Background

Breast cancer is the most common cancer in women with high morbidity and mortality rates [1]. Risk assessment tools estimating the individual’s absolute risk for developing breast cancer and identifying the women at high level of risk are crucial for decision-making about prevention and screening.

The Breast Cancer Risk Assessment Tool (BCRAT) [2], also known as the Gail model, was the most widely used appraisal tool for predicting the absolute risk of developing breast cancer. Individuals with 5-year risk exceeding 1.67% were considered high risk [3]. In 1992, the tool was modified to specifically predict invasive breast cancer, and this updated model, referred to as Gail model 2 (Caucasian-American Gail model) [4], has been used for determining the eligibility of subjects for chemoprevention of invasive breast cancer [5, 6]. In addition, this modified Gail model was also updated subsequently to predict the risk for other ethnic populations, such as African-American [7] and Asian-American [8] females.

A number of studies have been conducted to validate the Gail model in American [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27], European [28,29,30,31,32,33,34,35,36,37], Asian [38,39,40,41,42,43,44,45,46,47,48,49,50] and Oceanian [51, 52] women. However, these studies showed variability in their calibration (expected-to-observed (E/O) ratio) and discrimination (Concordance-statistic (C-statistic) or area under the curve (AUC)). Although three systematic review and meta-analyses validated the Gail model previously [53,54,55], 19 studies [13, 14, 17,18,19,20, 22,23,24, 32,33,34,35,36, 38, 40, 41, 51, 52] with inconsistent results have been published subsequently or were not included in the previous meta-analyses. However, the evaluation studies launched in China [39, 42,43,44,45,46,47,48,49,50] have not been incorporated before and the diagnostic accuracy of the Gail model has not been fully evaluated.

There is increasing awareness that a meta-analysis also needs sufficient sample size to get a stable conclusion. Trial sequential analysis (TSA) was introduced to calculate the required information size (RIS) for meta-analysis and to decide whether the evidence was sufficient and conclusive [56, 57].

Here, we conducted a systematic review and meta-analysis to comprehensively evaluate the performance of different versions of the Gail model from three different dimensions (calibration, discrimination and diagnostic accuracy). In addition, the meta-analysis for calibration of the Gail model was also challenged by TSA.

Methods

Study registration

The current systematic review and meta-analysis was performed according to MOOSE guidelines [58] and has been registered with the International Prospective Register of Systematic Reviews (PROSPERO; registration number CRD42016047215).

Literature search strategy

Two investigators conducted a literature search in the PubMed, Embase, WANFANG [59], VIP [60] and China National Knowledge Infrastructure (CNKI) [61] databases for all articles concerning the performance of the Gail model in females.

We used “mammary OR breast cancer OR carcinoma OR tumor OR neoplasm” AND “calibration OR validate OR validation OR screen OR screening OR expected-to-observed ratio OR E/O ratio” AND “Gail model OR breast cancer risk assessment tool OR BCRAT” as medical subject headings (MeSH) in searching for studies evaluating the calibration of the Gail model.

The terms “mammary OR breast cancer OR carcinoma OR tumor OR neoplasm” AND “discrimination OR validate OR validation OR screen OR screening OR sensitivity OR specificity OR area under the curve OR AUC OR C-statistic” AND “Gail model OR breast cancer risk assessment tool OR BCRAT” were used for retrieving publications assessing the discrimination and diagnostic accuracy of the Gail model.

Publications in English and Chinese language between 1 January 1989 (when the Gail model was developed [3]) and 31 July 2016 were included. Listed references were also manually checked for relevant papers.

Inclusion and exclusion criteria

The inclusion and exclusion criteria for this meta-analysis included the following: studies validating the performance of the original (Gail model 1) or modified (Gail model 2) Gail model in women [3, 4]; calibration of the Gail model was prospectively estimated focusing on cohort studies that provided the E/O ratio and its 95% confidence interval (CI) or offered sufficient data for calculating the expected and observed number of breast cancer; discrimination of the Gail model was estimated focusing on the studies providing the C-statistic or AUC and its 95% CI for the Gail model; the diagnostic meta-analysis included publications that provided sufficient data for calculating the true positive (TP), false positive (FP), false negative (FN) and true negative (TN) values of the Gail model, respectively; the threshold of the Gail model was limited to ≥ 1.67%; the sample size should be higher than 100 and the mean follow-up period for the cohort studies should be longer than 1 year; and when multiple publications included the same population, studies with larger sample size or longer follow-up period were incorporated and studies with independent validations in subsequent articles were included.

