Breast imaging: understanding how accuracy is measured when lesions are the unit of analysis

Medical imaging tests of breast cancer patients can be used to detect and provide information on the location of multiple malignant lesions within a patient. Within this context, it is often the case that one needs to evaluate the accuracy of an imaging test for finding the multiple lesions in a patient rather than simply detecting that a patient has disease. A natural way to approach this task is to estimate the accuracy of the test using a lesion-level analysis. Sensitivity, specificity, and receiver operating characteristic (ROC) curves are analytic measures that are frequently used to quantify the accuracy of medical tests. When the test or radiologist must first locate the lesions, however, it is not possible to directly estimate the specificity or an ROC curve keeping the individual lesions as the unit of analysis. The goal of this study is to demonstrate to clinicians conducting or reviewing studies evaluating breast imaging tests what measures of accuracy can and cannot be calculated in different types of studies and to describe in detail the difficulty with calculating specificity and ROC curves in a lesion-level analysis.