Factors associated with brain tumor risk : with focus on female sex hormones and allergic conditions

Abstract

Every year approximately 1200 people in Sweden are diagnosed with a brain tumor. The two main histological types of brain tumors, gliomas and meningiomas, differ in terms of their localization, histology, prognosis, and probably etiology. Exposure to ionizing radiation and a few rare genetic syndromes are the only unequivocally established risk factors.

The overall aim of this thesis was to look at some specific factors that potentially could be associated with glioma and meningioma risk, and to characterize and compare demographic and socioeconomic factors among participants and non-participants in a case-control study of brain tumors. Data from an international population-based case-control study of brain tumors formed the base for the papers included in the thesis. In papers I and IV only Swedish participants were included. Papers II and III included participants from Denmark, Finland, Norway, Sweden, and England.

We found an increased risk of meningioma associated with use of hormone replacement therapy. We found no association between use of oral contraceptives and meningioma risk, but results indicated an increased risk associated with use of other hormonal contraceptives. No associations were found between use of exogenous female sex hormones and glioma. Increasing number of pregnancies leading to a live birth was associated with an increased meningioma risk among women < 50 years of age, but not among older women. Ever having been pregnant was associated with a decreased glioma risk. Among parous women longer duration of breast-feeding and older age when giving birth for the fist time was associated with an increased glioma risk.

Allergic conditions (asthma, eczema, hay fever, other allergy) were associated with a reduced glioma risk, OR=0.70 (95% CI: 0.61-0.80), which is in accordance with previously published studies. We showed that the inverse association was primarily related to current allergic condition and was not influenced by anti-allergy treatment. Allergic conditions were not associated with meningioma risk, except for a decreased risk associated with eczema.

Records were linked to registries at Statistics Sweden to gather information on socioeconomic status, income, and education, for all participating and non-participating cases and controls in the Swedish part of the study. Working status and income level were positively associated with participation among both cases and controls. A high income level was associated with a slightly increased risk of glioma, but not related to meningioma risk.

Conclusions: Our findings imply that female sex hormones influence the occurrence of meningioma and glioma. The results also indicate that immunological factors are of importance for glioma tumorigenesis. Non-participation related to socioeconomic factors should always be acknowledged as a potential source of selection bias, but the influence was not large in our study due to the fact that the level of participation was comparable between cases and controls and participation was similarly influenced by socioeconomic factors among cases and controls.