Literature selection for the systematic review and meta-analysis

For the calibration of the Gail model, 435 studies were found in the electronic databases and 10 were manually retrieved. After careful examination, 419 publications were excluded: 62 were duplicated records, 235 were not related, 70 were reviews and 52 were conference abstracts. In addition, two studies were excluded [27, 62] as they focused on the same population but with smaller sample size than other studies [17, 31]. In the end, 24 studies with 29 datasets were included.

After excluding the duplicated records, 356 studies were retrieved for estimating the discrimination of the Gail model. Of these, 311 were excluded in the preliminary screening and 19 were further eliminated by full-text reading. Moreover, seven studies [31, 62,63,64,65,66,67] were also excluded as they focused on the same population but with a shorter study period or smaller sample size than other included studies [17, 27, 51]. In total, 26 studies incorporating 29 datasets were included in this meta-analysis.

For the diagnostic accuracy of the Gail model, 455 publications were retrieved at the beginning. After preliminary screening and the full-text reading, 13 studies were finally included (Fig. 1).

Fig. 1
figure 1

Flowchart of study selection in the meta-analyses for estimating the calibration, discrimination and diagnostic accuracy of the Gail model. AUC area under the curve, CI confidence interval, CNKI China National Knowledge Infrastructure

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Studies included in the aforementioned three meta-analyses overlapped to some extent, as some of them provided both the E/O ratio and AUC or the diagnostic accuracy of the Gail model [11, 15, 17, 18, 20, 30,31,32,33, 35, 39, 41, 44, 45].

Data abstraction

Two investigators independently extracted data. Relevant information included the first author, publication year, geographic region, versions of the Gail model (Gail model 1 or Gail model 2 for Caucasian-American, Asian-American and African-American women), risk prediction period, study design, study population, sample size, mean age of participants as well as the risk for breast cancer, study period, follow-up period, E/O ratio with 95% CI, C-statistic or AUC with 95% CI and number of true positive (TP), false positive (FP), false negative (FN) and true negative (TN) values. The quality of the included studies was assessed by Newcastle–Ottawa Scale (NOS) [68] and the studies incorporated in the diagnostic meta-analysis were assessed by Quality Assessment Diagnostic Accuracy Studies (QUADAS) [69]. Any discrepancies were resolved by consensus, and where needed the corresponding author was contacted.

Statistical analyses

Calibration assessed how closely the number of subjects predicted to develop breast cancer matched the observed number of breast cancer cases diagnosed during a specific period. This was calculated by E/O ratio and the 95% CI of the E/O ratio was computed as: E/O ratio × exp.(± 1.96 × 1/√(O)) [11]. A well-fitting calibration should be close to 1.0. The discrimination value was assessed by C-statistic, which measures the Gail model’s ability to discriminate the women who will and will not develop breast cancer; moreover, it was considered identical to the AUC in the current study [54]. A C-statistic/AUC of 0.5 was considered as no discrimination, whereas 1.0 indicates perfect discrimination.

The pooled E/O ratio and C-statistic/AUC of the Gail model were calculated using DerSimonian and Laird’s random-effects model [70]. The I2 value was employed to evaluate the heterogeneity among the studies, and subgroup analyses were carried out to identify the source of the heterogeneity. Sensitivity analysis was conducted to assess the influence of each study on the combined effects by sequentially omitting each dataset [71]. Cumulative meta-analyses were launched to investigate the trend of the pooled E/O ratio and C-statistic/AUC ranked by the publication year and sample size [72]. Visualized asymmetry of the funnel plot and Egger’s regression test were assessed to detect publication bias. Pooled effects were also adjusted by the Duval and Tweedie trim-and-fill method [73,74,75].

