Biological factors account for most of the variations in breast cancer survival rates

Among nearly 375,000 US women with invasive breast cancer, the likelihood of diagnosis occurring at an early stage, and survival after stage I diagnosis, varied by race and ethnicity, with much of the difference accounted for by biological differences, according to a study in JAMA (2015; doi:10.1001/jama.2014.17322).

In the United States, incidence rates of breast cancer among women vary substantially by racial/ethnic group. Race/ethnicity and sociodemographic factors may influence a woman's adherence to recommendations for clinical breast examination, breast self-examination, or screening mammogram and the likelihood of her seeking appropriate care in the event that a breast mass is noticed.

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 and other or unknown pathologic features).

Javaid Iqbal, MD, of Women's College Hospital, Toronto, and colleagues examined the proportion of breast cancers that were identified at an early stage (stage I) in different racial/ethnic groups in the United States and whether ethnic differences may be better explained by early detection or by intrinsic biological differences in tumor aggressiveness.

The study included women with an invasive breast cancer diagnosis from 2004 to 2011 who were identified in the Surveillance, Epidemiology, and End Results (SEER) 18 registries database (n = 452,215). For each of eight racial/ethnic groups, biological aggressiveness (triple-negative cancers [negative for estrogen receptor, progesterone receptor, and HER2], lymph node metastases, and distant metastases) of small-sized tumors of 2.0 cm or less was estimated. In addition, the odds were determined for diagnosis at stage I compared with a later stage, as was the risk of death from stage I breast cancer by racial/ethnic group.

Of 373,563 women with invasive breast cancer, 268,675 (71.9%) were non-Hispanic white; 34,928 (9.4%), Hispanic white; 38,751 (10.4%), black; 25,211(6.7%), Asian; and 5,998 (1.6%), other ethnicities. Average follow-up time was 40.6 months.

The researchers found that diagnoses in Japanese women were significantly more likely to be stage I (56.1%) than non-Hispanic white women (50.8%), while diagnoses in black women were less likely to be at stage I (37.0%), as well as in women of South Asian (Asian Indian, Pakistani) ethnicity (40.4%).

The 7-year actuarial risk for death from stage I breast cancer was highest for black women (6.2%) compared with white women (3.0%); it was 1.7% for South Asian women. The probability of a woman dying due to small-sized breast cancer tumors (2.0 cm or less) was significantly higher for black women (9.0%) compared with non-Hispanic white women (4.6%).

The authors wrote that much of the difference in diagnosis and survival could be statistically accounted for by intrinsic biological differences such as lymph node metastasis, distant metastasis, and triple-negative behavior of tumors.

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