Ionizing radiation
At present, the primary environmental risk factor identified for meningioma is exposure to ionizing radiation (IR) with risks from six fold to ten fold reported [
20‐
23]. At high dose levels, data exist for atomic bomb survivors showing a greatly increased risk for meningioma [
21]. Evidence also exists for lower dose levels. In one of the most well-known studies of ionizing radiation and meningioma risk, children who were given radiation therapy for scalp ringworm in Israel between 1948 and 1960 (the Tinea Capitis Cohort), were observed to have a relative risk of almost 10 for meningioma [
24]. A number of studies have linked the number of full-mouth dental radiographs to risk of meningioma (reviewed in [
25]) although the sample sizes are limited and some subsequent studies (also small in size) did not replicate earlier studies [
26,
27]. However, the most recent case/control study of 200 meningioma patients reported that patients reporting full-mouth X-rays had a significantly increased risk of meningioma (OR 2.06, 95% CI 1.03, 4.17) although evidence for a dose response relation was lacking (
P for trend = 0.33) [
28]. Radiation therapy for intra-cranial tumors has also been linked to meningioma risk [
22]. No recent large-scale studies of meningioma risk relative to ionizing radiation exist. Such studies are still highly relevant in the current era in which X-ray doses for dental and other procedures have decreased, since new radiographic procedures with significant exposure risks have been introduced, including computed tomography (CT).
Hormones
An association between hormones and meningioma risk has been suggested by a number of findings including the increased incidence of post-pubertal disease in women versus men (2:1) with the highest ratio of 3.15:1 during the peak reproductive years (Fig.
1), the presence of estrogen, progesterone, and androgen receptors on some meningiomas, an association between breast cancer and meningiomas (see below), indications that meningiomas change in size during the luteal phase of the menstrual cycle and pregnancy, and the regression of multiple meningiomas in a patient following cessation of estrogen agonist therapy [
29,
30]. Despite these sentinel clues, meningioma is far from exhibiting a “hormone-fed” character in the clinic and epidemiologic measures of endogenous and exogenous hormones are not consistently associated with meningioma incidence.
Researchers have only begun to address the question of whether the use of exogenous hormones such as oral contraceptives (OC) and/or hormone replacement therapy (HRT) is associated with an increased risk of meningioma [
5,
29,
31‐
35]. Data from two cohort studies and several case/control studies exist. In a case/control study nested within the Nurse’s Health Study (NHS) (including 125 cases of meningioma), the relative risk of meningioma associated with hormone use for pre-menopausal women was 2.48 (95% CI 1.29, 4.77) when compared with postmenopausal women who had never used hormones [
34]. For postmenopausal women who were hormone users the relative risk was 1.86 (95% CI 1.07, 3.24). No excess risk was associated with past hormone use. No association was found for past or current use of oral contraceptives. Recently published data from a cohort study of 1.3 million women with a mean age of 55.9 and recruited from 1996 to 2001 (The Million Women Study) did not find an association between OC use (OR 1.06, 95% CI 0.81–1.38 for use within the past five years) and meningioma risk (
n = 390) but did not report results for HRT use [
5]. In a large and recent case/control study, the Interphone Group reported an increased relative risk of meningioma (
n = 178) among postmenopausal women for ever-use of HRT (OR 1.7, 95% CI 1.0–2.8) [
35]. Women who had used long-acting hormonal contraceptives also had an increased risk of meningioma; the odds ratio for at least 10 years of use was 2.7 (95% CI 0.9–7.5). A retrospective records-based cohort study using the Mayo Clinic Jacksonville patient database between 1993 and 2003 confirms the positive NHS findings (OR 2.2, 95% CI 1.9–2.6) of an association between HRT use and meningioma risk, comparing the frequency of HRT use among a case population of 1,390 with over 350,000 other women in the health system [
31], while a case/control study including 219 meningioma cases identified from three Chicago area hospitals between 1987 and 1992 reports a protective effect for oral contraceptive use (OR 0.2, 95% CI 0.0–0.8) and a non-statistically significant protective effect associated with HRT use [
32]. This latter study used the spouses of men with back pain as controls. Hence at present, there is limited statistical evidence of an increased risk of meningioma among users of oral contraceptives. Although not definitive, available data suggest an association between the use of hormone replacement therapy and increased meningioma risk. Further evaluation of exogenous hormone use in women with meningioma in a larger study is needed with particular attention to stratification by hormone composition (i.e. estrogen and/or progesterone), duration of and age at use as well as meningioma subgroups defined by tumor receptor expression (see below).
