Breast Cancer Among Indigenous and Non-Indigenous Women at the Mexican Teachers' Cohort

Hispanics and Indigenous women are underrepresented in cancer research. We aimed to estimate the incidence of breast cancer (BC) among indigenous and non-indigenous women and describe reproductive and lifestyle risk factors. METHODS: The baseline questionnaire was completed by 115,307 women (2006-2008). Indigenous ancestry was defined by self-adscription and/or speaking an indigenous language. Incident BC-cases were confirmed using self-reports, administrative and clinical databases, cancer registries, and death certificates. We calculated person-years from the baseline questionnaire to the date of diagnosis, death, or the end of follow-up (December 31, 2019). We age-standardized reproductive and lifestyle information. RESULTS: After a median follow-up of 10.8 years, we confirmed 1,212 BC-cases. The crude incidence rate per 100,000 person-years was 55 for indigenous and 95 for non-indigenous women; the mean age at diagnosis was 48.2 and 50.8, respectively. In indigenous BC-cases, early menarche (11.9 v 26.7%), first pregnancy > 25 years (36.8% v 51.7%), and nulliparity (11.7% v 14.1%) were less frequent compared to non-indigenous. The number of children (3.0 v 2.6) and breastfeeding > 12 months (71.6% v 45.2%) were higher among indigenous. Indigenous had earlier menopause (44.8 v 47.5 years) and more premenopausal-BC (27.8% v 25.1%). Oral contraceptives use (42.6% v 48.0%), hormone-replacement therapy (12.4% v 24.6%), family history of BC (11.9% v 15.1%) and benign breast-disease (17.8% v 23.0%) were less frequent in indigenous cases. Physical activity (> 150 min/week) was higher in indigenous women (34.3% v 27.8%). Smoking (6.6% v 11.2%) and alcohol consumption (47.9% v 61.3%) were lower in indigenous women; however, they had more diabetes (14.6% v 6.0%) and were at the highest tertile of the dietary glycemic index (40.2% v 35.0%). CONCLUSION: In the MTC, BC-incidence in indigenous women is lower than in non-indigenous; this might be explained by a lower prevalence of hormonal and reproductive risk factors and higher physical activity among indigenous women.