An analysis of recently available registry data including HER2 status for breast cancer patients confirms higher proportions of more aggressive breast cancer subtypes among younger women, black women, and Hispanic women, and notable differences in clinical presentation across subtypes, investigators reported in the Journal of the National Cancer Institute.
Although the reasons underlying these ethnic and age differences are not yet clear, the new data "are directly relevant to individualized treatment decisions that influence clinical outcomes," said Nadia Howlader of the National Cancer Institute, Bethesda, Md., and her colleagues.
They analyzed nationally representative data from 17 population-based Surveillance, Epidemiology, and End Results (SEER) registries, identifying 36,810 women diagnosed as having invasive breast cancer in 2010, the first year for which SEER data regarding tumor HER2 status were available. In addition to information on tumor receptor status, stage, and grade, the database includes information on patient age, race/ethnicity, and socioeconomic status.
Ms. Howlader, a mathematical statistician in the division of cancer control and population sciences, and her associates found that 72.7% of these breast cancers were HR-positive (expressing either estrogen receptors or progesterone receptors) and HER2-negative (not expressing human epidermal growth factor 2-neu); 12.2% were triple-negative (negative for estrogen, progesterone, and HER2 receptors); 10.3% were HR-positive and HER2-positive; and 4.6% were HR-negative but HER2-positive (J. Natl. Cancer Inst. 2014 April 28 [doi: 10.1093/jnci/dju055]).
The receptor status was unknown for 12% of the invasive breast cancer cases in the registries.
Compared with HR-positive and HER2-negative patients, those diagnosed with the other three subtypes were somewhat more likely to be younger, belong to minority racial or ethnic groups, live in counties with higher poverty levels, and have later-stage and higher Bloom-Richardson grade disease, the investigators reported.
Non-Hispanic black women had the highest incidence rates of triple-negative breast cancer across all age groups, with the difference in rates reaching its widest point at ages 60-64 and 65-69 years, when non-Hispanic black women were much more likely to be diagnosed with this subtype than were the other racial/ethnic groups. Triple-negative cancers were substantially more likely to be high-grade tumors (75% vs. 17%) and to present at an advanced stage than was the predominant HR-positive HER2-negative subtype.
The HER2-overexpressing tumors were less common subtypes with fewer observed variations by race/ethnicity, compared with both the HR-positive and HER2-negative, and triple-negative subtypes.
Compared with the predominant HR-positive and HER2-negative subtype, the proportion of women with the other three subtypes decreased with advancing age; these subtypes comprised 35% of case patients under age 50, but represented only 20% of case patients among those aged 75 years and older, the investigators reported.
"Understanding of the biological basis for differences in breast cancer subtype incidence and mortality across population groups is limited and warrants continued intensive study," the authors wrote. SEER data will continue to support this research, Ms. Howlader and her colleagues said.
This study was supported by the National Cancer Institute and the participating SEER registries. Ms. Howlader and her associates reported no financial conflicts of interest.