Survival after breast conservation versus mastectomy in ductal carcinoma in situ of the breast: a population-based Swedish register study

Abstract Introduction In invasive breast cancer, studies indicate superior overall survival after breast-conserving surgery (BCS) than mastectomy (Mx). BCS is increasingly used also in ductal carcinoma in situ (DCIS). The aim was to compare survival and local recurrence rates after BCS versus Mx for DCIS. Method This population-based register study included 7229 women with DCIS receiving surgery in Sweden 2008-2019 from the Breast Cancer Database Sweden (BCBaSe 3.0). Cases without information on radiotherapy (RT) or surgery were excluded. Locoregional treatment was BCS with RT, Mx without RT, or BCS without RT. Associations between treatment and outcomes were modelled using adjusted Cox regression models implementing multiple imputation. Result 3854 women received BCS with RT (53.3%), 2408 (33.3%) Mx without RT, and 967 (13.4%) BCS without RT. Median follow-up was 69.1 months (range 0.6-155.9). 320 local recurrences and 376 deaths occurred. In the imputation analysis, Mx without RT was significantly associated with a lower risk of local recurrence than BCS with RT (HR 0.20, 95% CI 0.13-0.32); the opposite was found for BCS without RT (HR 1.72, 95% CI 1.24-2.28). Compared to BCS with RT, Mx without RT was not significantly associated with overall death in the imputation analysis (HR 1.37, 95% CI 0.95-1.96) but in the complete case analysis (N=3169; HR 2.09, 95% CI 1.19-3.66). BCS without RT had inferior overall survival. Discussion BCS for DCIS is associated with higher local recurrence rates than Mx. Superior survival after BCS with RT was not confirmed in the imputation analysis of all patients, suggesting selection bias.