In situ breast cancer: incidence trend and organised screening programmes in Italy.

The effect of mammography screening programmes on the incidence of in situ breast cancer (CIS) is described by analysis of the CIS incidence trend in the 1990s and comparison of pre-screening and screening periods in six areas of Italy. All 1069 CIS arising in women aged 40-79 years between 1988 and 1999 were analysed through age-standardised rates and Poisson regression models. The results show that, for the whole series, ductal carcinoma in situ (DCIS) represented 89% and lobular carcinoma in situ (LCIS) 11% of CIS detected. For all six areas, the introduction of screening increased the incidence of DCIS (screening/pre-screening ratio, range 1.12-1.77). Overall, DCIS represented 11% (226/2022) of all screening-detected cancers. A significant increasing trend in DCIS incidence during the 1990s and a modification in pattern of age-specific incidence rates after the beginning of screening programmes were observed. This increase can largely be explained by screening programmes. The incidence observed during the screening period was a persistent 39% higher than during the pre-screening period, after adjustment for the "percentage of cases diagnosed by screening". The increase also involves women at an age not targeted by screening programmes. In conclusion, as the increasing trend in DCIS is not completely explained by the effect of the screening programmes, this supports the use of mammography as a "spontaneous" preventive practice during ongoing organised screening programmes, particularly among age groups not usually invited for screening. Therefore, the effect of mammography on stage-specific incidence of CIS may be more marked than expected on the basis of the effect of screening programmes.