[Oral contraception and benign breast diseases: point of view of the epidemiologist].

The study of the association between oral contraceptives (OC) and benign breast disease (BBD) is of great interest for the epidemiologist, because of the impact that this association may have on the risk of breast cancer. In fact, three factors have to be analysed simultaneously: OC use, the occurrence of BBD and the occurrence of breast cancer. This analysis is complex because two out of the three factors are not univocal in their definition (various composition and doses for pills, numerous histologic types of BBD). Moreover, a fourth parameter is to take into account: the time factor. This last factor intervenes in the chronological order of events (OC before or after BBD), and for the use of different types of pills (duration of use, age at the start of OC use before or after the first full-term pregnancy, etc). Although all the relationships between these different factors are still insufficiently known at the present time, certain points are acquired: 1) OC use decreases the risk of BBD (particularly when pills contain high doses of progestogens); 2) a history of BBD increases the risk of breast cancer (particularly when BBD is a fibrocystic mastosis with cellular atypia); 3) in spite of the decrease in the risk of BBD associated with OC use, OC use does not decrease the risk of breast cancer. For a long time, this third proposition appeared as paradoxal. In 1986, Stadel and Schlesselman tried to estimate the terms of this paradoxe. Their estimations allowed to show that the risk of breast cancer associated with OC use, for women with a history of BBD, was overestimated in all previous reports. Indeed, the decrease of the occurrence of BBD due to OC use, which was associated with an important reduction in the number of breast cancer cases, was not taken into account in the estimation of this risk. Other epidemiological investigations with a greater number of subjects are needed to further analyse complex relationships between OC use, BBD and breast cancer.