Published online before print November 16, 2007, 10.1148/radiol.2461061298
(Radiology 2008;246:116-124.)
© RSNA, 2007
Meta-Analysis of MR Imaging in the Diagnosis of Breast Lesions1
Nicky H. G. M. Peters, MD,
Inne H. M. Borel Rinkes, MD, PhD,
Nicolaas P. A. Zuithoff, MSc,
Willem P. T. M. Mali, MD, PhD,
Karel G. M. Moons, MSc, PhD, and
Petra H. M. Peeters, MD, PhD
1 From the Department of Radiology (N.H.G.M.P., W.P.T.M.M.); Department of Surgical Oncology (I.H.M.B.R.); and Department of Clinical Epidemiology, Julius Center for Health Sciences and Primary Care (N.H.G.M.P., N.P.A.Z., K.G.M.M., P.H.M.P.), University Medical Center Utrecht, Heidelberglaan 100, E01.132, 3584 CX Utrecht, the Netherlands. Received July 28, 2006; revision requested October 3; revision received December 21; accepted January 30, 2007; final version accepted June 1.
Address correspondence to N.H.G.M.P. (e-mail: n.peters{at}umcutrecht.nl).
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ABSTRACT
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Purpose: To determine, in a meta-analysis, the diagnostic performance of contrast material–enhanced magnetic resonance (MR) imaging in patients with breast lesions.
Materials and Methods: Studies to assess the diagnostic performance of MR imaging in patients suspected of having breast cancer who underwent MR imaging and biopsy from January 1985 through March 2005 were reviewed for inclusion. A summary receiver operating characteristic curve was constructed, and pooled weighted estimates of sensitivity and specificity were calculated by using the recently developed bivariate approach for diagnostic meta-analysis.
Results: Of 251 eligible studies, 44 were included in the meta-analysis (sample size range, 14–821; cancer prevalence, 23%–84%). Pooled weighted estimates of sensitivity and specificity were 0.90 (95% confidence interval: 0.88, 0.92) and 0.72 (95% confidence interval: 0.67, 0.77), respectively. The performance of breast MR imaging was influenced by the prevalence of cancer in the studied population (P = .05) and by whether two criteria (ie, morphology, enhancement, and kinetic enhancement pattern)—versus one or three criteria—were used to differentiate benign from malignant lesions (P = .02).
Conclusion: MR imaging of the breast has high sensitivity and lower specificity in the evaluation of breast lesions.
Supplemental material: http://radiology.rsnajnls.org/cgi/content/full/2461061298/DC1
© RSNA, 2007
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INTRODUCTION
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The value of magnetic resonance (MR) imaging of the breast is based on the capability of this modality to depict (a) multicentric and multifocal disease (1–7), (b) an invasive component in ductal carcinoma in situ lesions (8,9), (c) the tumor in a three-dimensional way (3,5,10,11), and (d) cancer in dense breast tissue (12–14). Thus, MR imaging has the potential to facilitate improved diagnosis and treatment in patients with breast lesions. Yet, for all of these applications, the level of diagnostic performance is important.
Numerous studies have been performed to assess the diagnostic performance of MR imaging in the evaluation of breast lesions (15–58). Hrung et al (59) performed a meta-analysis of studies performed between 1994 and 1997. Sensitivity and specificity varied widely among the included studies: Sensitivity ranged from 0.63 to 1.00, and specificity ranged from 0.21 to 1.00. At a sensitivity of 0.95, the corresponding specificity was 0.67. However, the patient characteristics, MR imaging techniques, and diagnostic criteria for malignancy in the studies differed substantially, and this may have compromised the comparison of the diagnostic performance of breast MR imaging between the studies. After 1997, many additional studies of the diagnostic performance of breast MR imaging were performed (15–17,19,20,22–25,27–29,33–35,37,38,47–50,54–57). The aim of our study was to determine, in a meta-analysis, the diagnostic performance of contrast material–enhanced MR imaging in patients with breast lesions.
