Most patients reported positively or neutrally of having served as controls in the trials within cohorts design

To evaluate patients' experience of having served as controls without a notification at the time of randomization in the context of the trial within cohorts (TwiCs) design.

Patients were asked for their opinion on having served as controls in TwiCs, before and after having been provided the trial results. Patients had provided broad consent to randomization at cohort entry and had served as controls in one of two TwiCs (an exercise program after breast cancer treatment or radiotherapy dose-escalation for rectal cancer).

Two to 6 years after cohort entry, 15% (n = 16) of all patients remembered having provided broad consent to randomization. Before disclosure of trial results, 47% (n = 52) of patients thought positively, 45% (n = 50) neutrally, and 2% (n = 2) negatively of having served as controls in one of the two trials. Seventeen percent (n = 18) of patients were positive, 65% (n = 71) neutral, and 11% (n = 12) negative about not having been notified when serving as controls. The survey results were comparable after disclosure of trial results.

These results support the use of the TwiCs design with the staged-informed consent procedure. Keeping patients engaged and aware of the consents provided might further improve patients' experience of serving as controls in TwiCs.