Evolution of breast cancer incidence in relation to hormone replacement therapy use in Belgium

Background Recent randomized studies concluded that an increased risk of breast cancer is associated with hormone replacement therapy (HRT). Since then, HRT use has decreased in many countries. Several studies have reported a subsequent decrease in breast cancer incidence. Aim As Belgium has one of the highest incidences of breast cancer in Europe and has a high rate of HRT use, with differences between regions, we assessed the evolution of breast cancer incidence and HRT sales per region and per regimen. Materials and methods Breast cancer incidence rates (provided by the Belgian Cancer Registry) and HRT sales data (provided by IMS Health (R)) were analyzed by region, age class and HRT regimens. We also calculated the correlation between breast cancer incidence and HRT sales. Results Breast cancer incidence decreased from 2003 onwards in Flanders, Brussels and Wallonia, especially in the age group 50-69 years. In the same three regions, HRT use decreased by half from 2002 onwards. Greater decreases were observed for estrogens combined with androgenic progestins, estrogens only and estrogens prescribed with a separate progestin. The correlation between breast cancer incidence rates and HRT sales in the previous year was 0.55 (p = 0.04), but, when adjusted for the number of women in the age class 4069 years in each region, the correlation was no longer statistically significant (r = 0.39, p = 0.17). Conclusion Although many arguments support the hypothesis that the drop in breast cancer incidence can be partly explained by the decrease in HRT use, we were unable to find a strong association between the two in Belgium.