A reversal of fortune: Practical problems in the monitoring and interpretation of an EORTC breast cancer trial

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Abstract
An example of some of the practical problems associated with the interim monitoring of accumulating time to event data is illustrated through an EORTC clinical trial in locally advanced breast cancer which studied the effects of both hormonal therapy and chemotherapy using a 2 × 2 factorial design. The data monitoring process is described and summaries of the interim statistical analyses and the analysis for publication of the clinical results are provided. The results 2.5 years after closing the trial to patient entry differed quite markedly from those observed during the period of patient accrual, thus showing the potential dangers of drawing conclusions based on only short term follow‐up data.