Second events following ductal carcinoma in situ of the breast: a register-based cohort study.

The incidence of ductal carcinoma in situ (DCIS) of the breast has increased in recent decades, particularly, in counties offering mammography screening. The aims of the present study are to examine factors that may predict subsequent breast malignancy amongst patients with DCIS, and to compare the incidence of the subsequent malignancy and mortality with that of the general population. This population-based study includes all primary cases of pure DCIS diagnosed in Norway in the period 1993 to 2007 (N = 3167). The patients were followed to subsequent malignancy (DCIS or invasive cancer) or death. Risk estimates within 10 years of follow-up were calculated using Kaplan-Meier methods adjusting for competing risks, Cox regression models and Standard Incidence and Mortality Ratios. Patients with DCIS had a 11.2% risk of being diagnosed with a subsequent breast malignancy within 10 years (9.4% for invasive cancer), implying that they were five times as likely to be diagnosed with breast malignancy as the general female population in Norway. The risk was dependent on the treatment of the DCIS; patients treated with mastectomy and breast-conserving treatment had a 3.8 and 9.8% risk of ipsilateral invasive cancer within 10 years, respectively. Breast cancer mortality was 2.5% within 10 years of follow-up, a fourfold risk compared with the general population. Patients with DCIS have an increased risk of both subsequent breast malignancy and breast cancer death compared with women in the general population. Our results support previous knowledge of DCIS as a heterogeneous disease.