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MATERIALS AND METHODS
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Search Strategy and Selection Criteria
A Medline search was performed (N.H.G.M.P.; February 23, 2005) by using the terms "Magnetic Resonance Imaging [MeSH] or MRI or MR mammography or MR-mammography" for the diagnostic test and "Breast Neoplasms [MeSH] or breast cancer or breast lesions or breast" for the clinical domain. To avoid missing relevant studies, we did not include terms regarding diagnostic performance in our search criteria. We limited our search to the English language, results had to apply to human female subjects older than 19 years, and the search terms had to be present in the title or abstract of the article. Since contrast-enhanced MR imaging of the breast was not introduced before 1985, we limited the search to publication dates from January 1985 through March 2005. We did not define a minimal subject sample size for inclusion. In addition, with use of the same criteria applied in the Medline search, published meta-analyses and reviews on this subject were searched for (N.H.G.M.P.; July 7, 2005) in PubMed, the DARE (Database of Abstracts of Reviews of Effectiveness) database, and the Cochrane library to find additional studies.
Eligibility Criteria and Data Extraction
Studies were excluded after exclusion criteria based on quality assessment of diagnostic accuracy studies, or QUADAS (60), and standards for reporting of diagnostic accuracy, or STARD (61), guidelines were prospectively defined and applied. First, the inclusion and exclusion criteria were applied to the titles of the study articles that fulfilled the search criteria. Then, the same criteria were applied to the article abstracts of the studies that were still eligible after the titles were reviewed. Finally, for the remaining studies, we applied inclusion and exclusion criteria to the full extent of the remaining studies to obtain the final studies to be included in the meta-analysis.
Patients.—Our main interest was the diagnostic performance of MR imaging of small lesions detected at mammographic screening. Because a large proportion of these lesions are nonpalpable, we included only those studies that included at least one patient with a nonpalpable lesion. Articles with data on patients with only palpable or otherwise symptomatic lesions were excluded since sensitivity and specificity vary with lesion size and thus with disease severity (62,63). Such variation can affect the overall estimates of the sensitivity and specificity of breast MR imaging (62–64).
Index test.—The diagnostic test evaluated in this meta-analysis was contrast-enhanced MR imaging of the breast. There is substantial heterogeneity among the studies in the types of MR imaging systems, image acquisition protocols, contrast agents, contrast agent doses, and image interpretation methods, all of which may affect the sensitivity and specificity of MR imaging in breast lesion characterization. We included only those studies that were performed by using an MR system with a field strength of at least 1.5 T, in which T1-weighted images were acquired before and after contrast agent administration, and in which no complex imaging modalities such as MR spectroscopy or diffusion-weighted imaging were used. By applying these criteria, we sought to include only those studies performed according to current clinical practice standards and to increase the comparability of the studies.
Reference standard.—To be eligible for inclusion, the results of breast MR imaging had to be compared with those of an appropriate reference-standard examination. Hence, to minimize the effects of partial verification bias, we included only those studies in which a valid reference test was used in more than 75% of the included patients (60,65). Histologic analysis (of a large core-needle biopsy specimen or surgical specimen) and mammographic and clinical follow-up of more than 2 years were considered valid reference standards.
Data extraction.—We included only those studies in which the numbers of true-positive, false-positive, false-negative, and true-negative findings, as well as the numbers of studies on which these values were based, were reported. If in one article, the results were presented for several observers (to obtain the lowest number of missed cancers or the lowest number of false-positive cases), those results that were "most likely in accordance with clinical practice," as judged by two authors (P.H.M.P., W.P.T.M.M.), were abstracted. A prospectively constructed data extraction sheet based on quality assessment of diagnostic accuracy studies guidelines for diagnostic performance studies included in systematic reviews (60) extended with topic-specific items was applied to the eligible studies (45 items).
Data were extracted by two reviewers (N.H.G.M.P. and I.H.M.B.R., W.P.T.M.M., or P.H.M.P.) independently. In cases of discrepancy between the first two reviewers, a third reviewer (P.H.M.P. or W.P.T.M.M.) was consulted and a consensus was reached. The following data were abstracted from the eligible articles: country where the study was performed, year of publication, study design (case-control or patient cohort), mean patient age, cancer prevalence, prevalences of palpable lesions and invasive ductal carcinoma, contrast agent type and dose, whether the criteria for malignancy (lesion morphology, enhancement characteristics, and contrast enhancement over time) were reported, number of criteria used to differentiate benign from malignant lesions, whether the threshold for malignancy was determined a priori or post hoc during analysis of the study results, percentage of patients in whom an appropriate reference standard was used, number of different reference standards used, whether the interpreter of the MR images was blinded to clinical information and/or the reference-standard examination results, and lesion-to-patient ratio (number of included lesions divided by number of included patients).
