How do x-rays detect cancers of the breast?

Sooner than a lump: X-rays pass through tissues with different densities, and leave corresponding shadow images on the film or digital plate. Cancers show as lighter shadows (bone looks white, since few x-rays pass through dense bone) than breast tissue, since cancers are often slightly denser than breast tissue. Cancers can also have micro-calcifications that look like tiny white spots to the radiologist. Dr. Tholen.

Breast cancer: Xrays that pass through the body are absorbed on their way through based on the density of the structures they encounter. The more dense, the more the xray gets absorbed. Structures that are dense show up whiter on the image, stuctures less dense darker. Cancers are denser than average breast tissue, and tend to have a different shape and margins than normal tissue.