Answer to Question #4850 Submitted to "Ask the Experts"

Often I see that mammography doses are expressed as effective doses. In the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000) report it is noted (on page 301, http://www.unscear.org/docs/reports/annexd.pdf) that "Effective doses from mammography are included in the present analysis for completeness, although this quantity is not an appropriate indicator of risk for such exposures of female patients. Estimates of risk should be based on the mean dose to glandular tissue and age-specific risk factors."

Does this mean that one should not use the effective dose (e.g., 0.5 mSv; see p. 394 of UNSCEAR report) to calcuate excess cancer risk from radiation from mammography, but rather the glandular dose (e.g., US mean of 2.6 mGy)?

The effective dose concept was designed to estimate risk from exposures where dose was delivered to multiple organs, as in whole- or partial-body external exposures or from internalized radioactive nuclides. Where a single organ is irradiated, as in the case where only the breast is in the field of view, and where the scatter dose to organs other than the breast is much lower, it is not appropriate to use computed effective dose as the predictor of a health decrement which includes dose to organs other than the breast.

The question then becomes, What tissue specific-risk coefficient should one use to evaluate risk from the limited field exposure being considered? To estimate breast cancer risk to an otherwise normal woman, then the breast cancer risk coefficients from the atomic bomb survivors could be used, where the data are tabulated as risk per unit exposure as a function of age from total body exposures to a mixed gamma (predominantly) and neutron field (Land et al. 1980). In this case one needs to accept that Land et al. were able to take in account and remove confounding factors, including the effect of the small neutron dose in their analyses. Then one could use their age-specific risk factors. If one looks at the data from pooled analysis of eight cohorts, one of which is the atomic bomb survivors plus seven other populations with different reasons for their radiation exposure, one sees that there is a wide range of risk coefficients from which to choose (Preston et al. 2002). Alternatively, one could use risk coefficients used by groups such as the American Cancer Society in analyses of the risk to women from mammography.

A. Bertrand (Randy) Brill, PhD


References
Land CE, Boice JD, Shore RE. J. National Cancer Institute 65:353-376; 1980.

Preston DL, Mattson A, Holmberg E, et al. Radiation Research 158:220-235; 2002, and Errata Radiation Research 158:666; 2002.