Why don't cysts show up on a mammogram?

They may: But always clarified by ultrasound. Wouldn't it be great if they started with circular array, non-compession, prone u/s to start? Under development if the big companies do not kill it.

.: Most cysts show up on mammograms.

Cysts do show up ...: Cysts very commonly get detected by mammography. These are benign fluid-filled masses that can appear in breast tissue. On mammogram cyst appears as a round, oval, or lobulated mass with circumscribed margins. However, cyst margins may be obscured on and can't be reliably diagnosed with mammogram alone and further evaluation with spot-compression mammographic views and ultrasound is necessary.

They can, sort of: Sometimes they do, but since the density of cyst fluid is close(in the world of X-rays) to the density of soft tissue, a cyst and a solid nodule may look exactly the same on mammo.Due to the sonographic properties of fluid, ultrasound, on the other hand, can easily distinguish simple cysts from solid nodules.So although you might see a cyst on mammo, you don't know it's a cyst until the ultrasound