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Cigarette smoking and risk of adult glioma: a meta-analysis of 24 observational studies involving more than 2.3 million individuals

Background Cigarette smoking has been shown to be a risk factor for adult glioma by some but not all studies. We conducted a meta-analysis to systematically assess the potential association. Methods PubMed and EMBASE were searched from the date of their inception to October 1, 2015, to identify relevant articles. Reference lists from these articles were reviewed to identify additional studies. Both cohort and case–control studies were included. Fixed-effects models were used to calculate the overall relative risk (RR) with corresponding 95% confidence intervals (CIs). Results The final analysis included 24 studies (seven cohort and 17 case–control studies), involving more than 2.3 million individuals. The combined RR was 1.04 (95% CI: 1.00, 1.09; P=0.073) for ever-smokers, 0.97 (95% CI: 0.88, 1.07; P=0.574) for current-smokers, and 1.07 (95% CI: 0.98, 1.16; P=0.130) for past smokers, with little evidence of heterogeneity. Omission of any single study from the analysis had little effect on the result. No evidence of publication bias was found. A small but statistically significant increase was found in past smokers in females (RR: 1.13, 95% CI: 1.00, 1.28; P=0.046) but not in males. Conclusion In general, there was no association between cigarette smoking and adult glioma. The small but statistically significant association in females requires further investigation.


Introduction
Glioma accounts for 70% of all brain tumors in adults, with an estimated incidence of 6.0/100,000 per year. 1,2 The development of glioma is clearly associated with ionizing radiation. 3,4 Putative association with other factors, including pesticide exposure, 5 alcohol consumption, 6 smoking, 7 obesity, 8 and sex hormones, 9 has also been noted. Cigarette smoking is the most important modifiable cause of many types of human cancers, including the respiratory, digestive, hematologic, and urinary systems. 10 Association between adult glioma and cigarette smoking has been reported both in females 11,12 and in males. 13 However, increased risk of glioma in smokers has not been replicated in many other studies, including case-control studies 11,[14][15][16] and cohort studies. [17][18][19][20][21] A meta-analysis of 17 studies in 2009 failed to show significant association, but a small and significant increased risk was noted in a cohort study. 7 Recently, a series of cohort studies and case-control studies have been published, [21][22][23][24][25] again, with conflicting results. In particular, a recent large population-based case-control study involving 13,000 individuals in the People's Republic of China supported a positive association between cigarette smoking and gliomas. 25 As a result, a reanalysis is appropriate.

Materials and methods literature search
Two investigators (HXL and XXP) independently conducted a systematic search of the PubMed and EMBASE databases to identify relevant publications from the inception of the databases to October 1, 2015. The search terms were (glioma OR brain neoplasm OR brain cancer OR brain tumor) AND (smoke OR smoking OR cigarette OR tobacco OR smoker). The references of the retrieved articles were checked to identify additional studies.

inclusion and exclusion criteria
The criteria for data inclusion were: 1) studies investigating the association between cigarette smoking and risk of adult glioma; 2) the study design being cohort or case-control study; 3) odds ratio (OR), relative risk (RR), or hazard ratio (HR), with corresponding 95% confidence intervals (CIs), being provided or calculated from raw data; and 4) at least one of the following smoking exposure variables being provided: ever-versus never-smokers, current versus never-smokers, past versus never-smokers, duration, intensity, or cumulative smoking (pack-year). In the cases of multiple publications, only the one with the most complete information was used. Case report, meta-analysis, reviews, comments, and editorials were not included.

Data extraction
Two investigators (HXL and XXP) independently extracted the following data: first author's name, year of publication, study design, study country/period, data source (cohort study), follow-up years (cohort study), sample size, age of subjects, diagnostic criteria of glioma, sex strata, research instrument, control source (case-control study), smoking variables and adjustment factors, and risk estimate (RR, OR, HR, and 95% CI). Disagreements were resolved through discussion.

