A retrospective analysis of changes in distant and breast cancer related disease-free survival events in adjuvant breast cancer trials over time

Disease-free survival (DFS) comprises both breast cancer and non-breast cancer events. DFS has not been validated as a surrogate endpoint for overall survival (OS) in most breast cancer subtypes. We assessed changes to the type of events contributing to DFS over time. We identified adjuvant studies in breast cancer (BC) from 2000 to 2020 where the endpoint was DFS. We examined change in distant DFS events and the BC-related DFS using univariable and multivariable linear regression. Data were reported quantitatively using the Burnand criteria irrespective of statistical significance. We included 84 studies (88 cohorts), comprising 212,191 participants, 41,604 DFS events and 23,205 distant DFS events. The DFS event rate/100 participants/year has declined modestly over time (ß − 0.34, p = 0.001). Start year was negatively associated with distant DFS events (ß − 0.58, p < 0.0001); however, the effect was lost after adjusting for follow-up time (ß − 0.18, p = 0.096). The average number of BC-related events/100 participants/year also declined over time (ß − 0.28, p = 0.009). In multivariable analysis, start year and ER expression were quantitatively associated with distant DFS events and BC-related DFS events. DFS events have declined over time driven by a reduction in BC related events. As DFS events are increasingly defined by non-BC events, there will be limited surrogacy between DFS and OS.