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Change in Breast Cancer Prevalence
Abstract & Commentary
By Leon Speroff, MD, Editor, Professor of Obstetrics and Gynecology Oregon Health and Science University, Portland, is Editor for OB/GYN Clinical Alert.
Synopsis: U.S. data indicate a decline in breast cancer incidence that parallels decreases in screening mammography and use of postmenopausal hormone therapy.
Source: Glass AG, et al. Breast cancer incidence, 1980-2006: combined roles of menopausal hormone therapy, screening mammography, and estrogen receptor status. J Natl Cancer Inst. 2007;99:1152-1161.
Glass and colleagues from the Kaiser Permanente Northwest Center for Health Research compared breast cancer incidence rates with use of screening mammography and postmenopausal hormone therapy prescriptions between 1980 and 2006.1 The data were derived from tumor registry statistics and from clinical, pathological, and pharmacy records. In the 1980s, age-adjusted breast cancer increased in incidence by 25%, and through 2001, another 15%. In 2003-2004, the overall incidence decreased by 18%, then increased slightly in 2005-2006. These changes were concentrated in postmenopausal women with estrogen-receptor-positive tumors. Mammography screening increased up to 1992, then leveled off. Hormone therapy prescriptions decreased by about 75% after 2002 in this Kaiser health plan. The authors concluded that the changes in breast cancer prevalence paralleled changes in both mammography screening and use of postmenopausal hormone therapy.
We now have 3 reports that have consistently indicated a decline in breast cancer incidence that began in 2003. An analysis of the U.S. national SEER data concluded that there was a 7% decrease in 9 regions of the U.S. in 2003, with the sharpest decline in estrogen-receptor-positive tumors in women ages 50-69.2 In the Northern California Kaiser program, there was a 10% decline in 2003 and 2004 in the Kaiser members and 11% in the area's population. The decline in this current report from the Portland, Oregon, area is even greater.
The authors of all three reports carefully highlighted the small, but important decrease in screening mammography that has occurred in the U.S. since the year 2000.3 This national change has been estimated to equal about a 4% decrease.4 This is attributed to a phenomenon called "saturation of the population," close to everyone who can get screening mammography is doing so. The critical question then is whether the decrease in breast cancer incidence reflects the change in mammography or the decline in use of hormone therapy. In my view, the answer is: both!
If postmenopausal hormone therapy is affecting pre-existing tumors, an argument I have made often in the OB/GYN Alert, then one would expect small undetectable tumors to stop changing (at least temporarily) when women discontinue hormone therapy, and thus be below the detection limit. This would be consistent with the effect being reported: a decrease in estrogen-receptor-positive tumors in younger postmenopausal women.
There are good reasons to suspect that a decrease in the use of hormone therapy is not the only explanation. The decrease in breast cancer incidence rates did not begin in 2003, but actually started in 1999 in all postmenopausal age groups.5 Between 2002 and 2003 there was a sharper decrease in estrogen-receptor-positive tumors in women age 50-69. The overall decrease may reflect the change in screening mammography and the 2002-2003 decrease is the response to discontinuation of hormone therapy. Appropriately for both of these influences, the decreases were noted only in small and localized tumors.
How much of the decrease is due to mammography and how much is due to hormone therapy? This question cannot be answered with any accuracy. Two observations give mammography an important role. In the national SEER data, the decline in breast cancer incidence was not only in women age 50-69, but also in women over age 70, a population that most likely had a much lower use of hormone therapy.2 In the United Kingdom where postmenopausal women discontinued hormone therapy at a rate similar to that of U.S. women, there has been no decrease in breast cancer incidence in any age group, most notably in women ages 50-64.6
The concern is that at least some results on breast cancer prevalence reflects the behavior of existing tumors, and does this mean that tumors will emerge later of greater stage and grade of disease with poorer outcomes? Only time will tell, but this may prove to be another harmful effect of the publicity associated with the Women's Health Initiative.