More than 10 years after diagnosis, women who had the most common and least aggressive subtype of breast cancer were still at risk of death from the disease, according to a Kaiser Permanente study.

Breast cancer tumors are often divided into four molecular subtypes. Luminal A tends to have the best prognosis and has fairly high survival rates. Luminal B typically occurs in younger women and has a poor prognosis. The basal-type subtype also tends to occur in younger women and in African American women, and it has a poor prognosis. The HER2-enriched subtype has a fairly poor prognosis and is prone to early and frequent recurrence and metastases. Of these four subtypes, luminal A is the most common and accounts for 42% to 59% of all breast cancer cases.

The 21-year study included nearly 1,000 women from Kaiser Permanente Southern California. The researchers found that molecular subtypes of breast cancer are important predictors of breast cancer mortality. Although luminal A tumors are generally thought to have the best prognosis, the study found that women with luminal A tumors were still at risk for death from the cancer more than 10 years after diagnosis. The researchers also found a roughly two-fold increased risk of death from breast cancer for women with HER2-enriched and luminal B tumors compared with luminal A tumors, which agrees with previous studies.

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“The findings of this study indicate that it is important to consider breast cancer molecular subtypes in determining the optimal treatment for women with breast cancer,” said study lead author Reina Haque, PhD, MPH, from Kaiser Permanente Southern California’s Department of Research & Evaluation. “Women with luminal A tumors—the least aggressive but most common cancerous breast tumor—could benefit from extended treatment to improve their chances for long-term survival.”

The authors of the study have suggested that future breast cancer studies should focus on identifying the factors that are associated with longer survival in women with luminal A tumors. Future studies should also seek to understand how the association between breast cancer and survival varies by race and ethnicity, particularly in minority women who are likely to have aggressive tumor subtypes. The study was published in Cancer Epidemiology, Biomarkers & Prevention.