Oral Contraceptive Use and Breast Cancer Risk Assessment: A Systematic Review and Meta-Analysis of Case-Control Studies, 2009–2020

Simple Summary Breast cancer (BrCa) is a heterogeneous disease and has important variability according to ethnicity and race with respect to incidence, clinical characteristics, and prognosis. Numerous epidemiological studies indicate that BrCa and it’s also related to environmental factors. We, therefore, undertook a systematic review of the literature regarding BrCa risk in women who used OCs based on case-control studies carried out in the years 2009–March 2020 and then performed a meta-analysis of relevant data. Increased BrCa risk was associated with early menarche, nulliparous, non-breastfeeding, older age at first parity, postmenopause, obesity, smoking, and family history of BrCa. Abstract To perform a meta-analysis of case-control studies that addressed the association between oral contraceptive pills (OC) use and breast cancer (BrCa), PubMED (MEDLINE), Embase, and the Cochrane Library were searched to identify case-control studies of OC and BrCa published between 2009 and 2020. We used the DerSimonian–Laird method to compute pooled odds ratios (ORs) and confidence intervals (CIs), and the Mantel–Haenszel test to assess the association between OC use and cancer. Forty-two studies were identified that met the inclusion criteria and we included a total of 110,580 women (30,778 into the BrCa group and 79,802 into the control group, of which 15,722 and 38,334 were using OC, respectively). The conducted meta-analysis showed that the use of OC was associated with a significantly increased risk of BrCa in general, OR = 1.15, 95% CI: 1.01 to 1.31, p = 0.0358. Regarding other risk factors for BrCa, we found that increased risk was associated significantly with early menarche, nulliparous, non-breastfeeding, older age at first parity, postmenopause, obesity, smoking, and family history of BrCa. Despite our conclusion that birth control pills increase the cancer risk being supported by extensive previous studies and meta-analyzes, further confirmation is required.


Introduction
Among malignant tumors, BrCa is the leading type of cancer that affects women in most countries. According to the predictions of the WHO's International Agency for Research on Cancer, the specific age-standardized incidence rate for cancer of the female breast worldwide is 46 cases per 100,000 women. Herein, incidence rates are elevated in Australia/New Zealand (94), Western Europe (93), Northern Europe (90), and North America (85). In contrast, incidence rates in sub-Saharan African regions, particularly in A systematic review and meta-analysis of published case-control studies assessing the impact of OC on the risk of female BrCa development was performed based on the guidance "Preferred Reporting Items for Systematic Reviews and Meta-analysis for 2015 protocols (PRISMA-P 2015)" [19]. Our research was limited to articles published from 2009 to March 2020.
The contents of the medical databases of PubMed (MEDLINE), Embase, and the Cochrane Library were reviewed to identify studies related to the assumptions of our work. To provide a complete overview of the available relevant studies, we additionally scrutinized references to previously published review articles, meta-analyses, and other publications.
The following inclusion criteria were established in the selection of studies: (i) full-text articles published between 2009 and 2020; (ii) case-control design (design (populationand hospital-based)); (iii) data on the correlation between OC use and BrCa; (iv) articles written in English; (v) data included in the articles were sufficient to calculate the odds ratio (OR) and 95% confidence interval (CI); (vi) at least 20 subjects use of OC included in the case group; and (vii) if there was an overlap in the cases included, only the latest and most comprehensive data were selected. The exclusion criteria were as follows: insufficient quantitative data; the results were reported as graphics; duplicate reports; and articles published in languages other than English [20].
Two reviewers independently checked the titles and abstracts of all papers retrieved from databases. They next extracted relevant study data from the full-text papers selected for inclusion. Any disagreements were resolved by consensus. Articles were initially evaluated according to title and/or abstract. Subsequently, the decision was made to include or exclude after independent and double analysis and full tests of selected research. After approval, the studies were qualified for meta-analysis and collection of data on their clinical and methodological characteristics.

Data Extraction
The following data were extracted for each study: (i) clinical and methodological study characteristics such as last name of the first author, publication year, the country in which the study was performed, name of the study, years of data collection, number of cases and control subjects, and source of cases; (ii) information on the usage of OC in both groups (ever/never); and (iii) BrCa incidence depending on menarche, parity, breastfeeding, menopausal status, family history of BrCa, nutritional status, diabetes, and tobacco smoking.

