Recent studies suggest that the use of combined estrogen and progestin hormone replacement therapy is associated with an increased risk of invasive lobular carcinoma (ILC), but that it has little association with risk of invasive ductal carcinoma (IDC). Also, the incidence rates of ILC have risen over the past 10 years while those of IDC have remained constant. Differences in survival rates by histologic types of tumor have been reported, but few of the published studies were population based or had adequate power to address this issue.
We conducted a retrospective cohort study spanning the years 1974 through 1998 using data from the 9 cancer registries that have participated in the Surveillance, Epidemiology, and End Results Program since 1974. The cohort consisted of 164 958 women aged 50 to 79 years who had been diagnosed as having 1 of 7 histologic types of invasive breast cancer. Risks of mortality due to any cause were estimated using the Cox proportional hazards model.
Women with ILC had a risk of mortality 11% lower than women with IDC. The magnitude of this difference has increased over the past 10 years and, from 1994 through 1998, the risk of mortality was 26% lower for women with ILC. Also, the risk of mortality was between 8% and 34% lower in women with mucinous carcinoma, comedocarcinoma, or medullary, tubular, and papillary carcinomas compared with women with IDC.
Differences in prognosis by histologic type of breast cancer were identified. The survival rate of women 50 to 79 years old who have ILC, the cancer whose histologic type is the most closely linked with the use of combined estrogen and progestin hormone replacement therapy, is more favorable than that of women with IDC and appears to be improving over time.