Serendipity in diagnostic imaging: Magnetic resonance imaging of the breast

Background: Magnetic resonance imaging (MRI) of the breast has been propose d as a noninvasive diagnostic test for evaluation of suspicious ("index") l esions noted on mammography and/or clinical breast examination (CBE), Howev er, women may have incidental ("serendipitous") lesions detected by MRI tha t are not found on mammography or CBE, To understand better whether or not biopsy procedures should be performed to evaluate serendipitous lesions, we estimated the breast cancer risk for women with this type of lesion, Metho ds: A decision analysis model was used to estimate the positive predictive value (i,e,, the chance that a woman with a serendipitous lesion has cancer ) of MRI for serendipitous lesions in women who had an abnormal mammogram a nd/or CBE suspicious for cancer (where a biopsy procedure is recommended). We restricted the analysis to data from women whose index lesions were nonc ancerous and used meta-analysis of published medical literature to determin e the likelihood ratios (measures of how test results change the probabilit y of having cancer) for MRI and the combination of CBE and mammography, The positive predictive value of MRI was calculated using the U,S, population prevalence of cancer (derived from registry data) and the likelihood ratios of the diagnostic tests, Results: Under a wide variety of assumptions, the positive predictive value of MRI was extremely low for serendipitous lesio ns. For instance, assuming sensitivity and specificity values for MRI of 95 .6% and 68.6%, respectively, approximately four of 1000 55- to 59-year-old women with serendipitous lesions would be expected to have cancer (positive predictive value 0.44%, 95% confidence interval = 0.24%-0.67%). Conclusion : In women with a suspicious lesion discovered by mammography and/or CBE th at is found to be benign, serendipitous breast lesions detected by MRI are extremely unlikely to represent invasive breast cancer, Immediate biopsy of such serendipitous lesions may, therefore, not be required.