Biology of primary breast cancer in older women treated by surgery: with correlation with long-term clinical outcome and comparison with their younger counterparts

Background: As age advances breast cancer appears to change its biological characteristics, however, very limited data are available to define the precise differences between older and younger patients. Methods: Over 36 years (1973-2009), 1758 older (>= 70 years) women with early operable primary breast cancer were managed in a dedicated clinic. In all, 813 underwent primary surgery and 575 good quality tumour samples were available for biological analysis. The pattern of biomarkers was analysed using indirect immunohistochemistry on tissue microarrays. Comparison was made with a previously characterised series of younger (<70 years) patients. Results: There was high expression of oestrogen receptor (ER), PgR, Bcl2, Muc1, BRCA1 and 2, E-cadherin, luminal cytokeratins, HER3, HER4, MDM2 and 4 and low expression of human epidermal growth factor receptor (HER)-2, Ki67, p53, EGFR and CK17. Oestrogen receptor and axillary stage appeared as independent prognostic factors. Unsupervised partitional clustering showed six biological clusters in older patients, five of which were common in the younger patients, whereas the low ER luminal cluster was distinct in the older series. The luminal phenotype showed better breast cancer-specific survival, whereas basal and HER2-overexpressing tumours were associated with poor outcome. Conclusion: Early operable primary breast cancer in older women appears as a distinct biological entity, with existence of a novel cluster. Overall older women showed less aggressive tumour biology and ER appeared as an independent prognostic factor alongside the time-dependent axillary stage. These biological characteristics may explain the differences in clinical outcome and should be considered in making therapeutic decisions.