The impact of body mass index on breast cancer incidence among women at increased risk: an observational study from the International Breast Intervention Studies

Background:

We investigated the association between body mass index (BMI) and breast cancer risk in women at increased risk of breast cancer receiving tamoxifen or anastrozole compared with placebo using data from the International Breast Cancer Intervention Studies [IBIS-I (tamoxifen) and IBIS-II (anastrozole)].

Methods:

Baseline BMI was calculated from nurse assessed height and weight measurements for premenopausal (n = 3138) and postmenopausal (n = 3731) women in IBIS-I and postmenopausal women in IBIS-II (n = 3787). The primary endpoint was any breast cancer event (invasive and ductal carcinoma in situ). We used Cox proportional hazards regression to calculate hazard ratios (HRs) for risk after adjustment for covariates.

Results:

There were 582 (IBIS-I) and 248 (IBIS-II) breast cancer events [median follow-up = 16.2 years (IQR 14.4–17.7) and 10.9 years (IQR 8.8–13.0), respectively]. In adjusted analysis, women with a higher BMI had an increased breast cancer risk in both IBIS-I [HR = 1.06 per 5 kg/m2 (0.99–1.15), p = 0.114] and in IBIS-II [HR per 5 kg/m2 = 1.21 (1.09–1.35), p < 0.001]. In IBIS-I, the association between BMI and breast cancer risk was positive in postmenopausal women [adjusted HR per 5 kg/m2 = 1.14 (1.03–1.26), p = 0.01] but not premenopausal women [adjusted HR per 5 kg/m2 = 0.97 (0.86–1.09), p = 0.628]. There was no interaction between BMI and treatment group for breast cancer risk in either IBIS-I (p = 0.62) or IBIS-II (p = 0.55).

Conclusions:

Higher BMI is associated with greater breast cancer risk in postmenopausal women at increased risk of the disease, but no effect was observed in premenopausal women. The lack of interaction between BMI and treatment group on breast cancer risk suggests women are likely to experience benefit from preventive therapy regardless of their BMI.

Trial registration: Both trials were registered [IBIS-I: ISRCTN91879928 on 24/02/2006, retrospectively registered (http://www.isrctn.com/ISRCTN91879928); IBIS-II: ISRCTN31488319 on 07/01/2005, retrospectively registered (http://www.isrctn.com/ISRCTN31488319)]