Abstract S3-05: Higher 10-year overall survival after breast conserving therapy compared to mastectomy in early stage breast cancer: A population-based study with 37,207 patients

Background: Randomised controlled trials have shown that breast conserving therapy (conserving surgery with radiation therapy, BCT) has equal overall survival (OS) rates as mastectomy without radiation therapy (MAST) in early stage breast cancer. However, 10-year disease-free survival (DFS) in a population-based study was not investigated before. The aim of this study was to compare 10-year OS and DFS after BCT with MAST in Dutch women with early stage breast cancer.

Methods: Data of all women diagnosed with primary invasive T1-2N0-1M0 stage breast cancer between 1 January 2000 and 31 December 2004, treated with either BCT or MAST, were selected from the Netherlands Cancer Registry. Multivariable Cox proportional hazard analysis was performed to estimate 10-year OS, stratified for T and N stage. Ten-year DFS was determined in a subgroup of patients diagnosed in 2003, of which an active follow-up was conducted registering all recurrent events within 10 years. Multiple imputation was performed to account for missing data.

Results: Of in total 37,207 patients, 21,734 patients (58.4%) received BCT and 15,473 patients (41.6%) received MAST. The subcohort of 2003 consisted of 7,552 patients, with similar distributions of treatments and characteristics. In the total cohort, 10-year OS was 76.8% (99% CI: 76.1-77.5%) after BCT and 59.7 (99% CI: 58.7-60.7%) after MAST. After correction for confounding, 10-year OS was better after BCT than after MAST (HR_adjusted: 0.79 [99% CI 0.75-0.83]). In the 2003 cohort, 10-year DFS was 83.6% (99% CI: 82.5-84.7%) after BCT and 81.5% (99% CI: 79.6-83.4%) after MAST. After correction for confounding, 10-year DFS was comparable for both treatments (HR_adjusted 0.91 [99% CI 0.77-1.07]). All results were similar for all subgroups (Table). In the 2003 cohort, 11.0% of the patients experienced distant metastases (DM) after BCT compared to 14.7% after MAST (p<0.001). Regional recurrences (RR) were diagnosed in 2.1% of patients treated with BCT and in 4.0% of patients treated with MAST (p<0.001). Percentages of local recurrences (LR) did not differ between the treatment groups.

Conclusion: BCT showed substantially improved OS compared to MAST. However, while DFS was similar, patients treated with BCT less often developed RR and DM. Although residual factors might explain part of the difference in recurrences, we hypothesise that radiation therapy might largely be responsible for better OS by eliminating residual tumour cells.

Citation Format: van Maaren MC, de Munck L, de Bock GH, Jobsen JJ, van Dalen T, Poortmans P, Linn SC, Strobbe LJA, Siesling S. Higher 10-year overall survival after breast conserving therapy compared to mastectomy in early stage breast cancer: A population-based study with 37,207 patients. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr S3-05.