The pooled estimations of sensitivity, specificity and diagnostic odds ratio (DOR) were calculated using a bivariate mixed-effects model. The DOR is the ratio of risk odds in breast cancer cases relative to that in controls [76]. Publication bias was detected by Deeks’ funnel plot, using 1/root (effective sample size) vs log DOR. P < 0.05 for the slope coefficient indicates significant asymmetry [77].

In the current study, TSA was conducted to determine whether the sample size incorporated in the meta-analysis was sufficient for evaluating the calibration of the Gail model. The included cohort studies are identified as trials for calculating the difference in breast cancer incidence between the expected and observed groups, and accordingly the total sample size is doubled. For the TSA, when the Z-curve crosses the conventional boundary, a significant difference is considered to exist between the expected group and the observed group for breast cancer incidence. Moreover, if the Z-curve passes through the trial sequential monitoring boundary or required information size (RIS) boundary, the evidence is considered sufficient and conclusive. Otherwise, the evidence is adjudged inconclusive and more studies were required to further verify the results [56, 57]. Furthermore, in order to evaluate the effect of the Chinese studies on the performance of the Gail model, a sensitivity analysis was conducted by eliminating the studies retrieved from the WANFANG, VIP and CNKI databases.

Pooled E/O ratio and AUC were synthesized using Comprehensive Meta Analysis version 2.0 (Biostat, Englewood, NJ, USA). Pooled sensitivity, specificity and DOR were conducted with Stata statistical software version 14.0 (StataCorp, College Station, TX, USA). The trial sequential analyses program (version 0.9 beta) was used for the TSA [78] (Copenhagen Trial Unit, Centre for Clinical Intervention Research, Copenhagen, Denmark, 2011).

Results

Calibration of the Gail model

Twenty-four studies incorporating 29 records were included to evaluate the calibration of the Gail model [9,10,11,12,13,14,15,16,17,18,19,20, 28,29,30,31,32,33,34,35, 38, 39, 41, 52] (Table 1). The pooled E/O ratio was 1.16 (95% CI 1.05–1.30) with a high level of heterogeneity between studies (I2 = 98.8%; p < 0.01) (Fig. 2a). Sensitivity analysis showed that the combined E/O ratio and 95% CI were not significantly altered before and after the omission of each dataset (see Additional file 1A). Cumulative analysis showed that by continually increasing the publication year and the sample size, the 95% CI became narrower and the pooled E/O ratio was closer to 1.0, which indicates that the precision of the pooled E/O ratio was gradually improved (see Additional file 1B, C). Publication bias was detected by funnel plot (regression coefficient = 5.38; p = 0.027) (see Additional file 2A). According to the trim-and-fill method, the adjusted pooled E/O ratio was 1.25 (95% CI 1.11–1.40) after trimming (see Additional file 2B).

Table 1 Characteristics of the included studies for estimating the calibration of the Gail model
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Fig. 2
figure 2

Calibration of the Gail model in total and stratified by geographic region with the trial sequential analysis. Forest plot of the pooled E/O ratio for the Gail model in total (a) and studies from America (b), Europe (c) and Asia (d), respectively. Trial sequential analysis (TSA) for pooled E/O ratio in total (e) and studies from America (f), Europe (g) and Asia (g), respectively. E/O expected-to-observed ratio, CI confidence interval

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Subgroup analysis suggested the geographic region (see Additional file 3) could partly explain the heterogeneities between these studies (p < 0.01). The Gail model exhibited a tendency to overpredict breast cancer risk for Asian women (pooled E/O = 1.98; 95% CI 1.58–2.48) compared to American (pooled E/O = 1.02; 95% CI 0.93–1.12) and European (pooled E/O = 1.05; 95% CI 0.68–1.63) women (Fig. 2b–d). Publication bias did not exist in each of these subgroups (see Additional file 4).

