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.
Most importantly, breast cancer represents a new process in the breast and can be recognized as a change from a prior mammogram. This is the reason a comparison with a prior mammogram is so important.