Meta-analysis in hematology and oncology.

In 1992, a review article about meta-analysis identified only 15 meta-analyses of randomized, controlled trials of cancer therapy. Since then, the total number of meta-analyses in this field has increased almost sixfold. More importantly, the number of randomized, controlled trials in this discipline has also grown tremendously. The expansion in the literature will provide a fertile ground for future meta-analyses. The quality of the recent publications has also improved. An ongoing world-wide effort, the Cochrane Collaboration, is systematically assembling and synthesizing several hundred thousand randomized, controlled trials to improve the delivery of health care. Meta-analysis has many important advantages. It allows the viewing of the complete picture of the evidence. The advent of meta-analysis has sensitized researchers to issues of quality and has improved methodology in clinical research. Detection and explanation of bias and heterogeneity are prime objectives of meta-analysis in clinical research. An array of methods has been developed that allows a better understanding of bias and heterogeneity, beyond simple averaging of results from diverse studies. Meta-analyses of individual patient data, in particular, may promote the development of international collaborations. Several examples of their application are already available in oncology. Meta-analysis may point out deficiencies in the study design of past and current studies, suggest the need for new studies, and inform researchers about the size and design of these studies. In the end, meta-analysis helps to integrate evidence and make recommendations for medical care and medical practice.