Abstract
A study has been made of the way in which the number of events available for analysis in a clinical trial was dependent on the recruitment period, the maximum follow-up time on individual patients and the length of time between the start of the trial and its analysis. The events considered were deaths, local recurrences and late radiation effects on normal tissue in patients treated for cancer of the laryngo-pharynx by two different fractionation regimes. The relationship is demonstrated between the number of events and the 95% confidence intervals that can be placed on differences between results in the two arms of the trial. It was found, in this particular trial, that no significant improvement in precision was gained by following up patients beyond 5 years or carrying out the analysis later than 2 years after the end of recruitment. The results are discussed in the context of the initial design of clinical trials, particularly those in which the aim is to test therapeutic equivalence.
Highlights
Definitive analysis of a trial should take place when the number of events has reached that necessary for the required power to be achieved
Increased recruitment time leads to a corresponding increase in the number of events available for the final analysis
The curve for local recurrences lies below that for deaths because a considerable number of deaths were without record of local recurrence and have been assumed to be either due to distant metastases or to intercurrent disease other than cancer of the laryngopharynx
Summary
The two regimes differed in their fractionation schedules. One employed five fractions per week (one each weekday), which was common radiotherapy practice at that time. The other used three fractions per week (on Monday, Wednesday and Friday), which, provided any difference in its therapeutic effect was clinically unimportant, would be beneficial to the patient because of a reduced number of attendances, and would be more economical in the use of radiotherapy machines and associated staff. Because of the known radiobiological effects of changing fractionation, the total dose to patients treated with three fractions per week was set at 11 or 13% less than that given to patients treated with five fractions per week, for the longer or shorter schedules respectively.
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