Assessment of Study Quality
Methodological quality was evaluated by means of the Newcastle-Ottawa scale (NOS) for quality assessment of the included studies. In this evaluation, scores from 0 to 3, from 4 to 6, and from 7 to 9 were given for low, medium, and high quality, respectively [21]. With this tool, each study in the meta-analysis was assessed in three separate categories: selection of cases and controls, comparability of cases and controls on the basis of the design or analysis, and ascertainment of exposure. The NOS quality stars ranged between 4-8, and the average score was 5.76 for included studies. Fifteen (35.71%) studies were regarded as high quality (NOS ≥ 7 points), and 27 (64.29%) studies were regarded as medium quality (NOS ≥ 4 points).

Statistical Analysis
The meta-analysis of summary statistics from individual studies was performed through STATISTICA 13.3 software (StatSoft Poland, Krakow, Poland), using the Medical Package program. For each study, we constructed separate two-by-two (2 × 2) contingency tables to calculate the odds ratios (OR) and 95% confidence intervals (CIs), cross-classifying OC users and occurrence of BrCa. The Mantel-Haenszel test was used to assess the relationship between OC use and BrCa. The meta-analysis combining the ORs across studies was conducted using the DerSimonian-Laird random-effects model [22]. The random-effects meta-analysis model was used due to the diversity of research in terms of, for example, design and population. In the random-effects model, it is assumed that there is no common effect size for independent studies. Instead, each study is assumed to have a different population effect size, which is a random variable and has a normal distribution. Therefore, there is a difference between the size of the effects of individual studies. Thus, the variance of the effect size in the random-effects model is the sum of the variance within and between studies. The weighting of the studies in the meta-analysis was calculated on the basis of the inverse of the sum of 'within study' and between studies variances.
Heterogeneity was assessed graphically by employing a forest plot and statistically by applying the Q test and I 2 index. I 2 values of 25%, 50%, and 75% have been regarded as respectively representing low, moderate, and high heterogeneity between studies [23].
Publication bias was explored via funnel plots and estimated using Begg's and Egger's tests [24,25]. For all the analyses, a forest plot was generated to display results, whereby di- amonds represent study-specific odds ratios; 95% CIs for individual studies are represented by horizontal lines.

Results
We identified 443 references through the medical electronic databases PubMed (MED-LINE), Embase, and the Cochrane Library up to March 2020. We excluded 346 citations on the basis of titles and abstracts, and 99 citations after reviewing the full texts. These studies were excluded because the full text was not available in the English language, they were letters to the editor, commentaries, review articles, of inappropriate designs (studies of cross-sectional, randomized controlled trials, prospective/retrospective cohort studies), or BrCa was only reported. Moreover, exclusion occurred because the subjects were BRCA mutation carriers, women with BrCa were the control group, the papers were not about female BrCa, the researchers were investigating mortality or prognosis of BrCa, etc. Eventually, 42 case-control studies were qualified in the review and meta-analysis ( Figure 1).   Figure 1. Flowchart of the selection procedure for studies included in the current review and meta-analysis. In total, 110,580 women (30,778 in the group with BrCa and 79,802 in the control group, of which 15,722 and 38,334 used of OC, respectively) were included in the meta-analysis. Of the studies included: • seven (7) case-control studies were conducted in the Americas, (1) was an international, multicenter research study.
Characteristics of selected works are shown in Table 1.  Figure 2 shows the change incidence of BrCa risk related to OC use in each study and overall. An increase in the risk of BrCa among women using OC was reported in 23 of 42 trials, including the increase being statistically significant at 14 trials. In comparison, a decrease in the risk of BrCa was observed in participants not using OC in 19 of 42 studies, with the reduction being statistically significant in 6 trials. Meta-analysis using a randomeffects model show moderate, statistically significant increase of BrCa risk: OR = 1.15 (95% CI, 1.01 to 1.31), p = 0.0358. This was accompanied by high heterogeneity: I 2 = 92.32%.

