Racial Disparities in Breast Cancer Mortality Are Not Driven by Estrogen Receptor Status Alone

Black women who are diagnosed with breast cancer have a higher probability of dying from the disease than white women, regardless of their estrogen receptor status, according to research from the National Cancer Institute (NCI). Differences in breast cancer mortality may reflect racial differences in access and response to innovative breast cancer treatments, as well as other biologic and nonbiologic factors, according to the report. In addition, the researchers found that differences in outcomes in the first few years postdiagnosis make up nearly all of the disparity. These results were published online July 7, 2009, in the Journal of the National Cancer Institute.

Recent years have seen an improvement in overall breast cancer mortality rates, but disparities remain between black and white women, with black women experiencing higher breast cancer mortality rates despite lower incidence rates. Researchers have been studying biologic, environmental, and socioeconomic factors, but the underlying cause of this disparity remains unclear.

Black women are more likely than white women to develop estrogen receptor–negative (ER–) breast cancers. Previously, it has been thought that the difference in the proportion of estrogen receptor–positive (ER+) to ER– tumors was a major contributor to the gap in mortality rates.

“We were surprised to discover that estrogen-receptor status only accounted for a portion of the disparity,” said Idan Menashe, PhD, lead author and research fellow in NCI’s Division of Cancer Epidemiology and Genetics. “Looking further at the data we were able to pinpoint that disparate outcomes between black and white women, particularly in the first few years following a breast cancer diagnosis, are the driving factor behind this racial gap.”

Study Details
The NCI researchers compared breast cancer rates for black and white women using data from the NCI’s Surveillance, Epidemiology and End Result (SEER) program. They found that, from 1990 to 2004, incidence rate ratios remained fairly stable while the breast cancer mortality rate ratios persistently increased.

Furthermore, the researchers observed that, regardless of ER status, black women with breast cancer were still more likely to die of the disease than white women. This disparity remained even when the researchers adjusted for age at diagnosis, stage and grade of the tumor, year of diagnosis, and socioeconomic status. When the researchers examined the hazard rate trends in black and white women, they noticed that the largest differences occurred in the first 3 years after diagnosis in both ER– and ER+ tumors.

The researchers also examined the incidence-based mortality of breast cancer among black and white women using a novel counterfactual (what if) approach to further explore this disparity. In this case, the researchers explored what the black-to-white mortality rate would have been if the proportion of ER+/– tumors were similar in both the black and white populations, as well as what the mortality rates would have been if the two groups had similar hazard rates.

This analysis confirmed that, despite the higher incidence of ER– tumors (which are associated with a less favorable prognosis) among black women, the differences in survival outcomes between the two populations were driven largely by differences in the hazard rate as opposed to differences in ER status.