Survival of breast cancer patients in German-Danish border regions – A registry-based cohort study

1

Introduction

Globally, breast cancer is the most common malignant neoplasm and the leading cause of cancer death in women [ 1 ]. In Europe, recent trends show increasing incidence rates with simultaneously decreasing mortality rates [ 2 ] and improvements in survival among female breast cancer patients [ 3 ]. Denmark was and is known to have lower overall and breast cancer-specific survival than other Nordic countries [ 3 , 4 ] and its neighbouring country Germany [ 3 , 5 ], though remarkable improvements have been observed on a national level in recent years [ 6 ].

In past decades, significant efforts were made to compare cancer survival and explain survival differences across nations with constantly evolving studies such as EUROCARE [ 7 ] and CONCORD [ 8 ], but these also came with limitations. The major limitation of previously published research based on the most recent EUROCARE-5-data, including amongst others Danish and German cancer registry data, is the lack of information about stage and treatment as the key prognostic factors for cancer survival [ 3 , 5 ]. Within EUROCARE and CONCORD protocols, high-resolution studies were conducted and clinical data on stage and treatment were collected successfully from many countries including Denmark, but German cancer registry data were not included [ 9 , 10 ]. However, a study by Storm et al. showed that meaningful survival comparisons of cancer registry data from Germany and Denmark are possible if additional variables like stage and treatment are taken into account as exemplified by cancer registry data for colorectal cancers [ 11 ]. Taking stage and treatment into account is important, as Germany’s and Denmark’s universal health care systems differ substantially, e.g. in terms of accessibility to specialized care (gatekeeper function of Danish general practitioners (GPs) versus free choice of physicians in Germany), implementation of early detection programs, treatment capacities and funding [ 12 ].

Cross-border collaboration in cancer research and care between the German federal state Schleswig-Holstein and the bordering Danish regions Southern Denmark and Zealand was established in 2007 [ 13 ] and expanded since. In light of this program, our aim was to investigate regional differences in breast cancer survival using cancer registry data, including clinical information, to assess i) whether regional survival differences exist and ii) the extent to which they can be explained by tumour stage at diagnosis and type of treatment.