Generalizability of cancer clinical trial results: prognostic differences between participants and nonparticipants

The generalizability of clinical trial results is questionable, because fewer than 5% of cancer patients participate. The authors examined the comparability of clinical trial participants and nonparticipants and the potential impact of differences.

A retrospective cohort of 19,340 cancer patients who were diagnosed between January 1990 and December 1997 was characterized by trial participation. The distributions of prognostically important factors among trial participants were compared with the distributions among nonparticipants and the population of patients diagnosed during the same period in the Surveillance, Epidemiology, and End Results population. The impact of these factors on survival was examined by using a Cox proportional hazards analysis.

Trial participants were younger and had better performance status and fewer comorbid conditions compared with nonparticipants. However, participants were more likely to have locally advanced disease, positive lymph node status, poorly differentiated tumors, liver metastases, and multiple metastatic sites. The former factors were associated with significantly longer survival, whereas the later factors were associated with significantly shorter survival.

The lack of comparability between trial participants and nonparticipants called into question the generalizability of clinical trial results. Although selective recruitment for clinical trials is justified, the authors encourage the use of population-based trials of effectiveness in "all comers."