statistical analysis
The meta-analysis was conducted using STATA 12.0 (StataCorp LP, College Station, TX, USA). Since glioma is a rare disease, OR and HR are practically equivalent to RR. 26 RR was used throughout this study. Heterogeneity was evaluated with Cochran's Q statistic 27 and I 2 statistic, 28 and defined as low (I 2 25%), moderate (I 2 =25%-50%), or high (I 2 50%). RR was calculated using a fixed-effects model when I 2 50%, and using a random-effects model otherwise. Sensitivity analysis was carried out to evaluate the potential effects of the individual study to the overall results, as described previously. 29 Publication bias was assessed using Begg's funnel plots and Egger's regression test. 30,31 Sensitivity analysis and publication bias were assessed for ever-versus never-smokers only. All analyses were two sided, with P0.05 indicating statistical significance.
Ever-, current-, and past smokers were defined as in the original publications. To probe into potential heterogeneity, several subgroup analyses were performed according to study design (cohort versus case-control study), control source (population-based versus hospital-based), research instrument (questionnaire versus interview for case-control study; mailed versus self-administered questionnaire for cohort studies), sex, geographical area, age at which smoking was started (20 versus 20 years), smoking intensity (20 versus 20 cigarettes per day), duration (20 versus 20 years), cumulative smoking (15 versus 15 pack-years), and year of study publication (before 1990 versus 1990-2000 versus 2000-2010 versus 2010-now).

search results and characteristics of studies
The initial screening identified a total of 873 articles (n=417 from PubMed and n=456 from EMBASE). After reading the titles and abstracts, 32 papers were selected for further processing. Reviewing the full text, eleven articles were eliminated for the following reasons: six for irrelevant articles; 32-37 two articles did not provide the data "neversmokers", "ever-smokers", or "current-smoker"; 38,39 and two articles did not have sufficient data for the variable "ever-, current-, or past smokers", but were included the subgroup analysis based on smoking "intensity". 40,41 Two articles were from the same study, 11,42 and so only the one with most complete information was included. 11 One article included three case-control studies. 22 The final analysis in this study included a total of 24 studies from 22 articles. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][43][44][45][46][47][48][49] The selection process is shown in Figure 1.

Stratified analysis
To examine the potential heterogeneity, several subgroup analyses were performed. The results of "ever smoker versus never smoker" are listed in Table 2.
In the stratified analysis by study design, more than 2,756,887 participants and 1,956 glioma cases were included in cohort studies, and more than 6,977 participants and 15,063 glioma cases were included in case-control studies. The pooled RR for ever-smokers (n1,329,577) versus never-smokers (n913,450) was 1.05 (95% CI: 0.97, 1.13; P=0.200) in cohort studies, and the RR for ever-smokers (n9,999) versus never-smokers (n12,283) was 1.04 (95% CI: 0.98, 1.10; P=0.201) in case-control studies. Within the case-control studies, the RR for ever-smokers was 0.98 (95% CI: 0.87, 1.11) in hospital-based studies and 1.06 (95% CI: 0.99, 1.13) in population-based studies.

sensitivity analysis and publication bias
Removing one study at a time did not significantly alter the results, resulting in the pooled RR having a narrow variation range from 1.01 (95% CI: 0.81, 1.26) to 1.06 (95% CI: 0.88, 1.28; Figure 5). No evidence of publication bias was detected for Begg's (P=0.862) and Egger's test (P=0.630; Figure 6).

