Abstract P2-11-24: Disparities in the application of post-mastectomy radiotherapy in Switzerland: A pooled analysis of 7 cancer registries over the 2003-2005 period
Authors: Zwahlen DR , Ess S , Zimmermann M , Bordoni A , Bouchardy C , Frick H , Konzelmann I , Mousavi M , Rohrmann S , Oehler C 
Published: February 2018

Abstract
Background: The aim of this study was to identify factors that influence the delivery of post-mastectomy radiotherapy (PMRT) in Switzerland, and to analyze the adherence to consensus guidelines.
Methods: Based on 7 regional cancer registries covering 45% of the Swiss population, we identified 1408 women which underwent mastectomy for stage I-III breast cancer between January 1, 2003 and December 31, 2005. We categorized patients according to ASCO grouping in similar fashion to other comparable studies: low-risk group (T1/T2 N0): PMRT not routinely recommended; intermediate-risk group (T1/T2 N1): PMRT controversial; high risk group (T3-T4 and/or N2-N3): PMRT recommended. We further investigated factors leading to potential overtreatment (PMRT in low-risk group) or undertreatment (absence of PMRT in high-risk group). Data analysis was performed for the entire cohort, and separately for patients &lt;70 years and ≥ 70 years of age. Probability of receiving PMRT was assessed using multivariable logistic regression.
Results: A total of 421 patients (29.9%) received adjuvant RT after mastectomy. The rate of PMRT delivery was 67% in the high-risk group, compared to 6% and 18% in the low-risk and intermediate-risk groups, respectively.
For patients at high-risk of chest wall recurrence after mastectomy (T3-T4 or N2-N3 disease), the risk of PMRT omission wassignificantly associated to older age (OR 4.25 [95% CI: 2.27-7.95] for patients ≥ 70 years) and to the absence of chemotherapy (OR 4.30 [95% CI: 1.97-9.36]). In patients with T3-T4 disease, PMRT was delivered in 77% of patients &lt; 70 years and in 42% of patients ≥ 70 years (p&lt;0.001). In patients with N2-N3 disease, PMRT was delivered in 82% of patients &lt; 70 years and in 51% of patients ≥70 years (p&lt;0.001).
PMRT was delivered to 28 patients (7%) at low-risk of recurrence after mastectomy (T1-T2 N0, negative margins). It was more frequently offered to patients &lt;40 years of age (OR 3.86 [95% CI: 1.01-14.76]), with T2 tumors (OR 3.43 [95% CI: 1.45-8.11]) and negative hormone receptor status (OR 2.60 [95% CI: 1.04-6.50]).
Positive or close surgical margins (&lt; 1mm) were a strong indicator for PMRT (p=0.001) and chest wall boost (p&lt;0.03).
Conclusions: After mastectomy, one third of patients (33.26%) with high-risk disease did not receive PMRT. Even if we consider only patients &lt; 70 years, a non-trivial proportion of patients with clear indication for treatment delivery did not receive PMRT (T3-T4 disease: 23%; N2-N3 disease: 18%). Further analyses are planned to explain the apparent failure of evidence-based guidelines to impact the adoption of PMRT in women with high-risk breast cancer.
Citation Format: Zwahlen DR, Ess S, Zimmermann M, Bordoni A, Bouchardy C, Frick H, Konzelmann I, Mousavi M, Rohrmann S, Oehler C. Disparities in the application of post-mastectomy radiotherapy in Switzerland: A pooled analysis of 7 cancer registries over the 2003-2005 period [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-11-24.