Interpretation of cancer prevention trials

Principles and methods to guide interpretation require a different emphasis for cancer trials. Assumptions used to design a trial must be validated and modified during the trial to avoid limitations. To maximize information from such trials, recruitment strategies for commonly free-living subjects, measurements of safety and compliance, and ascertainment with pathologic review of endpoints must be obtained. Consideration of multiple endpoints may provide a better interpretation of cancer prevention for skin, colon, and transient occurrences illustrated by cervical dysplasia or biochemical precursors. A careful definition of the limitations of preventive trials is required. These include the actual size of the intervention groups, completeness and duration of follow-up, and comparison between trial participants and a defined source population. To obtain a valid interpretation with adequate precision of intervention effectiveness, time to endpoints should be evaluated using statistical multivariate methods such as Cox proportional hazard or relative risk models. These permit adjustment for important confounding and risk modifiers such as compliance, dietary intake, and drift in control group. The magnitude of the intervention efficacy and the generalizability of results of the trial will be negatively impacted if the intervention has a delayed (latent) effect. Such delay in intervention effect requires added considerations with possible extension of trial duration. Use of confidence limits for intervention effectiveness provides added insight and improved interpretation of prevention trials. The final component of a cancer prevention trial, as with any study, is to interpret and report its results. Providing a valid interpretation with adequate precision to hypotheses of a cancer prevention trial requires added emphasis on the accuracy of the assumptions made to design the trial and the duration of the trial. Design assumptions regarding compliance to the prescribed interventions, time until the experimental intervention achieves full effect, and the frequency of endpoints directly impact on the number of endpoints observed. Terminating a cancer prevention trial before adequate information is obtained, thus severely flawing its interpretation, requires ongoing awareness. Interpretation of a cancer prevention trial should include several added steps: first, investigators to critically review the actual manner in which the trial was conducted; second, carry out an appropriate analysis of the data; and third, review the results and note exceptions or limitations in the data. The results of the trial should be contrasted with previous studies. Implications of the results to future trials should be considered. Finally, these interpretations should be documented in a written report and made available to the scientific community.