Statistical Analyses
Overall analysis.—Sensitivity was calculated as TP/(TP + FN), where TP is the number of true-positive findings and FN is the number of false-negative findings. Specificity was calculated as TN/(FP + TN), where TN is the number of true-negative findings and FP is the number of false-positive findings. The diagnostic odds ratio (DOR) was calculated as (TP · TN)/(FP · FN). If the DOR could not be calculated because one of the cells in the 2 x 2 table was empty, 0.5 was added to all cells so that an approximate DOR could be calculated (66,67). The DOR is a single overall indicator of diagnostic performance and is the ratio of the odds of positivity in disease relative to the odds of positivity in nondiseased subjects (68). If the DOR is equal across studies, the only cause of heterogeneity is a difference in cutoff levels for malignancy. If the DOR varies across studies, factors other than cutoff differences exist as well (68). With use of statistical software (SAS, version 8; SAS Institute, Cary, NC), Forest plots for sensitivity and specificity were constructed to graphically present the sensitivity and specificity values, with corresponding 95% confidence intervals (CIs), for the individual studies.
A summary receiver operating characteristic (sROC) curve was constructed by using Stata, version 7 (Stata, College Station, Tex), software. The sROC curve is a method of summarizing the true- and false-positive rates from different diagnostic studies (66,69). If all of the sensitivity–1 – specificity combinations in the individual studies lie on the best fit line in the sROC curve or the deviation from this line can be assumed to be a random variation, the observed differences between studies can be explained by differences in cutoff levels for malignancy (66,68). If, however, the line does not fit the data points, heterogeneity between the studies due to causes other than cutoff differences—such as differences in disease severity, study design, or mean age—could be present.
The goodness of fit of the sROC curve was evaluated by using the (unweighted) R2 statistic of the underlying model, in which the logarithm of the DOR was the dependent variable and the sum of the logit sensitivity and logit specificity was the independent variable (proc reg, SAS, version 9) (69,70). If all observed values fall on the fit regression line, R2 is 1. If there is no relationship between the observed and predicted values, R2 is 0.
Overall estimates of sensitivity and specificity cannot be calculated separately with the sROC method. Furthermore, the effect of covariates cannot be studied adequately with either the DOR or the sROC approach, as neither method takes into account the correlation between the sensitivity and specificity values within studies (66,68,69,71). Hence, we applied the random effects model by using the relatively recently introduced bivariate analysis for diagnostic meta-analysis (proc mixed routine) to obtain an overall sensitivity and an overall specificity with a 95% confidence ellipse (SAS, version 9) (71). The 95% confidence ellipse is a combination of the 95% CI for the overall sensitivity and the 95% CI for the overall specificity and can be interpreted as the region that contains the most likely combinations of summary sensitivity and specificity values.
Subgroup analysis.—Finally, we studied the effects of heterogeneity between studies on overall sensitivity and overall specificity. We included each of the following covariates separately in the bivariate model to compare the overall sensitivity and overall specificity between different strata: country where the study was performed, year of publication, study design, mean patient age, cancer prevalence, palpable lesion prevalence, invasive ductal carcinoma prevalence, contrast agent type, contrast agent dose, number of criteria used to differentiate benign from malignant lesions (ie, morphology, enhancement, kinetic enhancement pattern; any one, any two, or all three of these criteria), whether the criteria for malignancy were reported, whether the threshold for malignancy was determined before or after the study was conducted, percentage of patients in whom an appropriate reference standard was used, number of different reference standards used, whether the interpreters of the MR images were blinded to clinical information and histologic findings, and lesion-to-patient ratio. Stratum-specific sensitivities and specificities were compared for all characteristics.
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RESULTS
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Evaluated Studies
By using the search terms described earlier, we identified 1069 studies. After reviewing the article titles for these studies, we excluded 818 studies. After review of the article abstracts of the remaining 251 studies, 117 studies were left. After cross-referencing the references from a previous meta-analysis (59), two extra studies were included. After review of the 119 articles, 44 studies remained for inclusion in our meta-analysis (Fig 1). The abstracted data of these individual studies are summarized in Table E1 (http://radiology.rsnajnls.org/cgi/content/full/2461061298/DC1).

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Figure 1: Flowchart of eligible studies. *Other clinical domains such as patients with metastatic breast cancer, patients with recurrent cancer, and patients at high risk. Other aims such as MR-guided interventions, MR evaluation of effect chemotherapy, and MR evaluation of breast implants.