In addition, results showed that Gail model 1 accurately predicted breast cancer risk in American women (pooled E/O = 1.03; 95% CI 0.76–1.40). However, Gail model 2 overpredicted the risk for breast cancer (pooled E/O = 1.20; 95% CI 1.07–1.35) (see Additional file 3). When further stratified by different versions of Gail model 2, the pooled E/O ratios of Caucasian-American Gail model 2 in American [11, 12, 15,16,17,18,19,20, 31], European [28,29,30, 32,33,34,35] and Asian [39, 41] women were 0.98 (95% CI 0.91–1.06), 1.07 (95% CI 0.66–1.74) and 2.29 (95% CI 1.95–2.68), respectively. The pooled E/O ratio for Asian women was significantly higher than that in American and European females (p < 0.001). Moreover, only two studies clearly stated that they used the Asian-American Gail model [38, 41], and the results indicated that it overestimated the risk for Asian women about two times (pooled E/O = 1.82; 95% CI 1.31–2.51) (see Additional file 5).

When excluding studies conducted in Asian women [38, 39, 41], results showed that the Gail model precisely predicted the risk for developing breast cancer (pooled E/O = 1.04; 95% CI 0.93–1.16) (see Additional file 6A). Sensitivity analysis by singly eliminating each study showed no significant fluctuation, which indicated the stability of the results (see Additional file 6B). Cumulative analysis showed that the pooled E/O ratio became progressively closer to 1.0 according to accumulation of the publication year and sample size (see Additional file 6C, D). When stratified by different versions of the Gail model, the combined E/O ratios of Gail model 1 and Caucasian-American Gail model 2 were reported to be 1.03 (95% CI 0.76–1.40) and 1.05 (95% CI 0.93–1.17), respectively, with no significant difference (p = 0.93) (see Additional file 7). Stratified analysis showed that the studies with high reporting quality were prone to have a precise estimate of breast cancer risk (pooled E/O = 0.88; 95% CI 0.71–1.10 vs pooled E/O = 1.13; 95% CI 1.00–1.29; p = 0.06). However, no difference was found when stratified by the geographic region and other factors (see Additional file 8).

Trial sequential analysis

In the TSA, the cumulative Z-curve passed through both the conventional and the trial sequential monitoring boundary, which suggested the evidence was sufficient to verify the overprediction of the Gail model (Fig. 2e). When stratified by geographic region, the cumulative Z-curve did not cross the conventional and RIS boundary in American (Fig. 2f) and European (Fig. 2g) studies, demonstrating the accurate prediction of the Gail model. However, the evidence was insufficient to draw a firm conclusion and more related studies were required to confirm the results. With respect to Asian women, the Z-score crossed both the conventional and TSA-adjusted boundary, which showed the overestimation of breast cancer risk in Asian females and the evidence was sufficient and conclusive (Fig. 2h).

Discrimination of the Gail model

Twenty-six articles with 29 datasets describing the C-statistic/AUC of the Gail model were combined to evaluate its pooled discrimination [11, 15, 18,19,20,21,22,23,24, 27, 29,30,31,32, 34,35,36, 39,40,41,42,43,44,45,46, 51] (Table 2). The pooled AUC was 0.60 (95% CI 0.58–0.62) with substantial heterogeneity (I2 = 97.0%; p < 0.01) (Fig. 3a). Sensitivity analysis suggested that the results were stable, and cumulative analysis indicated that the 95% CI became narrower and the pooled AUC progressively rose toward 0.60 with the accumulation of data ranked by publication year and sample size (see Additional file 9A–C).

Table 2 Characteristics of the included studies for estimating the discrimination of the Gail model
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Fig. 3
figure 3

Pooled discrimination and diagnostic accuracy of the Gail model in total or stratified by geographic region. Pooled AUC/C-statistic of the Gail model in total (a) and studies from America (b), Europe (c) and Asia (d), respectively. Pooled sensitivity, specificity and diagnostic odds ratio (DOR) of the Gail model in total (e) and studies from America and Europe (f) and Asia (g), respectively. AUC area under the curve, CI confidence interval

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When stratified by geographic region, the pooled AUCs in American, European and Asian women were 0.60 (95% CI 0.58–0.62), 0.58 (95% CI 0.55–0.60) and 0.61 (95% CI 0.52–0.69), respectively, with no significant heterogeneities (p = 0.30) (Fig. 3b–d and see Additional file 10). Subgroup analysis also showed that the pooled AUC in studies with sample size ≥ 10,000 was lower (0.57 vs 0.64; p = 0.01). However, the combined AUC was not markedly changed when stratified by other factors (see Additional file 10). The funnel plot indicated no publication bias (Egger’s regression coefficient = −1.25; p = 0.54) (see Additional file 11A). According to the trim-and-fill method, eight studies had to be trimmed and the adjusted pooled AUC was 0.63 (95% CI 0.60–0.65) after trimming (see Additional file 11B). In addition, when stratified by geographic region, the funnel plot found significant publication bias across the studies in Europe (Egger’s regression coefficient = 4.45; p = 0.01) (see Additional file 12). After trimming, the adjusted AUC in European women was 0.59 (95% CI 0.56–0.62).

