The Trials within Cohorts design faced methodological advantages and disadvantages in the exercise oncology setting

The Trials within Cohorts (TwiCs) design is an alternative for pragmatic randomized controlled trials (RCTs) and might overcome disadvantages such as difficult recruitment, dropout after randomization to control, and contamination. We investigated the applicability of the TwiCs design in an exercise oncology study regarding the recruitment process, representativeness of the study sample, contamination, participation, and dropout.

The Utrecht cohort for Multiple BREast cancer intervention studies and Long-term evaLuAtion (UMBRELLA) Fit TwiCs evaluates an exercise intervention in inactive breast cancer patients. Eligible patients participating in the prospective UMBRELLA were identified and randomized. Patients randomized to the intervention (n = 130) were offered the intervention, whereas controls (n = 130) were not informed.

Fifty-two percent (n = 68) accepted the intervention. Because this rate was lower than expected, a larger sample size was required than initially estimated (n = 166). However, recruitment of 260 patients was still completed by one researcher within 30 months. Unselective eligibility screening and randomization before invitation improved representativeness. Disadvantage of the design might be inclusion of ineligible patients when cohort information is limited. Furthermore, the design faced higher noncompliance in the intervention group, but prevention of contamination.

The TwiCs design improved logistics in recruitment and prevented contamination, but noncompliance due to refusal of the intervention was higher compared with conventional pragmatic exercise oncology RCTs, which may dilute the estimated intervention effect.