Abstract P1-10-21: Influence of timing of radiation therapy following breast-conserving surgery on 10-year disease-free survival
Authors: van Maaren MC , Bretveld RW , Jobsen JJ , Veenstra R , Groothuis-Oudshoorn KCGM , Struikmans H , Maduro JH , Strobbe LJA , Poortmans P , Siesling S 
Published: March 2017

Abstract
Background
In the Netherlands, one of the indicators of quality of care is that radiation therapy (RT) should start within six weeks following breast-conserving surgery (BCS). However, there is still much controversy regarding timing of RT in literature. This study investigated the effect of timing of RT on disease-free survival (DFS) in a Dutch nationwide population-based cohort.
Methods
All women diagnosed with primary invasive stage I-IIIA breast cancer in 2003, treated with BCS plus RT, of whom the start date of RT was known, were included. Patients who received chemotherapy between surgery and RT were excluded, as this affects delay. Patients were categorised into three groups: &lt;42 days, 42-55 days and &gt;55 days, between surgery and start of RT. The primary outcome was 10-year DFS. Secondary outcomes were 10-year locoregional recurrence-free (LRRFS) and distant metastasis-free survival (DMFS). Multivariable Cox regression was used to correct for confounding. Since adjuvant systemic therapy largely influences DFS, all analyses were stratified for use of adjuvant systemic therapy (chemotherapy and/or endocrine therapy).
Results
In total, 2,759 patients were included. The median number of days between BCS and RT was 45 (IQR 37-54 days). The hazard ratio (HR) for 10-year DFS was 0.79 (95% CI: 0.65-0.96) for 42-55 days and 0.71 (95% CI: 0.56-0.90) for &gt;55 days, both compared to &lt;42 days. While no significant differences in 10-year LRRFS were found, 10-year DMFS (HR 0.64 [95% CI: 0.45-0.91]) was significantly higher for BCS-RT interval &gt;55 days compared to &lt;42 days. After stratification, no significant differences were found for any outcome in patients not treated with adjuvant systemic therapy, while in patients who were treated with adjuvant systemic therapy, 10-year DFS was significantly improved for 42-55 days (HR 0.70 (95% CI: 0.51-0.97) and &gt;55 days (HR 0.63 (95% CI: 0.42-0.96) compared to &lt;42 days. Significantly improved 10-year DMFS was confirmed after stratification for longer delays (HR 0.69 (95% CI: 0.47-1.00 for 42-55 days) and 0.59 (95% CI: 0.36-0.96) for &gt;55 days, compared to &lt;42 days)
10-year DFS, LRRFS and DMFS for different time intervals between BCS and RT, stratified for use of adjuvant systemic therapy Entire cohort (n=2,759)No adjuvant treatment (n=1,761)Adjuvant treatment (n=998)Time intervalHR [95% CI]*HR [95% CI]*HR [95% CI]*10-year DFS&lt;42 days11142-55 days0.79 [0.65-0.96]0.83 [0.65-1.05]0.70 [0.51-0.97]&gt;55 days0.71 [0.56-0.90]0.77 [0.57-1.03]0.63 [0.42-0.96]10-year LRRFS&lt;42 days11142-55 days0.74 [0.51-1.06]0.79 [0.52-1.21]0.55 [0.28-1.11]&gt;55 days0.90 [0.59-1.37]0.99 [0.61-1.60]0.67 [0.29-1.57]10-year DMFS&lt;42 days11142-55 days0.90 [0.69-1.17]1.13 [0.77-1.66]0.69 [0.47-1.00]&gt;55 days0.64 [0.45-0.91]0.73 [0.44-1.22]0.59 [0.36-0.96]* Corrected for all relevant confounders. HRs in bold are statistically significant (p&lt;0.05).
Conclusion and interpretation
In conclusion, patients treated with adjuvant systemic therapy following RT showed increasing 10-year DFS and DMFS with longer BCS-RT intervals, which was not the case for patients not receiving adjuvant systemic therapy. Possible explanations for these results have to be explored further.
Citation Format: van Maaren MC, Bretveld RW, Jobsen JJ, Veenstra R, Groothuis-Oudshoorn KCGM, Struikmans H, Maduro JH, Strobbe LJA, Poortmans P, Siesling S. Influence of timing of radiation therapy following breast-conserving surgery on 10-year disease-free survival [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-10-21.