Representativeness of trial participants: linking the EORTC boost-no boost trial to the Netherlands cancer registry

Background and Objectives: To evaluate the representativeness of Dutch patients participating in the European Organization for Research and Treatment of Cancer EORTC boost-no-boost trial to the target breast cancer patient population. Methods: All female breast cancer patients diagnosed between 1989 and 1996, aged ≤70 years, treated with breast-conserving surgery and radiation therapy, were selected from the Netherlands Cancer Registry (NCR) and linked to the EORTC trial database. Baseline characteristics were compared between trial and non-trial participants, for the Dutch population and according to seven participating institutions. Kaplan-Meier curves and multivariable Cox regression were used to explore potential heterogeneity in overall survival between low, medium and high-volume institutes. Results: Overall, 20,880 patients were identified from the NCR: 2,445 of 2,602 (94%) trial participants could be linked, and 18,435 were treated outside the trial. Trial participants had similar age, morphology, topography, laterality and socioeconomic status as non-trial participants, but more often stage I (62.7% vs. 56.4%) tumours and less often adjuvant treatment (22.9% vs. 26.5%). Crude 20-year survival ranged from 52.5% to 57.4%, without significant differences in multivariable analyses. Conclusion: This case study showed that participants in the boost-no-boost trial well represented the Dutch target population. Data linkage comes with challenges, but can close the gap between research and clinical practice.