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Answer to Question #342 Submitted to "Ask the Experts"Category: Doses and Dose Calculations — External dose calculations The following question was answered by an expert in the appropriate field: Q
I have not been able to locate DCFs (dose conversion factors) for the lens of the eye. I am wondering, since we have dose conversion factors for skin, would it be possible to craft a logical argument that says that the dose to the lens of the eye is always less than the skin dose? Or if this is not the case, how about an argument that says that the dose to the lens of the eye is xx percent greater than the skin dose? Or, will I have to analyze the particle spectrum and use energy cutoffs to determine the relationship between skin dose and lens dose? This is spent fuel that I am dealing with.
A
The term dose conversion factors (DCFs) generally refers to radionuclide-specific factors or coefficients derived for specific exposure geometries, for example, exposure to contaminated ground surface. We know of no compilation of coefficients specific to spent fuel. Presumably the skin dose coefficients noted in the question were derived with consideration of the spectrum of radiations emerging from the spent fuel and an appropriate exposure geometry. Publication 74 of the International Commission on Radiological Protection provides some insight into the relationship between the skin and lens of the eye absorbed dose for photons. Table A.18 (p. 176) of Publication 74 provides conversion coefficients (absorbed dose per unit air kerma) for the lens of the eye for monoenergetic photons incident on the body under various exposure geometries. Table A.14 (p. 172) provides corresponding coefficients for the skin. With the exception of the anterior-posterior (AP) geometry (photons are incident on the front of the body in a direction perpendicular to the body's long axis) the lens of the eye coefficients are comparable to the skin coefficients (that is, within about 10 percent in either direction for energies from 10 keV to 10 MeV). In the AP case, the skin coefficients are lower than the lens coefficients by about 30 percent. Publication 74 presents very limited electron data. Tables A.43 (p. 201) and A.44 (p. 202) give electron data for organ (including skin but not lens of the eye) and directional dose equivalent in the International Commission on Radiation Units and Measurements sphere, respectively. Since the lens of the eye is deeper than the 7 mg/cm2 depth assumed for the skin, one can be assured that the skin dose coefficient for electrons is a conservative estimator of the lens of the eye absorbed dose. Publication 74 provides no information on the lens of the eye dose from neutrons. It is noted that in practice the limitation on the effective dose provides adequate protection for the lens of the eye (p. 70). Assuming that the beta and neutron radiations can be neglected, then it is quite reasonable to take the skin dose as an estimator of the dose to the lens of the eye. Keith F. Eckerman, PhD Editor's Note: Publications of the ICRP can be ordered at http://www.icrp.org/products.asp.
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