Cancers can be hidden or “masked” by dense tissue. On a mammogram, cancer is white. Normal dense tissue also appears white. If a cancer develops in an area of normal dense tissue, it can be harder or sometimes impossible to see it on the mammogram, like trying to see a snowman in a blizzard. If a cancer (white) develops in an area of fat (black or dark gray), it is usually easier to detect even when it is small. Because dense tissue can hide cancers, the more fatty a breast is, the more effective the mammogram is in showing the cancer. As breast density increases, the ability to see cancer on mammography decreases. The images below are examples of how cancer presents in each breast density category:
Mammographic Images Showing How Cancer Looks in Each of the Breast Density Categories. A) A small cancer (arrow) is easily seen in a fatty breast. B) In this breast with scattered fibroglandular density, a large cancer is easily seen (arrow) in the relatively fatty portion of the breast, though a small cancer could have been hidden by areas of normal dense tissue. C) In this heterogeneously dense breast, a 4 cm cancer (arrows) is hidden by the dense breast tissue. Note the metastatic node in the left axilla (curved arrow). D) In this extremely dense breast, a cancer is seen because part of it is located in the back of the breast where there is a small amount of dark fat making it easier to see (arrow and triangle marker indicating lump). If this cancer had been located near the nipple and completely surrounded by white (dense) tissue, it probably would not have been seen on mammography.
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