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Overall Analysis
Specificity varied across studies more than sensitivity did (Fig 2). The fact that the fit line of the sROC curve did not precisely fit the data points (Fig 3) suggests that factors other than differences in cutoff points for malignancy caused variations in the sensitivity and specificity of breast MR imaging across the studies. The goodness-of-fit test revealed the same finding: The R2 statistic was 0.12, with a P value of .024, indicating significant heterogeneity across the studies. The overall sensitivity, overall specificity, and DOR based on the bivariate approach were 0.90 (95% CI: 0.88, 0.92), 0.72 (95% CI: 0.67, 0.77), and 23.5 (95% CI: 16.8, 32.9), respectively (Fig 3).

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Figure 2: Forest plot of sensitivity and specificity, with corresponding 95% CIs, of included studies. Numbers 15–58 are the reference numbers of the individual studies. n/N = number of studies included/total number of studies. p/P = number of patients included/total number of patients.
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Subgroup Analysis
The mean sample size for the 44 studies was 128 patients (range, 14–821 patients), and 26 of the 44 studies were performed after 1997. The mean prevalences of cancer, palpable lesions, and invasive ductal carcinoma were 54% (range, 23%–84%), 49% (range, 2%–86%), and 70% (range, 23%–100%), respectively. The mean patient age was 51 years (range, 46–58 years). The number of criteria used to differentiate benign from malignant lesions was unknown in 11, one in 14, two in nine, and three in 10 studies (Table, Table E1; http://radiology.rsnajnls.org/cgi/content/full/2461061298/DC1). Specificity varied with or across cancer prevalence and number of criteria used to differentiate benign from malignant lesions: It decreased with increasing cancer prevalence (specificities for low, moderate, and high prevalence: 0.81, 0.71, and 0.61, respectively) and was higher when two criteria (0.81) were used to differentiate benign from malignant lesions than when one (0.74) or all three (0.67) criteria were used (Table). These findings are consistent with the larger variability in specificity across studies (Fig 2).
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DISCUSSION
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In our meta-analysis, we calculated an overall sensitivity of 0.90 (95% CI: 0.88, 0.92) and an overall specificity of 0.72 (95% CI: 0.67, 0.77). Specificity varied across the individual studies more than sensitivity did, and it varied with or across cancer prevalence and number of criteria used to differentiate benign from malignant lesions.
In one study, Sherif et al (43) reported a sensitivity (0.52) that was substantially lower than the sensitivities in the other studies; however, the reported specificity was 1.00. We believe that this deviation was due to the fact that a cutoff level was chosen to optimize specificity, and this resulted in a low sensitivity. If the line of the sROC curve were extrapolated, it could be seen that this study also lay on the sROC curve; this finding suggests that there was a difference in the chosen threshold only.
In most diagnostic meta-analyses, results are summarized in an sROC curve, a DOR, or the Q point (the point where sensitivity equals specificity)—measures that are less useful to clinicians. By using the bivariate approach for diagnostic meta-analysis, we derived overall estimates of sensitivity and specificity. At a sensitivity of 0.90, we derived a specificity of 0.72, meaning that if women with a breast lesion detected at mammography, ultrasonography (US), or physical examination were referred for MR imaging, 28% of the lesions would be false-positive for cancer. However, if more precise mammography, US, and/or physical examination data were taken into account, our estimates of the diagnostic performance of breast MR imaging might have been influenced.
A previous meta-analysis included 16 studies that were published between 1994 and 1997 (59). The results were presented in an sROC curve, the Q point, and a likely optimal point (sensitivity of 0.95 with corresponding specificity of 0.67). We thought it relevant to perform a new meta-analysis of the diagnostic performance of breast MR imaging because numerous studies were performed after 1997, MR imaging technology has since improved substantially, and the bivariate approach for diagnostic meta-analysis, which facilitates more accurate summary estimates of sensitivity and specificity, has since become available. Thus, our results are difficult to compare with those of Hrung et al (59). At a sensitivity of 0.95, we derived a specificity of approximately 0.55 in our sROC curve analysis, compared with the specificity of 0.67 derived by Hrung et al. We included an additional 24 studies that were published after the previous meta-analysis. Despite improvements in MR technique and interpretation ability over time, our estimates for the diagnostic performance of breast MR imaging were slightly lower than those derived in the previous meta-analysis. We hypothesize that this can be explained by the inclusion of patients with more apparent breast lesions in the earlier studies; patients with more clinically relevant lesions were included in the later studies, and, thus, the study populations had higher proportions of nonpalpable and noninvasive lesions.
