Guidelines are advantageous, though not essential for improved survival among breast cancer patients.

The purpose of this retrospective multicenter study was to resolve the pseudo-paradox that the clinical outcome of women affected by breast cancer has improved during the last 20 years irrespective of whether they were treated in accordance with clinical guidelines or not. This retrospective German multicenter study included 9061 patients with primary breast cancer recruited from 1991 to 2009. We formed subgroups for the time intervals 1991-2000 (TI1) and 2001-2009 (TI2). In these subgroups, the risk of recurrence (RFS) and overall survival (OS) were compared between patients whose treatment was either 100% guideline-conforming or, respectively, non-guideline-conforming. The clinical outcome of all patients significantly improved in TI2 compared to TI1 [RFS: p < 0.001, HR = 0.57, 95% CI (0.49-0.67); OS: p < 0.001, HR = 0.76, 95% (CI 0.66-0.87)]. OS and RFS of guideline non-adherent patients also improved in TI2 compared to TI. Comparing risk profiles, determined by Nottingham Prognostic Score reveals a significant (p = 0.001) enhancement in the time cohort TI2. Furthermore, the percentage of guideline-conforming systemic therapy (endocrine therapy and chemotherapy) significantly increased (p < 0.001) in the time cohort TI2 to TI for the non-adherent group. The general improvement of clinical outcome of patients during the last 20 years is also valid in the subgroup of women who received treatments, which deviated from the guidelines. The shift in risk profiles as well as medical advances are major reasons for this improvement. Nevertheless, patients with 100% guideline-conforming therapy always had a better outcome compared to patients with guideline non-adherent therapy.