Researchers have also reported conflicting results when examining meningioma risk across categories of pregnancy, menstrual and anthropometric variables [
5,
29,
31‐
36]. When examining age at first menstrual period, investigators in the Nurse’s Health Study observed a relative risk for meningioma for women with age at menarche 12–14 years of 1.29 (95% CI 0.86–1.92) and for women with age at menarche after 14 years a relative risk of 1.97 (95% CI 1.06–3.66) compared to those with menarche before age 12 [
34]. A tendency for increased risk of meningioma for parous compared to non-parous women (RR = 2.39, 95% CI 0.76–7.53) was also observed, although this value was not statistically significant [
34]. In a second nested case/control study, Lambe et al
. examined 1088 patients with meningioma within the Swedish Cancer Registry and matched to data from the Swedish Fertility Registry [
36]. This group found no association between either parity or age at first birth and meningioma risk, however their analyses were not adjusted for other possible meningioma risk factors such as use of exogenous hormones or radiation exposure history. Data from the Interphone Study suggest that meningioma risk among women aged <50 years is increased with increasing number of livebirths (OR 1.8, 95% CI 1.1–2.8 for three versus no livebirths) but found no association with menopausal status [
35]. The Million Women Cohort reported an increasing risk of meningioma with increasing body mass index (OR 1.46, 95% CI 1.11–1.91) but no association with number of pregnancies or age at first birth [
5]. Body mass index, or more specifically body fat, is positively associated with aromatase activity resulting in higher endogenous estrogen exposure. An additional case/control study which included 219 cases found a protective effect for pregnancy which increased with number of pregnancies and age at first pregnancy [
32]. Neither age at menarche or menopause were reported to show any effect in unadjusted analyses although menopause showed an increased risk (OR 2.0, 95% CI 1.0–4.0) in adjusted analyses. In summary, the association between traditional hormone-based pregnancy and menstrual risk factors and meningioma risk is not consistent and deserve a more formal examination. Such an examination requires precise exposure assessments as well as an incorporation of a more thorough examination of biological features of individual patients data, including the expression of hormone receptors as described below, which may reveal subtypes with more clear evidence of hormone associations.
The functional significance of hormone receptors expressed on meningiomas is still controversial. Hormone receptors (estrogen, progesterone, and androgen) are expressed in an equivalent proportion of meningiomas derived from men and women [
37], but their assessment has only been performed once in an epidemiologic study [
38], and different laboratory methods for receptor expression capture widely varying proportions of positive patients [
39‐
41]. A pilot study of 31 meningioma samples reported that a specific gene expression pattern appeared more strongly associated with PR status than with ER status [
42]. Genes on the long arm of chromosome 22 and near the
NF2 gene (22q12) were most frequently noted to have expression variation, with significant up-regulation in PR positive versus PR negative lesions suggesting a higher rate of 22q loss in PR negative lesions. Pathway analyses indicated that genes in collagen and extracellular matrix pathways were most likely to be differentially expressed by PR status [
42]. The future incorporation of receptor expression into epidemiology studies may revolutionize the field as various molecular markers have done for cancers, for example hormone receptors in breast cancer. The field first needs to harmonize methodology to classify hormone receptors so that different studies can be compared and contrasted.
Association with breast cancer
An association between breast cancer and meningioma has been examined in several studies [
29,
52,
53]. A number of explanations have been proposed for this association including the presence of common risk factors such as endogenous and exogenous hormones as well as shared genetic predisposition, including variants in DNA repair polymorphisms [
52]. A review of the literature as well as an analysis of the association between breast cancer and meningioma using the western Washington State cancer registry data was provided by Custer et al. [
53]. The relative risks observed across existing studies range between 1.5 and 2.0 with the majority statistically significant. Most of these studies have been conducted with tumor registry data and have relatively small sample sizes and none have been able to examine the association while controlling for risk factors which are likely to be shared by the two tumors, such as pregnancy and menstrual variables and exogenous hormone use. The fact that studies which identify risk of breast cancer in women who had meningioma, and vice versa, both have similar magnitude increased risk suggests that there is not a causal relationship between these tumors, rather that they share the same risk factors such as gender, age, hormone induction, and possibly other demographic variables [
53].
Occupation/diet/allergy
Attempts to link specific chemicals with meningiomas in occupationally or industrially exposed groups have proved inconclusive (reviewed in [
54]). An international case/control study found no association between diet and meningioma (
n = 332) [
55]. Although a number of studies which examine the relationship between glial brain tumors and allergic disease such as asthma and eczema have found evidence for an association, little evidence has been found for such an association for meningioma [
6,
8,
56]. A meta-analysis however demonstrated a significant inverse relationship of meningioma with allergy when excluding the single study that was most heterogeneous from the others (pooled
RR = 0.84, 95% CI 0.72–0.98,
P = 0.029) [
57], and a large recent study showed consistent inverse risk with asthma, hayfever, and eczema [
7]. A study of innate immune genes did not find strong evidence of risk imparted by variants in such genes, but more investigation is warranted [
58].
Family history of meningioma
Few studies have examined the relationship between meningioma risk and family history of meningioma. Malmer et al. examined cancer risk in spouses and first degree relatives of brain tumor patients in Sweden and reported that a meningioma diagnosis conferred a two fold increase in meningioma risk to first degree relatives (standardized incidence ratio [SIR] 2.2, 95% CI 1.4, 3.1) but not to spouses of affected individuals [
59]. An inverse association between risk and age at onset was observed with an SIR of 2.5 (95% CI 1.5–4.0) for probands less than 50 years of age versus 1.3 (95% CI 0.6–2.6) for probands older than 50 years of age. Similar analyses by Hemminki et al
. using data from the Swedish and Norweigian Registry Databases, reveal an increased risk with increasing numbers of affected first degree relatives with persons having one or two first degree family members with meningioma (SIR 1.6, 95% CI 1.3–42.0, and SIR 5.0, 95% CI 0.9–14.8), respectively [
60]. Despite the fact that up to one to three percent of the adult population may harbor a meningioma [
2,
3], the total number of families with multiple members diagnosed with meningioma are relatively rare (indicating, in part, a wide spectrum of phenotypic expression with respect to clinical import and hence screening undertaken), and most such families are currently attributed to inherited
NF2 mutations. At present no family based linkage or segregation analyses studies of meningioma have been reported.