LAUSR

Recent studies from high‐risk countries such as the US, Denmark and Ireland have shown rising incidence rates of hormone receptor (HR)‐positive and falling rates of HR‐negative breast cancers (BC). However, it remains unclear whether a similar pattern occurs in low‐risk countries. Detailed clinical and risk factor data were collected from 2,977 female invasive BC patients (≥20 years) in Sarawak General Hospital, Malaysia, representing 93% of the population. The population‐at‐risk was obtained from the Department of Statistics Malaysia. Secular trends in age‐standardized incidence rates were assessed using estimated average annual percent changes. Associations between established BC risk factors and tumor subtypes defined by HR or joint human epidermal growth factor receptor 2 (HR/HER2) status were examined by case–case comparisons using logistic regression. From 2006 to 2015, incidence rates increased for HR‐positive cancers by 4.46%/year (95% CI = 2.19–6.78) and decreased for HR‐negative cancers by 2.29%/year (95% CI = −4.31 to −0.24). When further stratified by HER2, the most contrasting difference in linear trends was observed between HR+/HER2− and HR−/HER2− subtypes. After controlling for potential confounders, cases with excess body weight (ORoverweight vs. normal = 0.82; 95% CI = 0.69–0.98; ORobese vs. normal = 0.62; 95% CI = 0.48–0.80), later age at first birth (OR≥26 years vs. <23 years = 0.82; 95% CI = 0.66–1.02), nulliparity (ORnulliparous vs. <23 years = 0.74; 95% CI = 0.59–0.94) and never‐breastfeeding (ORnever vs. ever = 0.73; 95% CI = 0.55–0.97) were less frequent among HR‐negative cases than among HR‐positive cases. Diverging incidence trends by HR expression were similar in Sarawak and Western countries, possibly reflecting changes in the prevalence of risk factors with opposing effects by tumor subtypes in low‐ and high‐risk populations.