Optimisation of the selection of women with an increased risk of a masked tumour for supplementary screening

PURPOSE The sensitivity of mammograms is low for women with dense breasts, because cancers may be masked by dense tissue. In this study we investigate methods to identify women with density patterns associated with a high masking risk. Four methods based on quantitative volumetric breast density analysis are compared to an area based density measure. METHOD AND MATERIALS We used the last negative screening mammograms of 87 women who subsequently presented an interval cancer (IC), and 870 randomly selected normal screening exams from women without cancer as controls. Volumetric breast density maps (VBDMs) were computed using software provided by Matakina (Wellington, New Zealand). These maps provide the dense tissue thickness for each pixel location. We used the VBDMs to compute four masking measures: 1) Volume of glandular tissue (VGT), 2) Percent dense volume (PDV), 3) Percent area where dense tissue thickness exceeds 1cm (PA1), and 4) Possibility that there is a tumor, with diameter t, at a location with dense tissue thickness d, normalized to the breast area, taking into account the size distribution of screen detected cancers (PT). In addition we determined percentage dense area (PDA) after classifying pixels automatically in dense and non-dense classes using a random forest classifier. Methods were applied to MLO views and then averaged per exam. For each method, we selected cases with the highest masking measure by thresholding and computed the fraction of ICs as a function of the fraction of controls selected. Furthermore we computed the ratio between IC with supplemental screening offer and the supplemental screening rate. RESULTS The highest ratio between IC and supplemental screening rate has been observed for PA1 at a screening rate of 5% and a sensitivity of almost 20%. For screening rates above 20%, the highest response of IC can be observed when using PA1 and PT for stratification. CONCLUSION We showed that the different breast density measures are suited for stratification. A careful choice of the stratification criteria is necessary depending on the number of women that one is willing to offer supplemental screening. CLINICAL RELEVANCE/APPLICATION To make supplemental breast cancer screening feasible and cost efficient, it is necessary to have a high response in the target group while the selected women in the overall screening population remains low.