A sharp drop in breast cancer cases in 2003 has many researchers pointing to the fact that millions of women quit hormone replacement therapy in 2002.
But others have doubts that quitting HRT could alone produce such a steep drop.
The 7 percent drop in breast cancer cases between 2002 and 2003 means about 14,000 fewer women in the United States were diagnosed with the disease. Most of these women were between 50 and 69 years old.
"It's very, very compelling that this is not random variability, that there is something very clear and dramatic that happened," said Dr. Donald Berry, professor and chairman of biostatistics at the M.D. Anderson Cancer Center, during an interview with ABC News correspondent John McKenzie.
McKenzie also talked to Dr. Eric Winer of the Dana Farber Cancer Institute, who said, "Any downward trend would be important. But this drop, and a drop this size in a couple of years, is really very major news."
The drop is significant in that it could be the single largest year-on-year reduction in new breast cancer cases ever recorded.
"It is biologically plausible, and there is no other glaring change in public health to explain the change," said Dr. Clifford Hudis, chief of the Breast Cancer Medicine Service at Memorial Sloan-Kettering Cancer Center in New York. "This is more evidence that HRT is risky in terms of breast cancer."
Some experts, however, said the findings overlook the possible role that other factors might have played in the decrease.
"At this point, it's still an intriguing and promising observation, but very early," said Michael Thun, vice president of epidemiology and surveillance research of the American Cancer Society. He said a leveling off of the number of women getting mammograms could be responsible, since fewer women getting screened means fewer cancers getting detected.
This, he said, "creates the false impression that incidence rates are falling, whereas what is actually happening is that many early tumors are not being detected."
In short, he said, the decrease is just a detection gap; the same number of women may have breast cancer, but their tumors just aren't being detected.
"This is the most likely factor in the more gradual decrease in the last five years," Thun said.
Others said the rapid drop in incidence needs to be studied further before the cessation of HRT is confirmed as the primary cause.
"The suggested relationship with reduction in breast cancer incidence is too premature to trust with any confidence," said Dr. Gary Lyman, professor of medicine and oncology at the University of Rochester Medical Center. "There are many possible explanations for this apparent drop in incidence, including chance."
If it is true that stopping HRT alone leads to such a dramatic drop, "it sends a message to women who are postmenopausal to think long and hard before opting to take HRT for control of menopausal symptoms," said Lillie Shockney, administrative director of the Johns Hopkins Avon Foundation Breast Center.
"Because this is an indirect study, they can't establish the reason," said Dr. Lisa Carey, medical director of the UNC Breast Center at the University of North Carolina-Lineberger Comprehensive Cancer Center.