Discussion
To the best of our knowledge, this meta-analysis is the most comprehensive analysis of a putative association between cigarette smoking and adult glioma risk up to date. The analysis included seven cohort studies and 17 case-control studies involving 2.3 million subjects. The overall analysis did not find significant association between cigarette smoking and glioma, regardless of smoking definition (ever-, current-, or past smokers). Subgroup analysis also failed to find an association regardless of the stratification factors, with an    exception of a small but statistically significant increase in females for past smokers (RR: 1.13, 95% CI: 1.00, 1.28; P=0.046).
Subgroup analysis in this study identified a small but statistically significant association between smoking and glioma among females but not males. The association was in accordance with results from two studies, 20,43 but not others. 11,13,15,17,18,21,23,25 Efird et al 12 reported on a cohort of 133,811 subjects to the Kaiser Permanente Medical Care Program of Northern California, with follow-up of up to 21 years; 130 cases were diagnosed with glioma, and compared with never-smokers, an increased risk of gliomas was seen among female smokers smoking 1 pack (RR=1.7, 95% CI=0.9, 3.1), 1-2 packs (RR=1.8, 95% CI=0.8, 4.1), and 2 packs (RR=3.0, 95% CI=0.9, 10.6) per day, respectively; but no association was observed in males.
Our result stratified analysis by past and current smokers in females showed that past females smokers had increased risk of glioma, but not current smokers. The result was consistent with Silvera et al's study, 43 but our result could have been due to chance. Silvera et al 43 reported that pastsmokers were at increased risk of glioma compared with never-smokers in females, but upon stratifying past smokers by years since having quit smoking, they found an inverse association between past-smokers who stopped smoking 10 years prior to baseline compared with those who stopped smoking within the 10 years prior to baseline (HR =0.39, 95% CI: 0.19, 0.82), indicating that the association between past smokers and glioma may have been driven by females who recently stopped smoking. 43 The mechanism underlying the association between cigarette smoking and adult glioma in females but not in males is unknown. However, some causal relationships are conceivable. First, several animal studies showed that smoking increases the levels of certain sex hormones and sex hormones promote tumor progression, 50,51 including glioma. 9 Second, the risk of smoking-related cancers (such as in the esophagus, lungs, and oral cavity) has been observed to be higher in females compared to males in some studies, [52][53][54] and these phenomena may also appear in brain cancer. Third, females may be more susceptible to carcinogens in cigarette smoke than males in terms of a greater frequency for specific mutations in the p53 55 and K-RAS 56 genes, which increased the concentration of carcinogen adducts in smoking-affected tissue, 57 elevated the expression of certain enzymes in the cytochrome p450 family, 58 reduced capacity for DNA repair, 59 and may also induce glioma. Furthermore, cigarette smoking affects both the innate and adoptive immune arms, and leads to a series of immunological disorders (such as atopic diseases and asthma), 60 and females are more susceptible to the immunological disorders. 61 It may be considered that smoking affects immunological function, and immunological factors induce brain cancer. [62][63][64] Little evidence of heterogeneity was observed in the current analysis. High between-study heterogeneity was only observed from subgroup results: the group self-administered submit your manuscript | www.dovepress.com Dovepress Dovepress 3520 li et al questionnaire, duration (short-term, long-term), intensity (heavy), and year of study publication (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010). It is not surprising given the differences in study designs, characteristics of populations, definition of cigarette smoking, geographic area, and adjustment for confounding factors. As a result, a random-effects model, a conservative method to estimate the pooled effect, was used in these subgroup analyses.
As a crucial barrier that maintains brain homeostasis, the blood-brain barrier (BBB) selectively excludes many endogenous and xenobiotic substances, including some carcinogens, from entering the brain. 65,66 Whether N-nitroso compounds, the major carcinogens in cigarette smoke from cigarette smoking, could cross the BBB in adults remains uncertain. A previous study in rats showed that, upon intravenous administration, N-nitroso compounds could induce glioma formation. 67 Nicotine could also stimulate the malignant behavior of glioma cells. 68 There are some in vivo evidences that showed that nicotine could increase the permeability of the BBB, by allowing carcinogens (eg, nitrosamines) to reach the brain. 69 However, another study failed to show that N-nitrosamines could cause cancer in the brain. 70 Therefore, future studies should explore the biological mechanisms between cigarette smoking and glioma risk.

3521
cigarette smoking and risk of adult glioma Figure 6 Begg's funnel plot of all 24 studies for the associations between eversmoking and glioma. Note: each point represents separate study for the indicated association. Abbreviations: rr, relative risk; se, standard error. Several important strengths should be mentioned in our analysis. First, the major strength of this meta-analysis is the large number of participants and diversity of studied populations. Compared with the former meta-analysis in 2009, 7 our study included 17 case-control and seven cohort studies involving more than 2.3 million individuals. Second, a number of subgroup analyses were conducted in our study. No publication bias was found in this study, which further supported the robustness of the study results.
There are some potential limitations in this study. First, only English language publications were included, which may omit other languages studies. Second, both cohort and case-control studies were included, and so methodological differences and confounding factors were unavoidable. However, we conducted a separate analysis for the two types of studies, with consistent results. Third, moderate-to-high heterogeneity was observed in some subgroups, which could not be avoided because of the confounding factors from original studies.

Conclusion
Cigarette smoking is not significantly associated with adult glioma in the overall population. However, there is a small but statistically increased association in females smokers.