Our data show that specificity varied with or across cancer prevalence. However, from a mathematical point of view, sensitivity and specificity cannot be affected by prevalence. We believe that disease prevalence influences specificity indirectly—for example, through differences in disease severity or other characteristics across studies (62,63). We planned to perform a multivariable analysis to determine whether cancer prevalence still affected specificity after correction for other factors. We were not able to perform such an analysis, however, because of the relatively small number of included studies and because the number of missing data was too large.
We cannot fully explain why the specificity was higher when two criteria for malignancy were used than when all three criteria were used. We expected sensitivity and specificity to be highest in the subgroup of studies involving the use of all three criteria. Many studies were performed before the introduction of guidelines for interpretation of MR mammograms (72). It could be that the radiologists in the studies with higher sensitivity and specificity used all three criteria but did not describe them as three separate criteria. Furthermore, eight of the 10 studies with three criteria used were performed outside of the United States. Since specificity was lower in the non-U.S. studies, the apparent influence of the number of malignancy criteria on specificity could have been related to the difference in the country of publication between the studies. However, these effects could not be differentiated in our study because a multivariable analysis was not feasible owing to missing data and the relatively small number of included studies.
We did not make adjustments for multiple testing. The use of adjusted P values would have prevented false-positive findings—but at the cost of missed relevant associations (ie, higher rate of type 2 errors). In addition, adjustment of P values—if applied at all—should be considered in studies that are merely "fishing expeditions," "data dredging," or "hypotheses generating." This was not the case in our study. We therefore believe that the two positive findings for subgroup analyses that we report are not false (73,74).
Our study had limitations. First, the quality of a meta-analysis depends on the quality of the individual studies included in the meta-analysis (65). We did not include a quality score on our data extraction sheet because all available quality scores are highly subjective (60). By applying strict inclusion and exclusion criteria, we included only those studies that were of sufficient quality to allow estimates of the sensitivity and specificity of breast MR imaging. We searched the Medline database and cross-referenced published meta-analyses and reviews on this subject. Owing to the large number of articles identified, we did not search for nonpublished studies.
Second, predictive values, such as probability of cancer given a positive or negative MR result, are even more useful in clinical practice because patients and physicians are even more interested in the predictive value of combinations of diagnostic test results than they are in the predictive value of a single test result (64). In the case of breast cancer, such a set could include patient characteristics and conventional mammography, US, and breast MR imaging results. In the majority of studies included in our meta-analysis, combined results of different diagnostic tests were not reported, so it was impossible to report such combination data. However, there are as yet no available methods to properly pool the predictive values of test results—let alone the predictive values of combinations of test results.
Third, the results of our covariate analysis should be interpreted with caution because certain studies did not include information on several relevant characteristics, so there was a substantial number of missing values for some of the studied characteristics. Thus, further research is required to make more precise statements about the effect of covariates on sensitivity and specificity.
In conclusion, all currently available evidence indicates that MR imaging has high sensitivity (0.90) and lower specificity (0.72) in patients referred for biopsy of a breast lesion. We believe that further research is required to determine the diagnostic performance of breast MR imaging in patients with nonpalpable breast lesions.
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ADVANCES IN KNOWLEDGE
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- The overall sensitivity and overall specificity for MR imaging in the evaluation of breast lesions in 44 studies were 0.90 and 0.72, respectively.
- The overall specificity varied across cancer prevalence and number of criteria used to differentiate benign from malignant breast lesions.
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IMPLICATION FOR PATIENT CARE
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- For definitive characterization of breast lesions, biopsy cannot yet be replaced by MR imaging.
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ACKNOWLEDGMENTS
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The authors thank Evelyn Monninkhof, PhD, for assistance in planning the meta-analysis, Cees Haaring for constructing the database and technical support, Lukas Bachmann, MD, PhD, for help with the sROC analysis, and Johannes Reitsma, MD, PhD, for help with the bivariate analysis.
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FOOTNOTES
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Abbreviations: CI = confidence interval DOR = diagnostic odds ratio sROC = summary receiver operating characteristic
Guarantor of integrity of entire study, P.H.M.P.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, N.H.G.M.P., I.H.M.B.R., W.P.T.M.M., P.H.M.P.; statistical analysis, N.H.G.M.P., N.P.A.Z., K.G.M.M., P.H.M.P.; and manuscript editing, all authors
Authors stated no financial relationship to disclose.
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