Incidence and risk factors for breast cancer subtypes in three distinct South-East Asian ethnic groups: Chinese, Malay and natives of Sarawak, Malaysia

INTRODUCTION: We determined the incidences of the ER, PR and HER2 subtypes among breast cancer cases in Sarawak, Malaysia, and their correlation with various risk factors in the three ethnic groups: Chinese, Malay and native. METHODS: Subtype status was ascertained for 1034 cases of female breast cancer (93% of all cases diagnosed since 2003), and the age-standardized incidence rates (ASRs) of each subtype were inferred. Case-case comparisons across subtypes were performed for reproductive risk factors. RESULTS: We found 48% luminal A (ER+/PR+/HER2-), 29% triple-negative (ER-/PR-/HER2-), 12% triple-positive (ER+/PR+/HER2+) and 11% HER2-overexpressing (ER-/PR-/HER2+) subtypes, with ASRs of 10.6, 6.0, 2.8, 2.8 per 100 000, respectively. The proportions of subtypes and ASRs differed significantly by ethnic group: HER2-positive cases were more frequent in Malays (29%; 95% CI [23;35]) than Chinese (22%; [19;26] and natives (21%; [16;26]); triple-negative cases were less frequent among Chinese (23%; [20;27]) than Malays (33%; [27;39]) and natives (37%; [31;43]). The results of the case-case comparison were in accordance with those observed in western case series. Some uncommon associations, such as between triple-negative subtype and older age at menopause (OR, 1.59; P <0.05), were found. CONCLUSIONS: The triple-negative and HER2+ subtypes predominate in our region, with significant differences among ethnic groups. Our results support the idea that the risk factors for different subtypes vary markedly. Westernized populations are more likely to have factors that increase the risk for the luminal A type, while risk factors for the triple-negative type are more frequent in local populations. (c) 2012 Wiley-Liss, Inc.