The recent drop in breast cancer cases started in the late 1990s, before the plunge in hormone use that has been credited with the decline, researchers said.
The drop in cancer rates dovetails with a leveling off in the number of mammograms done on women age 40 and older since 1999, the study from the American Cancer Society found. The fall accelerated from 2002 to 2003 after another study found hormone replacement therapy could lead to heart disease and cancer, leading millions to abandon the pills.
Both the drop in mammograms, which detect breast cancer, and the use of hormones, which may fuel tumor growth, are probably responsible for the decrease in new cases, said Ahmedin Jemal, strategic director for cancer surveillance at the cancer group.
What's not clear is whether there are fewer tumors or smaller growths are simply going undetected, he said.
The study appears in the journal Breast Cancer Research.
Researchers from M.D. Anderson Cancer Center in Houston reported in December that breast cancer rates fell substantially in 2003, and theorized that the drop stemmed from reduced use of hormones sold by Wyeth, the industry leader. Prescriptions for Wyeth's Premparin and Prempro, which once generated $2 billion annually, plunged 74 percent from 2000 to 2006, sales data show.
Mammogram rates started leveling off even sooner. In 1999, about 70.3 percent of US women had a mammogram within the previous two years, according to data from the National Center for Health Statistics. That fell to 69.5 percent in 2003, the last year for which data are available.
Several things point to the drop in mammograms to explain the cancer decline, the study found. The decreases seen among all women age 45 and older were greatest for small tumors typically detected with screening, and coincided with the plateau in mammography use.
The number of small tumors fell by 4.1 percent a year from 2000 through 2003.