Adjuvant antihormonal treatment in breast cancer. Understanding how poor adherence affects survival

Abstract Purpose Adherence to antihormonal treatment is known to be suboptimal. The beneficial effects of antihormonal treatment are well documented. We investigate how poor adherence affects survival. Methods This single-centre retrospective cohort study included 1176 patients diagnosed with breast cancer from 2004-2013. These patients were all prescribed antihormonal treatment as part of their adjuvant treatment regimen. Adherence was calculated based on the medical possession ratio (MPR) where patients were defined as adherent if they reached a MPR ≥80%. Non-adherent patients were further subclassified into secondary non-adherent (SNA) where MPR measured 0.1-80% or primary non-adherent (PNA) where MPR was zero. Results A total of 24.4% (287) of the patients were non-adherent. Secondary non-adherent patients had poorer breast cancer specific survival (hazard ratio [HR] 3.04, 95% confidence interval [CI] 2.12-4.36) compared to adherent patients. Primary non-adherent patients had better breast cancer specific survival compared to adherent patients (HR 0.27, 95% CI 0.10-0.75). Conclusion Subdividing non-adherent patients into primary non-adherent or secondary non-adherent provides valuable insight into how different adherence-behaviours affect survival. Based on the favourable survival rates of the low-risk primary non-adherent patients, we suggest that some low-risk patients currently are being overtreated with antihormonal treatment.