A new study published in JAMA finds that among women in the US, the chance of being diagnosed with breast cancer in the early stages of the disease and the likelihood of surviving after such a diagnosis may be influenced by race and ethnicity, and this may be down to biological differences.

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According to the American Cancer Society, white women are more likely to develop breast cancer than African American women, although African American women are more likely to die from the disease.

According to the American Cancer Society, there will be around 231,840 new cases of invasive breast cancer - in which the cancer cells have spread beyond the breast ducts - diagnosed in the US this year and more than 40,000 deaths from the disease.

It is already known that breast cancer incidence in the US varies by race and ethnicity. Overall, white women are more likely to develop breast cancer than African American women, although African American women are more likely to die from the disease.

Among women under the age of 45, however, African Americans are more likely to develop breast cancer, while Asians, Hispanics and native American women are less likely to both develop and die from the condition.

Some past studies investigating the reasons behind variations in breast cancer incidence and survival between ethnicities say it may be explained by differences in exposure to breast screening and examination, as well as the likelihood of receiving the required care after a breast mass is found.

However, the researchers of this latest study - including Dr. Javaid Iqbal of Women's College Hospital in Toronto, Canada - say more and more research is pointing to biological factors as a potential explanation.

"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 explain.

Breast cancer data analyzed by eight race/ethnic groups

For their study, Dr. Iqbal and colleagues set out to identify the variation in early-stage breast cancer diagnosis (stage I) by race and ethnicity and determine whether biological differences in tumor aggressiveness play a role in these variations.

Fast facts about breast cancer
  • Breast cancer is the second leading cause of death among women in the US, although death rates from the disease have been declining since 1989
  • Around 2 in 3 invasive breast cancers are found in women over the age of 55
  • Around 5-10% of breast cancers are hereditary, primarily caused by mutations in the BRCA1 and BRCA2 genes.

Learn more about breast cancer

Using the Surveillance, Epidemiology and End Results (SEER) database, researchers assessed the data of 452,215 women who had been diagnosed with invasive breast cancer between 2004 and 2011. After excluding women with stage 0 or unknown breast cancer, the team were left with 373,563 women in their analysis, who they followed for an average of 40.6 months.

The researchers assessed the biological aggressiveness of small-sized tumors (2 cm or less) across all eight races and ethnicities identified: non-Hispanic white, Hispanic white, black, Chinese, Japanese, South Asian (Asian Indian, Asian Indian or Pakistani, Pakistani), other Asian (Filipino, Thai, Vietnamese, Korean, Kampuchean, Laotian, Hmong) and other ethnicities (including Native American and Alaska Native).

By race/ethnic group, the team also assessed the women's risk of being diagnosed with breast cancer in the early stages of the disease, their likelihood of being diagnosed at a later stage and their risk of death from the disease.

Japanese women more likely to be diagnosed with early-stage breast cancer

The results of the analysis revealed that Japanese women were much more likely to be diagnosed with breast cancer at stage I than non-Hispanic white women; 56.1% of Japanese women were diagnosed at stage I, compared with 50.8% of non-Hispanic white women.

Around 37% of black women and 40.4% of South Asian women were diagnosed with breast cancer at stage I, meaning they were more likely to be diagnosed with the disease at a later stage than non-Hispanic white women.

The risk of death from stage I breast cancer in the 7 years after diagnosis was higher among black women than both non-Hispanic white women and South Asian women, with 6.2%, 3% and 1.7% dying from the condition, respectively.

In addition, the researchers found that black women were much more likely to die from small-sized breast cancer tumors than non-Hispanic white women, at 9% compared with 4.6%.

They say the differences in diagnosis and survival of stage I breast cancer among race/ethnic groups may be explained by variations in biological variations in tumors, including lymph node metastasis, distant metastasis and triple-negative behavior (in which cancer growth is not driven by estrogen receptors, progesterone receptors and human epidermal growth factor receptor 2 (HER2).

The team explains:

"In support of this hypothesis, a black woman with small-sized breast cancer tumors was more likely to present with lymph node metastases, was more likely to have triple-negative cancer and was more likely to present with distant metastases than a non-Hispanic white woman with tumors of similar size."

The researchers note, however, that socioeconomic status, access to and use of health care, adherence to treatment and presence of additional health conditions may have also contributed to variations in diagnosis and survival of early-stage breast cancer.

The team notes several limitations to their study. For example, the SEER data used for the research only represents 28% of the US population, so the results may not be generalizable to all women of different race/ethnic groups in the US.

In addition, they point out that the women in the study may have been subject to different treatments and had various other illnesses, which may have influenced the results. Data on this information, however, was not available.

'Unprecedented opportunity' to provide high-quality care despite race, ethnicity

In an editorial linked to the study, Dr. Bobby Daly and Olufunmilayo I. Olopade, both of the University of Chicago, say that in order to effectively treat breast cancer going forward, there needs to be a better understanding of the drivers behind the variations of diagnosis and survival between race/ethnic groups.

They state, however, that there is an "unprecedented opportunity" to deliver high-quality breast cancer treatment regardless of a woman's race or ethnicity:

"Access to the use of genetic or molecular markers to guide choice of targeted therapy and reduce the costs of care can be made more equitable. For women with triple-negative disease, access to prompt diagnosis and initiation of chemotherapy can be lifesaving because these tumors metastasize early."

"Closing the survival gap will only occur once health care leaders initiate system changes that improve access to high-quality care along with a more comprehensive study of breast cancer biology through inclusion of a substantial number of minority patients in 'omics' research and in clinical trials," they add.

Medical News Today recently reported on a study published in Nature Communications, in which researchers from the UK revealed the discovery of the gene that drives triple-negative breast cancer.