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14 March 2010

Answer to Question #4836 Submitted to "Ask the Experts"

Category: Policy, Guidelines, and Regulations — Guidance Documents

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

Federal Guidance Report 13 was published in 1999, and yet I find relatively little awareness (or outright dislike) of its cancer risk coefficient approach towards estimating cancer risks. Would you consider these cancer risk coefficients to be just a refinement of dose conversion factors that account for cancer survival factors, or would you consider these risk coefficients to be a truly different method for cancer risk assessment? I'd appreciate knowing about any available information that describes the similarities and differences between the cancer risk coefficient approach compared to the dose-based approach, which relies upon using nominal probability coefficients.

Multiplying an effective dose by a dose-to-risk conversion factor is a quick technique for making "ballpark" estimates of risk. These conversion factors are based on either the adult worker or age-averaged population risk per sievert from uniform exposure to an external source of low linear energy transfer (LET) radiation. However, simple dose-to-risk conversions are not precise enough to satisfy the demands of a comprehensive radiation risk assessment. Instead, Federal Guidance Report No. 13 (FGR 13), "Cancer Risk Coefficients for Environmental Exposure to Radionuclides," is used by federal agencies and others to calculate the risk of excess cancer from ingestion, inhalation, and direct exposure to over 800 radionuclides. Risk coefficients are provided for estimating mortality (fatal cancers) or morbidity (cancer incidence).

FGR 13 risk coefficients represent the integration of age-, organ-, and gender-specific risk while accounting for population survivability (competing causes of mortality). The risk coefficients incorporate information from epidemiological studies, most notably the Life Span Study of the A-bomb survivors conducted by the Radiation Effects Research Foundation in Japan. They also are based on the latest biokinetic and dosimetric models available at the time of publication and other information necessary to account for the dependence of risks on factors such as type of radiation and dose rate. The methodology used to generate the FGR 13 risk coefficients is described in the document itself.

In 2002, the Interagency Steering Committee on Radiation Standards (ISCORS) published "A Method for Estimating Radiation Risk from Total Effective Dose Equivalent (TEDE)". This document provides the currently accepted dose-to-risk conversion factors for the general population with appropriate caveats for their use. It also gives more detail about why the risk-assessment methodology found in FGR 13 is used for determining compliance with risk-based regulations.

Michael A. Boyd, EPA

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