Discussion
The presented meta-analysis of the magnitude of effects of OC on BrCa risk revealed a statistically significant slight increase of cancer risk, OR = 1.15 (95% CI; 1.01 to 1.31). The results from our study were, to a large extent, consistent with the results described in previous studies. For example, in 2018, 1245 BrCa cases were found in the analysis of data derived from a prospective national population-based cohort study carried out from 1991 to 2007 of 74,862 Norwegian premenopausal women. These authors saw a statistically insignificant increased risk of BrCa in participants using hormonal contraception. Here, HR (hazard risk) = 1.12, 95%: CI 0.99 to 1.26 [68]. Moreover, over 11,000 BrCa cases were diagnosed from 1995 to 2012 in a large prospective cohort study published in 2017 involving 1.8 million Danish women. The authors of this work noted a moderate significant increase in the relative risk of BrCa (RR = 1.20, 95% CI: 1.14 to 1.26) among users of hormonal contraceptives [69]. Furthermore, the Royal College of General Practitioners Oral Contraception Study conducted in the years 1968-2012 involved a total of 28,993 OC taking women (cancer cases = 4661) and a total of 17,039 women who never took OC (cancer cases = 2341) and found a slight, statistically non-significant increase in the risk of BrCa was evident. Here, IRR (incidence rate ratio) = 1.04, 95% CI: 0.91 to 1.17 [70]. Our previous meta-analysis covering the period 1960-2010 [16] estimated 79 case-control studies conducted between 1960-2010, including a total of 72,030 incidents, histologically confirmed cases of BrCa and 123,650 population/hospital controls. A decrease was observed in cancer risk in OC users before age 25 years (0.91, 0.83-1.00). However, the use of OCs before the first full-term pregnancy significantly increased the risk of BrCa (OR, 1.14, 1.01-1.28, p = 0.04), as did OC use longer than 5 years (1.09, 1.01-1.18, p = 0.02). Pooled crude odds ratios of BrCa in everusers of OC was 1.01 [95% confidence interval (CI), 0.95-1.07], compared with never-users. There was no significant increase in risk among premenopausal women (1.06, 0.92-1.22), postmenopausal women (0.99, 0.89-1.10), or nulliparous women (1.02, 0.82-1.26) [16].
Gierisch et al. [71] also conducted a meta-analysis. This work covered fifteen casecontrol studies (38,682 women) and 8 cohort studies (317,341 women) published between 2000 and 2012. It assessed the association between OC use and BrCa incidence. Overall, in women taking OC, BrCa risk increased slightly, albeit statistically significantly. Here, OR = 1.08 (95% CI, 1.00-1.17). These results were similar to those derived from the metaanalysis by Zhu et al. [72], which included a total of 13 prospective cohort studies on OC use and BrCa risk (11,722 BrCa cases and 859,894 participants) that were identified by searching databases from 1960-2012. In the meta-analysis, the RR (relative risk) of BrCa in women using OC was 1.08 (95% CI: 0.99.17) when compared with never-users. What is more, in a recent publication by Del Puo et al. [73], which analyzed the results of two meta-analyses on the incidence of BrCa among users of hormonal OC, a statistically significant overall increase in the risk of BrCa (OR = 1.08 (95% CI 1.00 to 1.17)) was indicated.
The results of our meta-analyses of potentially modifying risk factors for BrCa indicate that menarche before the age of 12 years, nulliparous, non-breastfeeding, first pregnancy in old age, postmenopause, obesity, smoking, and family history of BrCa significantly increase the risk of development of BrCa. The majority of the above risk factors are associated with a longer duration of estrogen exposure [74][75][76][77][78][79].
We are aware that drawing final conclusions from the results of our meta-analysis requires caution given the number of limitations encountered in its construction. First, we limited the search to studies published in English, and these were identified through electronic databases. The possibility of not reaching out to all publications on this topic may have an effect on the value of the results [80]. Secondly, the studies, including summary estimates, are vulnerable to various types of bias. Retrospective self-reporting of using OC, for example, may be associated with overestimation or underestimation of actuality. Moreover, mistakes may be made in control group recruitment under hospital scenarios. Additionally, mistakes can be made when cases are recruited for study based on the admission of OC use. Thirdly, a lack of a uniform definition of "ever" use of OC exists (this affects the various periods of exposure to OCs), as defined by the times that pill self-administration was begun and ended. Such fuzziness may lead to misclassification, and this, in turn, may weaken the true association between OC use and BrCa affliction (as may occur equally among cases and controls). The limitations should also take into account other coexisting factors, such as the use of different OC and other genetic factors that were not explored in the studies analyzed, such as BRCA1 and BRCA2 mutations (participants were not tested for the presence of mutations in the BRCA1 and BRCA2 genes). Furthermore, a prospective study needs to be conducted to confirm our findings.

Conclusions
In conclusion, the results from this meta-analysis suggest that OC using is associated with a modest, statistically significant increase in the risk of BrCa. Despite our conclusion that birth control pills increase cancer risk, and that our conclusion is supported by extensive previous studies and meta-analyzes, further confirmation is required. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.