
Revisited" by Simpkin and Dixon (Health Phys 74(3):350-365, 1998).
Formula F.12 of Report 145 is taken directly from that article. The
unit of theta is in degrees. I doubt if the formula is valid at zero
degrees, since it is an empirical fit to measured data, and small angle
data is very difficult to measure. In any practical shielding
calculation the primary barrier should subtend up to 30 degrees of
scattered radiation. Note that the formula is scaled by the beam area;
hence it does not directly calculate the dimensionless fraction of
scattered radiation. The "0.1% Rule" is a conservative estimate (that
is, it is an over-estimate) of 90-degree scatter for field sizes
typical of diagnostic radiology (see Table 2 of the above referenced
article).
Answer posted on 23 March 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.
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