At the San Antonio Breast Cancer Conference in December 2006 a major presentation linked the drop in new cases (incidence) of breast cancer to the decline in the use of postmenopausal hormone replacement therapy.
Many were skeptical that this was a true cause and effect, and some postulated that the decline was not due to a decrease in the number of new cancers, but a decrease in their detection due to fewer women having mammograms.
If women were less vigilant and not undergoing mammography than the number of new cases would decline - although probably only for a few years. Fewer mammograms would also mean the cancers would be detected at more advanced stages.
A new study adds evidence that the decline is due to a decrease in hormone therapy rather than a decrease in mammography - published in the Journal of the National Cancer Institute.
Some of the controversy was raised in a series of letters to the New England Journal of Medicine. They pointed out that the decrease should have been confined to small tumors, the change in hormone therapy rate of 38% was not matched by a 38% reduction, the drop may have started in 1999, the same results were not seen in Norway, and that the same drop in breast cancer incidence was not seen in African American women. These are difficult to explain, which underscores the difficulty of epidemiologic association and inference (cause and effect).
Dr. Kerlikowske and colleagues examined breast cancer rates between 1997 and 2003 by studying women who participated in seven different mammographic registries. This represented a total of 600,000 mammograms during the six years. During this time, there was a decline in hormone therapy use of 7% between 2000 and 2002 and then by 34% a year in 2002 and 2003 - after the data linking hormone therapy to breast cancer was made public.
It is reassuring that the use of mammography has not been dropping as had previously been discussed. It is also reassuring that the linkage between hormone replacement therapy and breast cancer was made public and led to a change in behavior that perceptibly reduced the risk of breast cancer.
While further epidemiologic research will investigate these phenomena more, with more publications and more debate, I hope that the causes of breast cancer will further be elucidated to prevent even more cases.
Most of the women with breast cancer (2/3) still have no identifiable risk factors for breast cancer, and the hormone therapy data can not explain the development of breast cancer in premenopausal women.
I see that Tommy Thompson, the candidate who pledged to cure breast cancer and double the NCI budget as part of his campaign for president, has dropped out of the race after a poor showing in the Iowa straw poll. Let us hope that the other candidates of both parties pick up his pledge and carry it forward.
