Abstract
PURPOSE: The incidence of breast cancer has increased, and many more cases are being detected with favorable tumor characteristics and in older age. The guidelines for treating early breast cancer have evolved as new treatments have become available. Our aim was to study the extent to which the guidelines reflect the real-world setting and whether guideline non-adherence is more frequent in some patient groups. METHODS: Data from 803 women aged 40–84 years diagnosed with early breast cancer between 1992 and 2011 were retrieved from the clinical patient files and the Finnish Cancer Registry. To study adherence to existing national guidelines, undertreatment and overtreatment with respect to the guidelines were considered non-adherence, and adherence status was analyzed using multivariable regression models. RESULTS: During the study period, the proportion of patients with guideline-based indications for systemic adjuvant treatment increased from 43.8% to 98.9%, while adherence to the guidelines decreased from 82.2% to 70.1%. Undertreatment was more frequent in the later years of the study (odds ratio [OR] 2.77; confidence interval [CI] 95% 1.67–4.59) compared to the beginning of the study, and more frequent among patients 70 years or older (OR 1.86; CI 95% 1.22–2.84) compared to patients 50–69 years of age. Undertreatment was more common in node-negative (T2-4N0) cases than in node-positive cases (OR 2.14; CI 95% 1.16–3.96). CONCLUSION: As treatment indications expand, it should be acknowledged that the results of clinical trials can only be applied to a portion of real-world patients, resulting in a decrease in guideline adherence. This dynamic should be considered in future guidelines and clinical trials. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12885-026-15731-x.