Incidence of second cancers among women with in situ carcinoma of the breast.

A population-based cohort of 371 women with carcinoma in situ (CIS) of the breast, collected by the Tuscany Cancer Registry, has been analysed for further invasive cancers. All cases, diagnosed between 1985 and 1997, have been followed up to the end of 1997. During 1707 person-years of follow-up, 27 further invasive cancers were diagnosed while 13.7 were expected (Observed/Expected=2.0, P<0.05). The relative risk for invasive breast cancers was 3.7 (P<0.05). According to the surgical treatment for CIS and the site of further invasive breast cancer, no side specific difference was evident. No significant increase was evident for other cancer sites; only non-melanomatous skin cancers occurred more frequently than expected (O/E=4.2). The cumulative risk of developing an invasive cancer after CIS was 13.2% at 10 years. There were also two deaths due to breast cancer (O/E=8.3; P<0.05) corresponding to a cumulative mortality risk of 2% at 10 years. We have quantified the risk of developing an invasive breast cancer among women with CIS of the breast as four times that of the general population.