Breast cancer mammography patterns

Mammography studies, from 1963 through 1972, on 5,918 women over age 30 years with 327 breast cancers on initial studies, were prospectively categorized on a scale of 1 to 4 of increasing amounts of fibroglandular tissue. Approximately 60% of the cancers occurred in classes 1 and 2 breasts, about one‐third of the patients, while 40% of the cancers were in the remaining two‐thirds, comprising classes 3 and 4. There were 54 cancers that developed in breasts that previously were free of symptoms, clinical signs, and x‐ray abnormality. Up to 36 months one cancer was found in class 1, while 26 cancers were detected in class 4 breasts; two cancers developed in class 2 and seven in class 3. Cancers developing 38 to 88 months after normal examinations had an incidence of 0.23% in combined classes 1 and 2 and an incidence of 0.21% in classes 3 and 4. Dense fibroglandular tissue delays detection of breast cancer by mammography. Apparent increase in cancer risk in such breasts is due to this delay. More than a 3‐year follow‐up is required to assess the life history of breast cancer by mammography.