Highly reproducible results of breast cancer biomarkers when analysed in accordance with national guidelines - a Swedish survey with central re-assessment

Background. Biomarkers are crucial for decisions regarding adjuvant therapy in primary breast cancer, and their correct assessment is therefore of the utmost importance. Aims. To investigate the concordance between Swedish pathology departments and a reference laboratory, for routine analysis of oestrogen receptor (ER), progesterone receptor (PR), Ki67, and human epidermal growth factor receptor 2 (HER2), alone, and in combination (St Gallen subtypes). Methods. This survey included 27 of the 28 pathology laboratories in Sweden, covering 98% of cases of primary breast cancer surgery in Sweden. Paraffin-embedded tumour blocks (n = 270) were collected and sent to the central reference laboratory, together with the originally stained slides, for re-analysis. The primary evaluations were previously performed according to national Swedish guidelines, without any knowledge of the subsequent central assessment. Results. The agreement for ER, PR, and Ki67 was 99% [kappa value (kappa) = 0.95], 95% (kappa = 0.85), and 85% (kappa = 0.70), respectively. The agreement for HER2 (0/1 + vs. 2+/3+) was 85% (kappa = 0.64), but when equivocal tumours were further analysed with in situ hybridisation, only one discrepancy was observed. Discrepancies between results for ER and PR seem to be explained by analytical differences, whereas the interpretation of staining seems to be more critical for Ki67 and HER2 immunohistochemistry. The agreement between the results from the Swedish laboratories and the reference laboratory, based on the St Gallen subtypes, was 88% (kappa = 0.81). Conclusions. When applying national guidelines, highly reproducible results were obtained in routine assessment of breast cancer biomarkers, and the results of this study confirm the clinical utility of these markers for decisions regarding the treatment of primary breast cancer.