African-American women have a lower incidence of breast cancer compared with white women, but have a higher rate of breast cancer mortality. A prospective study by Rowan T. Chlebowski and colleagues finds that this is most likely explained by the aggressive nature of these cancers in a sizable proportion of African-American women.

The authors wanted to address the limited attention within the literature given to differences in the distribution of breast cancer risk factors in ethnic minorities. To this end, they analysed data from 156,570 postmenopausal women participating in the Women's Health Initiative (WHI).

The authors' calculations included established risk factors such as current age, age at menarche, previous breast biopsies, age at first live birth and family history of breast cancer in first-degree relatives — factors that are all incorporated in the often-used Gail model of breast cancer risk. Other presumptive risk factors such as income level, educational level, alcohol intake and body-mass index (BMI) were also taken into account.

After a median follow up of 6.3 years, 3,938 breast cancers were diagnosed. Age-adjusted incidences for all minority groups were lower than those in white women. However, adjustment for breast cancer risk factors accounted for all these differences except for the lower incidence and higher mortality evident in African Americans. The authors note that the tumours in this ethnic group were more often associated with a poor prognosis — they were high grade (poorly differentiated) and oestrogen-receptor negative. In fact, nearly one-third of all breast cancers in African-American women had this phenotype.

What lessons can be learned from this study? The authors propose that incorporating additional risk factors that vary with ethinicity into the widely used Gail model could provide a more accurate risk assessment in minority populations. Moreover, despite previous suggestions that reduced access to health care and a lower frequency of mammography were influencial in the outcome for African-American women, the authors indicate that genetic risk factors are more likely to have an influencial role and these should be investigated further in larger, more comprehensive studies.