Abstract P3-09-04: The associations between body mass index and breast cancer intrinsic subtypes in Japanese women

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Abstract Purpose: The purpose of this study was to examine the associations between body mass index (BMI) and breast cancer intrinsic subtypes in Japanese women. A more complete understanding of the subtypes of breast cancer may help elucidate mechanisms affecting etiology and mortality associated with each subtype. Methods: Tumor data on 531 invasive breast cancer cases subtyped by estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (Her2) status were obtained (luminal A, luminal B, TN, and Her2-type). Demographics (age at diagnosis, menopausal status, and BMI) were collected from medical records. Case-only odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using logistic regression, adjusting for age at diagnosis. Results: In all cases, the patients were Japanese. Of the 531 cases with IHC marker data, 333 (62.7%) were classified as luminal A, 85 (16.0%) were luminal B, 43 (8.1%) were Her2-type, and the remaining 70 cases (13.2%) were TN. The distribution of patient demographics (age at diagnosis, menopausal status, and BMI) did not differ significantly by breast cancer tumor subtype. Case-only ORs comparing each subtype to luminal A were caluculated. Of the TN cases, postmenopausal TN cases were more likely to be underweight (OR = 3.14, 95% CI = 1.19 to 8.01). Although some epidemiological studies have reported that higher BMI was associated with premenopausal TN cases compared with luminal A cases, this association was not found among premenopausal TN cases analyzed using BMI 18.5 to 24.9 kg/m2 as the reference in this analysis. However, there were no underweight cases (BMI < 18.5 kg/m2) among the premenopausal TN cases in the present study. Therefore, the association between BMI and the TN subtype was also analyzed using BMI < 25 kg/m2 as the reference. Compared to luminal A cases, premenopausal TN cases were more likely to be obese (OR = 4.11, 95% CI = 1.10 to 14.40), similar to reports from Western countries. Compared to luminal A cases, premenopausal luminal B cases were likely to be underweight (OR = 3.27, 95% CI = 0.88 to 11.39) or obese (≥ 25 kg/m2) (OR = 3.32, 95% CI = 0.98 to 10.81), yet this association was of borderline significance. Compared to luminal A cases, luminal B and Her2-type cases were likely to be underweight (BMI < 18.5 kg/m2), yet this association was of borderline significance (luminal B: OR = 2.12, 95% CI = 0.97 to 4.46; Her2-type: OR = 2.53, 95% CI = 0.92 to 6.36). Conclusions: In the present study, significant heterogeneity of associations between BMI and tumor subtypes was observed. Breast cancer subtypes may have different etiologies associated with each subtype. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P3-09-04.