According to a presentation at the San Antonio Breast Cancer Symposium last week, there was apparently a 7% drop in the incidence of breast cancer in the U.S. in 2003. While some observers attributed the drop to a decline in hormone replacement therapy (HRT), an entirely different phenomenon may be at work.
As reported on AuntMinnie.com on December 15, researchers from the University of Texas M. D. Anderson Cancer Center in Houston attributed the decline in breast cancers to the fact that "millions of women" had stopped using HRT. They based their view on the finding from the 2002 Women's Health Initiative (WHI) study that HRT use resulted in a slight increase in breast cancer risk.
In the M. D. Anderson study, the largest decrease in breast cancer incidence was reported among women ages 50-69 (12%) for hormone receptor-positive tumors. In the WHI study, the actual increased risk of developing cancer from HRT was very small. Imbibing alcohol actually carries a higher risk, so it would be surprising if this were the actual cause for the decrease in breast cancers in the later study.
If, however, the cessation of HRT is the reason for the drop in incidence, then this would support the contention that, if there is any increased risk from HRT, it is as a growth supporter (promoter) and not as a cause (initiator) of breast cancer. In fact, it is only speculation that the decrease in breast cancer incidence is related to the cessation of HRT. There is, in fact, no direct evidence to prove that contention.
There is an alternative explanation for the decrease in breast cancers being diagnosed in the U.S. Since the 1950s, based on the Connecticut Tumor Registry and Surveillance, Epidemiology, and End Results (SEER) data, there has been a steady increase in the incidence of breast cancer that remains unexplained.
The middle of the 1980s saw a sudden increase in breast cancer diagnoses that was over and above the background steady increase. At the time, there was great concern that something had happened and that breast cancer had suddenly become an epidemic. Some argued that mammograms were causing an increase in breast cancer.
However, more careful analysis demonstrated that the sudden increase was not due to more cancers being caused, but rather to cancers being detected earlier. The sudden increase marked the start of mammography screening in the U.S. in sufficient numbers that it affected national statistics.
If, for example, cancer can be detected by clinical examination when it reaches 2 cm in size, and assuming one cancer is diagnosed each year in a population of women, there have to be many more smaller cancers undetected "beneath the surface," so that another cancer will reach 2 cm in size and come to the surface the next year, and another the year after, and so on.
When screening begins, the first mammography screen detects the cancers that would be found that year, but also finds some of the cancers below the surface that, without screening, would not have been diagnosed until next year or the year after.
Thus, in a population of women, more cancers are found the first year they are screened than would be found each year without screening, accounting for a sudden jump in incidence. These additional cancers are found at a smaller size and earlier stage, and that is why screening decreases the death rate.
As long as more and more women enter into the screening program each year, the increased number of cancers persists, but once the number of women stabilizes, a new "steady state" develops that is the same as before screening began (e.g., one cancer each year), but the cancers are now found at a smaller size. This would return the total numbers of cancer to the baseline rate that was present before screening began, but with cancers being found at a smaller size.
The SEER data actually show that the spike in incidence actually began to decline in the late 1990s before the publication of the Women's Health Initiative results. The decrease that was reported with the M. D. Anderson data may merely indicate that we are reaching a new "steady state," with all women who are going to be screened being screened. Data from the next few years will provide a more accurate picture.
By Dr. Daniel B. Kopans
AuntMinnie.com contributing writer
December 22, 2006
Dr. Kopans is a professor of radiology at Harvard Medical School and the director of the breast imaging division at Massachusetts General Hospital, both in Boston.
The opinions expressed in guest editorials are those of the author, and do not necessarily reflect the views of AuntMinnie.com.
Related Reading
Drop in breast cancer tied to less HRT, December 15, 2006
RSNA studies delve deeper into DMIST results, December 14, 2006
Regular mammo screening reduces risk of false-positive results, needless biopsy, August 9, 2006
Sidestepping screening: What factors make women avoid annual mammography? October 10, 2005
Benefit of breast cancer screening small - study, July 20, 2005
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