Breast cancers invisible on mammography

Background: A proportion of the cancers 'missed' at mammography are invisib le even with the benefit of hindsight. The aim of the present study was to identify a group of women with proven breast cancer whose mammograms did no t show a suspicious lesion even in retrospect (i.e. the truly mammographica lly occult cancers), and to compare them with mammogram-positive cancers. Methods: A total of 1757 breast cancers was diagnosed at the Wesley Breast Clinic's Screening or Diagnostic Services between July 1987 and August 1997 . One hundred and twenty cases were identified where, after independent rev iew by two of the authors, no mammographic abnormality could be found in th e region where the cancer was subsequently found. These 120 cases were comp ared with 1548 cancers considered to have a lesion visible on mammography, whether benign, indeterminate, suspicious or malignant in appearance. Results: In 90% of the mammogram-negative cancers, a clinical abnormality l ed to further investigation, while the remainder were found incidentally on ultrasound. There were a higher proportion of dense breasts, and of women aged 40-49, in the mammogram-negative cancers than in the mammogram-positiv e cancers. The mammogram-negative cancers were of smaller size overall, but three of them were surprisingly large (7-11 cm). In both the mammogram-pos itive and -negative cancers similar to 60% were ductal invasive cancers. Conclusions: Where factors are present that make mammographically occult ma lignancy more likely (e.g. age 40-49 and dense breasts), women may be targe ted for further investigation by other modalities. This is essential in the presence of a clinical abnormality.