Results also showed the pooled AUC for Gail model 1 was 0.70 (95% CI 0.57–0.77), and when stratified by the geographic region the pooled AUCs for Gail model 1 in American and European women [22, 36] and Asian females [42, 44, 46] were 0.55 (95% CI 0.53–0.56) and 0.75 (95% CI 0.63–0.88), respectively (see Additional files 10 and 13). Additionally, the pooled AUC for Gail model 2 was 0.59 (95% CI 0.57–0.61), and when stratified by the geographic region and different versions of Gail model 2 the pooled AUCs for Caucasian-American Gail model 2 in American [15, 17,18,19,20,21, 23, 24, 27], Asian [39, 41, 43, 45] and European [29, 30, 32, 34, 35, 51] females were 0.61 (95% CI 0.59–0.63), 0.55 (95% CI 0.52–0.58) and 0.58 (95% CI 0.55–0.62), respectively (see Additional file 13). However, only one study clearly stated that they used Asian-American Gail model 2, and the AUC was reported to be 0.54 (95% CI 0.50–0.59) [41].

Diagnostic accuracy of the Gail model

Thirteen studies [11, 15, 25, 26, 31, 33, 37, 44, 45, 47,48,49,50] with 783,601 participants were included in this diagnostic meta-analysis (Table 3). The combined sensitivity, specificity and pooled DOR were 0.43 (95% CI 0.24–0.64), 0.88 (95% CI 0.81–0.92), and 5.44 (95% CI 2.17–13.63), respectively (Fig. 3e). Deeks’ funnel plot suggested that publication bias existed among the studies (p = 0.026) (see Additional file 14A).

Table 3 Characteristics of the included studies for estimating the diagnostic accuracy of the Gail model
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When stratified by geographic region, the pooled sensitivity, specificity and DOR in American and European women were 0.26 (95% CI 0.15–0.42), 0.84 (95% CI 0.72–0.91) and 1.79 (95% CI 1.57–2.05), respectively (Fig. 3f) and Deeks’ funnel plot showed no publication bias (p = 0.50) (see Additional file 14B). With respect to Asian women, the pooled sensitivity, specificity and DOR were 0.63 (95% CI 0.30–0.87), 0.91 (95% CI 0.84–0.95) and 17.56 (95% CI 4.13–74.59), respectively (Fig. 3g). However, publication bias persisted (p = 0.019) (see Additional file 14C).

When further stratified by different versions of the Gail model, the pooled sensitivity, specificity and DOR of Gail model 1 were 0.63 (95% CI 0.27–0.89), 0.91 (95% CI 0.87–0.94) and 17.38 (95% CI 2.66–113.70), respectively, and the corresponding indexes of Gail model 2 were 0.35 (95% CI 0.17–0.59), 0.86 (95% CI 0.76–0.92) and 3.38 (95% CI 1.40–8.17), respectively (see Additional file 15). When subgrouped by different versions of Gail model 2, the pooled sensitivity, specificity and DOR of the Caucasian-American Gail model for American and European women [11, 15, 25, 31, 33] were 0.36 (95% CI 0.27–0.45), 0.77 (95% CI 0.67–0.84) and 1.81 (95% CI 1.66–1.96), respectively, and for Asian females were 0.49 (95% CI 0.11–0.88), 0.90 (95% CI 0.76–0.96) and 8.80 (95% CI 1.19–64.81), respectively [37, 45, 47, 50] (see Additional file 16). However, only one study stated that they used the African-American Gail model and the sensitivity and specificity were reported to be 0.04 (95% CI 0.03–0.05) and 0.97 (95% CI 0.96–0.98), respectively [26]. Subgroup analysis also indicated that the pooled sensitivity with sample size < 1000 was higher than that in studies with ≥ 1000 samples, and the pooled specificity in studies with case–control design, sample size < 1000 and study quality < 8 points was higher than each of their counterparts (see Additional file 17).

