Increased risk of contralateral breast cancers among overweight and obese women: a time-dependent association.

Breast cancer (BC) survivors are at increased risk of second cancers. Obesity is commonly recognized as a risk factor of BC in postmenopausal period and a prognosis factor in BC regardless of menopausal status. Our aim was to study whether overweight BC survivors were at increased risk of contralateral BC (CBC). Our population was a large cohort of women followed since a first BC without distant spread and/or synchronous CBC. Body mass index (BMI) was assessed at diagnosis time. Binary codings of BMI were used to oppose overweight and obese patients to the others. Survival analyses were used including Cox models. Assumed hypothesis of proportional hazards was explored using graphical methods, Schoenfeld residuals and time-dependant covariates. In case of non-proportional hazards, survival models were computed over time periods. Over 15,000 patients were included in our study. Incidence of CBC was 8.8 (8.3-9.3)/1000 person-years and increased during follow-up. A significant time-dependent association between overweight and CBC was observed. After 10 years of follow-up, we found a significant increased hazard of CBC among patients with a BMI above 25 kg/m(2): the adjusted hazard ratio was 1.50(1.21-1.86), P = 0.001. After 10 years of follow-up, our study found a poorer prognosis among overweight BC survivors regarding CBC events. While benefits from diet habits and weight control may be expected during the long-term follow-up, they have yet to be established using randomized clinical trials.