Lead author Dr Diana S.M. Buist, a senior investigator at Group Health Research Institute in Seattle, Washington, and colleagues, write about their findings in the journal Cancer Epidemiology, Biomarkers, & Prevention.
Buist said in a statement released on Tuesday:
"This is the first study to look over time at screening mammography use among individual women by their hormone therapy status linked with their breast cancer diagnoses."
In 2002, the Women's Health Initiative published the results of a randomized controlled trial that showed breast cancer rates were higher in women on HT comprising estrogen and progestin than in those taking either a placebo or estrogen-only HT.
Since then, there has been a rapid fall in new breast cancer rates, accompanied by reductions in use of HT and screening mammography.
Two explanations have been put forward. Some say the fall in HT use caused the fall in breast cancer rate, perhaps by making tumors regress.
Others say the fall in breast cancer rates coincides with fall in HT use because former HT users are less concerned about developing breast cancer, so they make less use of screening mammography and also are less inclined to go to their doctor, than women who have never used HT.
But the study, which set out to test the second idea, appears to refute it.
Before 2002, former HT users had lower rates of screening mammography than did current users.
"But we found that this is no longer true," said Buist. In fact, the study suggests former HT users have slightly higher or the same screening rates as current users.
For the study, which forms part of the national Breast Cancer Surveillance Consortium, Buist and colleagues examine data on 163,490 women who were members of the Group Health Cooperative nonprofit health care system based in Seattle.
The participants, who were aged between 50 and 79, and enrolled between 1994 and 2009, had no previous history of breast cancer. The total number of woman-years included in the study comes to 741,681, with a median span of data coverage of 3.3 years per woman.
From the records, the researchers were able to determine HT dispensing information, use of mammography screening, and incident of breast cancer diagnosis.
They then compared mammography use and cancer rates by HT use over time.
The researchers converted the mammography screening information into what they describe as an "age-adjusted screening compliance time-varying variable" which basically showed whether, within the last 26 months, the participants in an age group had undergone screening or not.
The HT information allowed them to examine the data according to whether participants were never users of HT, former users of HT, or current users of HT. They also split HT usage into ETP (estrogen plus progestin) and ET (estrogen only).
When they analyzed the data, Buist and colleagues found:
- Before 2002, screening compliance differed significantly by HT use.
- Current EPT users had the highest rate of screening compliance (83%), followed by former EPT (77%), current ET (77%), former ET (72%), and never HT users (56%).
- After 2002, screening compliance was high (about 81%) among current and former EPT and ET users, and significantly increased among never users (about 62%).
- Rates of invasive breast cancer fell significantly over the whole study period for all HT users, except EPT current users.
- Rates of ductal carcinoma in situ or DCIS did not change in any of the groups, including never HT users.
Invasive breast cancer means it has spread from the duct or lobule to other breast tissue. Non-invasive breast cancer is also called "in situ".
This study shows that while rates of the most common non-invasive breast cancer did not change, rates of invasive breast cancer fell significantly over the whole period of the study.
"We concluded that differences in rates of screening mammography don't explain the declines in rates of the incidence of invasive breast cancer among women who've stopped using hormone therapy."
With her co-authors, she suggests the results show quitting HT has an" immediate effect on breast cancer rates", supporting the notion that it leads to tumor regression.
They recommend that when studies examine the effect of a changing exposure in relation to outcomes, they "should account for varying exposures, individuals' characteristics, as well as screening methods and frequency".
Funds from the National Cancer Institute, which supports the Breast Cancer Surveillance Consortium, paid for the study.
Additional support, for cancer data collection, came in part from several state public health departments and cancer registries throughout the US.