Subsequent risk of ipsilateral and contralateral invasive breast cancer after treatment for ductal carcinoma in situ: incidence and the effect of radiotherapy in a population-based cohort of 10,090 women

PURPOSE: To assess the effect of different treatment strategies on the risk of subsequent invasive breast cancer (IBC) in women diagnosed with ductal carcinoma in situ (DCIS). METHODS: Up to 15-year cumulative incidences of ipsilateral IBC (iIBC) and contralateral IBC (cIBC) were assessed among a population-based cohort of 10,090 women treated for DCIS in the Netherlands between 1989 and 2004. Multivariable Cox regression analyses were used to evaluate associations of treatment with iIBC risk. RESULTS: Fifteen years after DCIS diagnosis, cumulative incidence of iIBC was 1.9 % after mastectomy, 8.8 % after BCS+RT, and 15.4 % after BCS alone. Patients treated with BCS alone had a higher iIBC risk than those treated with BCS+RT during the first 5 years after treatment. This difference was less pronounced for patients <50 years [hazard ratio (HR) 2.11, 95 % confidence interval (CI) 1.35–3.29 for women <50, and HR 4.44, 95 % CI 3.11–6.36 for women ≥50, P (interaction) < 0.0001]. Beyond 5 years of follow-up, iIBC risk did not differ between patients treated with BCS+RT or BCS alone for women <50. Cumulative incidence of cIBC at 15 years was 6.4 %, compared to 3.4 % in the general population. CONCLUSIONS: We report an interaction of treatment with age and follow-up period on iIBC risk, indicating that the benefit of RT seems to be smaller among younger women, and stressing the importance of clinical studies with long follow-up. Finally, the low cIBC risk does not justify contralateral prophylactic mastectomies for many women with unilateral DCIS. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10549-016-3973-y) contains supplementary material, which is available to authorized users.