Reduction in hormone replacement therapy use and declining breast cancer incidence in the Belgian province of Limburg.

Breast cancer is the most common female cancer in Europe, but its incidence and mortality are rapidly changing across Europe. The early termination of the women's health initiative (WHI) trial, after the detection of an increased breast cancer risk in hormone replacement therapy (HRT) users, was followed by strong declines of HRT use worldwide. We investigated whether the reduction of HRT sales affected breast cancer incidence in the Belgian province Limburg. All women registered in the Limburg Cancer Registry with a diagnosis of invasive breast cancer diagnosed between 1/1/1996 and 31/12/2005 were included in the study. Data on the use of HRT in the population were obtained from the vendors and the social security system. For age-standardization using the direct method, the European standard population was taken. In 2003 and 2004, the breast cancer incidence rate decreased significantly as compared to 2002 for women aged between 50 and 69 years. This sudden drop in the incidence intercepted a markedly increasing trend until 2002, but was followed again by an increase in 2005. Between 2002 and 2006, the sales of HRT (about 75% to women aged 50-69 years) were reduced by 41%. Breast cancer incidence was maximally related to HRT use in the previous year (R(2) = 77%). The decrease of breast cancer incidence in the Belgian province of Limburg may largely be related to the fall of HRT use following the early termination of the WHI trial. This suggests that HRT stimulates the growth of pre-existing, clinically latent tumours that may not otherwise become clinically apparent.