Mammography screening and incidence of ductal carcinoma in situ of the breast in Italy: an age-period-cohort analysis.

Background: The increasing incidence of ductal carcinoma in situ (DCIS) of the breast is attributed to mammography screening, but the trend has so far been evaluated only descriptively.

Methods: We report an age-period-cohort modelling analysis of the incidence trend (1992-2017) observed among screening-aged women living in a region of northern Italy, where a mammography screening programme was implemented in 1996-98 (age 50-69 years) and 2010-14 (age 45-49 and 70-74 years).

Results: The incidence of DCIS increased by an annual average of 9.1% (age 60-64 years) to 14.9% (age 70-74 years). The incidence peak followed a complex age-time pattern indicating an interaction between age and period, which suggested a cohort effect explained by the screening programme. In the age-period-cohort analysis, the birth cohort had a 2-fold effect. The nonlinear effect consisted of an increase in incidence for the generations of 1933-42 and 1943-52, targeted by screening since 1997, and of a second increase for the cohort of 1963-72, first invited in 2010. Taking into account the early excess incidence due to the introduction of the screening programme, the linear effect consisted of an annual 4.0% increase in the risk of DCIS for all successive birth cohorts or calendar periods, which was only partially attributable to the programme.

Conclusions: The increase in incidence rates resulted from an increased detection of DCIS through the screening programme and from an uptrend in the risk of diagnosis that can be attributed either to long-term changes in diagnostic scrutiny independent of the programme or to an increased exposure to risk factors.