Completeness of breast cancer staging in the Danish Cancer Registry, 2004–2009

BACKGROUND The purpose of this study was to investigate the completeness of TNM (Tumor, Node, Metastasis) staging for breast cancer in the Danish Cancer Registry. METHODS We identified 26,488 patients with a first diagnosis of breast cancer between 2004 and 2009 from the Danish Cancer Registry. We obtained information on comorbidity through the Danish National Patient Registry. We estimated the completeness of TNM registration in the Danish Cancer Registry and stratified the analysis by gender, age, year of cancer diagnosis, and comorbidity. We designed an algorithm categorizing breast cancer into localized, regional, distant, or unknown stage based on TNM codes. RESULTS The overall completeness of TNM registration was 85.4%. The completeness varied little by gender and study year, but decreased from 91.3% in patients aged 0-39 years to 57.0% in patients aged 80 years or more, and from 87.9% among patients with a low level of comorbidity to 69.7% among patients with a high level of comorbidity. CONCLUSION The completeness of the TNM registration varied substantially by age and level of comorbidity. Thus, depending on the outcome under study, stage-specific analyses may yield biased results. The completeness of TNM should be considered in study designs using TNM information.