[BI-RADS] This is used to define the x-ray attenuation of the lesion relative to the expected attenuation of an equal volume of fibroglandular breast tissue. It is important that most breast cancers that form a visible mass are of equal or higher density than an equal volume of fibroglandular tissue. it is rare (although not impossible) for breast cancer to be lower in density. breast cancers are never fat-containing (radiolucent) although they may trap fat.