Correspondence to: Francois Eisinger, MD, Paoli-Calmettes Institute, Institut National de la Santé et de la Recherche Médicale E9939, Université de la Méditerranée, Marseille, France (e-mail: eisinger{at}marseille.inserm.fr)
The availability of an effective (1) but potentially risky (2) chemopreventive intervention makes the issue of breast cancer risk assessment a critical step in medical counseling. Because x-ray mammography is a widely used medical tool (3), breast density is therefore a risk index that is both accessible and relevant. In the study by Ziv et al. (4), data on women in whom the assessments of breast density were inconsistent were excluded from the analysis. However, the large number of women excluded (2519 of 8665 [29%]) and the potential usefulness of density measurements led me to question whether the inconsistency is associated with the phenomenon (i.e., variability in breast density) or with the assessment of the phenomenon itself (i.e., mammography or reader variability).
Different conclusions can be drawn depending on whether the inconsistency is associated with mammographic variability (i.e., different machines and radiologists) or subject variability. On one hand, if the measure of assessment is the main reason for the inconsistency, then digital mammography or magnetic resonance imaging may reduce the number of unclassified women by increasing reproducibility. Another solution may be to assign a mean risk value to women who were assessed in two different groups at two different times. On the other hand, if subject variability is the main reason for the inconsistency, then there is a subgroup of women with real variability in breast density, and the predictive value of such a characteristic (breast density variability) on the occurrence of breast cancer should be assessed.
In distinguishing transient/borderline hypertension, it was important to determine that transient variability in blood pressure was a prognostic indicator (5). Similarly, the first step in establishing where the variability in breast density comes from is to establish the reality of the phenomenon, i.e., whether it is a stable borderline state near the threshold value or a state with true intrinsic variability.
REFERENCES
1 Fisher B, Costantino JP, Wickerham DL, Redmond CK, Kavanah M, Cronin WM, et al. Tamoxifen for prevention of breast cancer: report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. J Natl Cancer Inst 1998;90: 137188.
2 Cuzick J. First results from the International Breast Cancer Intervention Study (IBIS-I): a randomised prevention trial. Lancet 2002;360: 81724.[CrossRef][ISI][Medline]
3 Brown ML, Kessler LG, Rueter FG. Is the supply of mammography machines outstripping need and demand? An economic analysis. Ann Intern Med 1990;113: 54752.[ISI][Medline]
4 Ziv E, Shepherd J, Smith-Bindman R, Kerlikowske K. Mammographic breast density and family history of breast cancer. J Natl Cancer Inst 2003;95: 5568.
5 Fujii I, Ueda K, Omae T, Shikata T, Yanai T, Hasuo Y, et al. Natural history of borderline hypertension in the Hisayama community, Japan-I. The relative prognostic importance of transient variability in blood pressure. J Chronic Dis 1984;37: 895902.[ISI][Medline]
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