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

PURPOSE 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.