A systematic review highlights threats to validity in studies of barriers to cancer trial participation.

OBJECTIVE: To investigate the barriers, modifiers, and benefits involved in participating in randomized controlled trials of cancer therapies as perceived by health care providers and patients. STUDY DESIGN AND
SETTING: We conducted a systematic review of the literature to identify published and unpublished studies in any language using electronic databases searched from 1996 to 2004, contact with experts, and reference lists. All study designs were acceptable provided relevant data were reported. Two reviewers were involved in the selection of studies, data extraction, and quality assessment processes. Studies were combined in a narrative synthesis.
RESULTS: Fifty-six studies met the inclusion criteria and represented the perspective of the patient or the health care provider or both. Although a range of barriers to trial participation were identified, a number of threats to the internal and external validity of the included studies limited interpretation of the evidence.
CONCLUSION: The limitations within the evidence base do not permit a clear interpretation of the barriers, moderators, and benefits involved in participation in cancer trials. We recommend that trialists prospectively identify the issues relevant to a particular trial using the current research as a starting point. We report checklists to guide this process.