Breast Cancer Outcomes From Ductal Carcinoma In Situ: A Population-Based Cohort Study

This population-based cohort study investigated whether the subsequent risk of breast cancer after ductal carcinoma in situ (DCIS) has changed over time in New Zealand women diagnosed between 2000 and 2022, using data from the New Zealand Breast Cancer Foundation National Register. The primary outcome was ipsilateral breast event, including invasive cancer (iIBC) or DCIS (iDCIS). Fine-Gray subdistributional hazard models assessed associations with demographic and clinical factors, and a 5-year landmark analysis evaluated subsequent risk among women who remained event-free at 5 years. Among 5830 patients (median follow-up 4.8 years), the overall 5-year cumulative risk of iIBC was 3.3% (95% CI: 2.7%, 3.9%). In the 5-year landmark cohort, the subsequent 5-year risk was 4.1% (3.3%, 5.0%). Corresponding iDCIS rates were 2.5% (2.0%, 3.0%) and 1.7% (1.2%, 2.3%), respectively. Age < 45 years at diagnosis and DCIS size > 20 mm were associated with a higher iIBC risk, but the association with younger age attenuated in the landmark analysis. Compared with breast conserving surgery (BCS) alone, both radiotherapy following BCS and mastectomy were associated with a lower iIBC risk; however, the association for radiotherapy was not evident at the 5-year landmark. Overall, women with DCIS had a 5.25-fold higher risk of invasive breast cancer than the general population (95% CI: 4.79, 5.73). Our findings support the importance of long-term mammographic surveillance, particularly for women with larger DCIS, those treated with BCS, and possibly younger women.