Image features of true positive and false negative cancers in screening mammograms

Abstract

The location, tissue background and imaging characteristics of true positive and false negative screens of breast cancers have been studied. These data can aid decisions in optimising the display of mammographic information, with the objective of minimizing false negative screens. Screening mammograms for four groups of women were digitised: those with screen detected cancers, those with false negative interval cancers, and matched normals for both groups. The optical density (OD) distribution in the main breast region of each mammogram was determined. The OD in three regions of interest around the cancers were also measured: corresponding to the cancer centre, its outline, and a ring of background tissue. Cancer locations were mapped and warped onto a typical image. Where a cancer was detected by calcifications alone it had a relatively low probability of being a false negative interval cancer. The mean OD differences between the cancer and the cancer background region were approximately a factor of two lower in dense breasts rather than in other breast types. Poorly defined masses that became interval cancers had mean OD differences that were approximately of 0.1 OD lower than those that were detectable by screening. Of the false negative cancers 22% were located near the chest wall edge of the mammograms, compared to 10% of the true positives. The results indicate the importance of effectively displaying information in the lighter areas of the mammogram, corresponding to glandular tissues. Where the cancer mean OD differences are low, as measured for some poorly defined masses, there is an increased risk of a false negative interval cancer. Particular attention should be given to the chest wall area of the film, especially in the lower retroglandular region, during routine screening.