Race and ethnicity play a role in both early diagnosis of invasive breast cancer and survival rates after diagnosis, according to a study published online Jan. 13 in the Journal of the American Medical Association.

Researchers, including Javaid Iqbal, MD, with the Women’s College Research Institute in Toronto, sought to explore whether the variance in incidence rates of breast cancer among different racial/ethnic groups were better statistically accounted for by early detection of by biological differences in tumor aggressiveness.

“A growing body of evidence suggests that biological factors may also be important in determining stage at diagnosis (ie, the growth rate and metastatic potential of small-sized breast cancer tumors may vary between women due to inherent differences in grade, receptor status, and other or unknown pathological features),” the authors wrote.

The researchers devised an observational study of women diagnosed with invasive breast cancer who were identified in the Surveillance, Epidemiology, and End Results (SEER) registries database between 2004 and 2011. For the study, the researchers grouped all 373,563 selected breast cancer patients into the following racial/ethnic groups:

• Non-Hispanic white
• Hispanic white
• Black
• Chinese
• Japanese
• South Asian
• Other Asian
• Other ethnicities

For each ethnic group, the authors studied biological aggressiveness (triple-negative cancers, lymph node metastases and distant metastases) of small-sized tumors that were 2 cm or less. Additionally, the team computed the odds ratio (OR) for being diagnosed in an early stage compared with later stages as well as the hazard ration (HR) for death from stage 1 breast cancer by racial/ethnic groups.

Results from the study found that the majority of women with invasive breast cancer were non-Hispanic white (71.9 percent). Hispanic white patients comprised of 9.4 percent of all cases, while black patients comprised 10.4 percent, Asian 6.7 percent and the remaining ethnicities totaled 1.6 percent of the cohort.

Iqbal and colleagues found that Japanese women were significantly more likely to be diagnosed at stage 1 (56.1 percent) than non-Hispanic white women (50.8 percent) and black women were significantly less likely (37 percent) to be diagnosed at stage I than non-Hispanic white women.

“It is important to identify factors associated with the diagnosis of stage I breast cancers and the groups for whom the proportion of cancers detected at stage I is less than optimal,” the authors wrote. “We found that probability of small-sized breast cancer tumors having spread to the regional lymph nodes or distantly varied between women with different racial/ethnic backgrounds and may reflect variations in the intrinsic biology of their tumors.”

They found that in all age groups, black women were the least likely to be diagnosed early in the stages of invasive breast cancer.

“This observation suggests that the stage disparity at diagnosis is not likely to be attributed to screening trends; rather, the paucity of stage I cancers appears to be explainable in large part by inherent biological factors,” the researchers wrote.

Among the different Asian subcategories, Japanese women had the highest number of stage I breast cancers and studies have suggested that Japanese women develop less aggressive  breast cancers and exhibit more vigorous host response—meaning fewer tumors are diagnosed at 2 cm or larger and with fewer lymph node metastases.

“In our study, survival was associated with biological differences in tumor characteristics (eg, between black women and women of other ethnicities) but factors such as socioeconomic status, access to and use of health care, adherence to treatment, and comorbidity might also contribute to breast cancer disparities,” Iqbal and colleagues concluded.