Performance of the Gail model after excluding studies published in Chinese

When excluding studies retrieved in the WANFANG, VIP and CNKI databases, no effect was found on the calibration of Gail model 1. The E/O ratios of the Caucasian-American Gail model and the Asian-American Gail model for Asian women were reported as 2.46 (95% CI 2.10–2.88) and 1.82 (95% CI 1.68–2.04), respectively (see Additional file 18A).

The pooled AUC for Gail model 1 was 0.55 (95% CI 0.53–0.56). After excluding studies published in Chinese, only one study validated discrimination of Asian-American Gail model 2 and Caucasian-American Gail model 2 for Asian females and the AUCs were shown as 0.54 (95% CI 0.50–0.58) and 0.55 (95% CI 0.50–0.60), respectively [41] (see Additional file 18B).

For the diagnostic accuracy of the Gail model, after excluding studies conducted in China, the pooled sensitivity, specificity and the DOR of the Gail model were 0.24 (95% CI 0.14–0.38), 0.85 (95% CI 0.75–0.92) and 1.79 (95% CI 1.58–2.03), respectively. When stratified by different versions of the Gail model, the sensitivity, specificity and the DOR of the Caucasian-American Gail model were 0.25 (95% CI 0.14–0.41), 0.85 (95% CI 0.72–0.93) and 1.89 (95% CI 1.68–2.13), respectively. Only one study remained to evaluate the performance of Gail model 1, and the sensitivity and specificity were reported as 0.15 (95% CI 0.18–0.21) and 0.86 (95% CI 0.83–0.88), respectively [26] (see Additional file 19).

Discussion

The current study comprehensively evaluated the calibration, discrimination and diagnostic accuracy of different versions of the Gail model. Gail model 1 and Caucasian-American Gail model 2 accurately predicted breast cancer risk for American and European women. However, the Caucasian-American and Asian-American Gail models overpredicted the risk for developing breast cancer about two times in Asian females. TSA showed that evidence in Asian women was sufficient; nonetheless, the results in American and European women need further verification. Moreover, the discrimination and the diagnostic accuracy of any versions of the Gail model were not satisfactory overall or stratified by geographic region.

The current study showed that both the Caucasian-American and the Asian-American Gail models overpredicted the risk for developing breast cancer in Asian women. To explain the results, firstly, the Gail model was constructed based on American white females, but the incidence of breast cancer in Asia (29.1/100,000) was much lower than that in American women (69.9/100,000) [1]. Accordingly, during a specific period, Asian women might not present with so many breast cancer incident cases as expected, leading to a higher E/O ratio. Secondly, the distributions of factors included in the Gail model were different between Asian and American women. Morabia and Costanza [79] conducted an international comparison on reproductive factors in 1998 and found age at first live birth in Asian women was older than that in American females, which may present a higher risk prediction in Asia according to the Gail model [3, 12]. Another potential explanation was the lack of regular breast cancer screening in Asian women. In America, conventional mammography examination would be conducted for women aged 45–74 years every 1 or 2 years [80, 81] and the Gail model was constructed based on women with annual screening [3, 12]. However, routine screening was seldom conducted in Asian women [82]; many of the breast cancer patients could not be detected and resulted in a lower number of observed breast cancer than actually existed, resulting in a higher E/O ratio.

Gail model 1 was designed for white women who were being screened annually [3]. The current version of Gail model 2 used Surveillance Epidemiology and End Results (SEER) breast cancer rates for Asian-American women and the relative and attributable risks were derived from Asian-American females [8]. The Breast Cancer Risk Assessment Tool program specifically warns against the use of the Gail model in Asian women, where breast cancer rates are lower than those in Asian-American women [1]. Accordingly, the risk prediction of the Gail model should be explained with caution when applying it to Asian women and it is necessary to modify the Gail model based on the special risk factors and incidence of breast cancer in Asia, to improve its performance.

