title: Study designs may influence results: the problems with questionnaire-based case-control studies on the epidemiology of glioma.
abstract: Glioma is a rare brain tumour with a very poor prognosis and the search for modifiable factors is intense. We reviewed the literature concerning risk factors for glioma obtained in case-control designed epidemiological studies in order to discuss the influence of this methodology on the observed results. When reviewing the association between three exposures, medical radiation, exogenous hormone use and allergy, we critically appraised the evidence from both case-control and cohort studies. For medical radiation and hormone replacement therapy (HRT), questionnaire-based case-control studies appeared to show an inverse association, whereas nested case-control and cohort studies showed no association. For allergies, the inverse association was observed irrespective of study design. We recommend that the questionnaire-based case-control design be placed lower in the hierarchy of studies for establishing cause-and-effect for diseases such as glioma. We suggest that a state-of-the-art case-control study should, as a minimum, be accompanied by extensive validation of the exposure assessment methods and the representativeness of the study sample with regard to the exposures of interest. Otherwise, such studies cannot be regarded as 'hypothesis testing' but only 'hypothesis generating'. We consider that this holds true for all questionnaire-based case-control studies on cancer and other chronic diseases, although perhaps not to the same extent for each exposure-outcome combination.
authors.affiliation: Oncology Clinic, Finsen Centre, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100 Copenhagen, Denmark.|Unit of Survivorship, Danish Cancer Society Research Center, Strandboulevarden 49, 2100 Copenhagen, Denmark.
authors.forename: Christoffer
authors.initials: C
authors.lastname: Johansen
authors.affiliation: Section of Environment and Radiation, International Agency for Research on Cancer, 150 Cours Albert Thomas, 69372 Lyon Cedex 08, France.
authors.forename: Joachim
authors.initials: J
authors.lastname: Schüz
authors.affiliation: Unit of Survivorship, Danish Cancer Society Research Center, Strandboulevarden 49, 2100 Copenhagen, Denmark.
authors.forename: Anne-Marie Serena
authors.initials: AS
authors.lastname: Andreasen
authors.affiliation: Unit of Survivorship, Danish Cancer Society Research Center, Strandboulevarden 49, 2100 Copenhagen, Denmark.
authors.forename: Susanne Oksbjerg
authors.initials: SO
authors.lastname: Dalton
mesh_terms: D001932:Brain Neoplasms / Q000453:epidemiology*; D016022:Case-Control Studies; D016021:Epidemiologic Studies; D005910:Glioma / Q000453:epidemiology*; D006801:Humans; D012107:Research Design / Q000592:standards*