Cancer is composed of water-like material, which means that it appears light gray on a mammogram.
Some cancers contain calcium.
The normal fibrous and glandular tissue of the breast is also composed of water-like material, and also may contain calcium. Tiny breast cancers usually look identical to normal breast tissue. That is why they are so difficult to detect. As cancers grow and become larger, they begin to change the appearance of the breast. Cancer contains more water-like material, so it can appear whiter than normal breast tissue.