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

S Meeson1, KC Young1, MG Wallis2, J Cooke3, A Cummin3 and M L Ramsdale1

1National Co-ordinating Centre for the Physics of Mammography, Department of Medical Physics, St. Luke’s Wing, Royal Surrey County Hospital, Guildford GU2 7XX, 2Warwickshire, Solihull & Coventry Breast Screening Centre, Coventry and Warwickshire Hospital, Stoney Stanton Road, Coventry CV1 4FH and 3Jarvis Breast Screening Centre, 60 Stoughton Road, Guildford GU1 1LJ, UK

The location, tissue background and imaging characteristics of true positive and false negative screens of breast cancers have been studied. This data can aid decisions in optimising the display of mammographic information with the objective of minimising 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 was also measured. Cancer locations were mapped and warped onto a typical image to show their spatial distribution. Where a cancer was detectable 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 (excluding calcifications) were approximately a factor of two lower in dense breasts compared to other breast types. Poorly defined masses that became interval cancers had mean OD differences that were approximately a factor of 0.1 OD lower than those that were detectable by screening. 22% of the false negative cancers 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, with sufficient contrast for suspicious mammographic details to be detected. Where the mean OD differences between the cancer and its background region 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.

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