Understanding Censoring Rules in Time to Event Calculations for Clinical Trials
INTRODUCTION
Time to event (TTE) is a critical endpoint in clinical trials, and it measures the duration between the start date of observation and the first date of documentation of an event. In cases where the event of interest does not occur within the observation period, censoring rules need to be applied to determine the TTE. This article aims to provide a comprehensive overview of censoring rules and their application in TTE calculations.
CENSORING RULES IN TTE CALCULATIONS
In TTE calculations, if the event of interest does not occur within the observation period, the TTE is calculated as the observation start date to the censor date. The censor date is the last available date or study completion date, or any other date as required by the study. Censoring rules are included in the statistical analysis plan (SAP) and detail how to handle subjects who do not experience the event of interest during the observation period.
APPLICATION OF CENSORING RULES IN TTE CALCULATIONS
Let us consider a study where subjects with lung cancer recently underwent surgery to remove their tumor. Disease-free survival (DFS) is considered as the primary endpoint in this study, and the researchers are observing to see how long it takes for the tumor to recur and for the patients to be disease-free. The duration of the study is from 3 months from the time of surgery to 1 year.
In this case, DFS is calculated as the duration from the start date of observation (the date of surgery) to the date of first recurrence of the tumor. If the date of first recurrence of the tumor is at 9 months, then DFS will be the time from the date of surgery to the date of the 9-month review. In this case, there is no censoring required as the event has been reached within the duration of the study.
However, if the event of interest (the recurrence of the tumor) does not occur within the duration of the study, censoring rules need to be applied. For example, if a subject’s follow-up ends at 8 months, and there is no evidence of tumor recurrence, the censoring date for this subject would be 8 months, and the TTE calculation would be:
TTE = censor date — observation start date + 1
= 8–3 + 1
= 6 months
In this case, the subject is censored as the event of interest did not occur within the duration of the study.
CONCLUSION
In conclusion, censoring rules are an essential aspect of TTE calculations in clinical trials. Censoring is used when the event of interest does not occur within the duration of the study, and it helps to handle incomplete data in survival analysis. Researchers should carefully consider the appropriate censoring rules to use in their study design and analysis to obtain accurate and reliable results. Censoring rules should be included in the statistical analysis plan, and they should detail how to handle subjects who do not experience the event of interest during the observation period.