For the discrimination of the Gail model, results showed that the pooled AUC was moderately acceptable, while substantial heterogeneities exist between studies. Sample size could partly explain the phenomenon, and two studies with extreme value markedly affected the results. Anothaisintawee et al. [40] reported that the AUC of the Gail model was 0.41 with sample size > 1000, while the study conducted by Wang et al. [44] showed the AUC was 0.93 with < 1000 participants. Subgroup analysis showed no heterogeneities in sample size (≥ 1000 and < 1000) when these two datasets were excluded (0.62 vs 0.58; p = 0.07).

Previous meta-analyses also showed similar results that the Gail model had a satisfactory calibration and moderately acceptable discrimination [53,54,55]. Besides, the current study evaluated the diagnostic accuracy of the Gail model and the results showed that the sensitivity of the Gail model was poor and the results were even worse when focusing on the studies in American and European women. Accordingly, many of the breast cancer cases were misdiagnosed and this may partly explain the modest discrimination of the Gail model to some extent. Other risk factors for breast cancer such as mammographic density [83] and genetic factors [84] should be added to the Gail model in the future to provide a more accurate prediction of breast cancer. Nonetheless, few studies were combined to evaluate the diagnostic accuracy of the Gail model; more related studies are required to further confirm the results in the future.

Diagnostic meta-analysis also showed that the pooled specificity was higher in Asian women than that in American and European women, and studies with a case–control design, sample size < 1000 and study quality < 8 points presented a higher specificity than each of their counterparts. All studies in Asia were conducted using the hospital-based case–control design and the healthy controls were prone to have fewer risk factors than the cases. For example, biopsy is required for breast cancer cases, but is rarely used in healthy women in Asia; this may lead to lower prediction of risk in controls according to the Gail model and may increase the true negative rate and the specificity value. Moreover, most of the case–control studies were conducted with smaller sample sizes and lower study quality, and thus the difference in these subgroups may be partly explained by the distorted distribution of the case–control studies.

Additionally, Deeks’ funnel plot showed publication bias exists in Chinese studies, some studies with small sample size and lower DOR may not be published, and the diagnostic accuracy of the Gail model may be overestimated to some extent. Sensitivity analysis showed that when excluding studies conducted in Chinese, the pooled specificity of the Gail model was not significantly altered but the pooled sensitivity and DOR were markedly decreased.

Limitations

The current study detected substantial heterogeneities across the studies for the three statistics that we summarized; these heterogeneities can be partially explained, but could not be markedly diminished by different geographic regions and various versions of the Gail model. Secondly, although many studies tried to evaluate the performance of different versions of the Gail model, they could not be included in this meta-analysis as they did not provide necessary indexes of the E/O ratio or the AUC with 95% CIs [85, 86]. This limits the power of this meta-analysis to evaluate the performance of different versions of the Gail model. Thirdly, most of the included studies did not clarify which version of Gail model 2 was utilized in their studies. In the current meta-analysis, the American and European studies who cited Constantino et al.’s paper [12] and the Asian studies which were published before the Asian-American Gail model was developed [8] were all deemed to be Caucasian-American Gail model 2. This may lead to misclassification to some extent and may partly affect the precision of the results. Finally, in order to comprehensively evaluate the performance of the Gail model in China, the WANFANG, VIP and CNKI databases were searched, which may partly overestimate the diagnostic accuracy of the Gail model, although it has no significant effect on the Gail model’s calibration and discrimination.

Conclusions

Although the original Gail model 1 and the Caucasian-American Gail model had a well-fitting calibration in American and European women, the Caucasian-American and Asian-American Gail models may overestimate the risk in Asian females about two times. Moreover, the discrimination and diagnostic accuracy of the Gail model were not satisfactory overall or stratified by geographic region and different versions of the Gail model. Accordingly, the Gail model was appropriate for predicting the incidence of breast cancer in American and European women, but not suitable for use in Asian women. Furthermore, this model cannot tell a woman whether she will or will not develop breast cancer precisely. Even so, it is still very valuable for women to have a well-calibrated risk assessment and select different prevention strategies that are suitable for their risk level.