Dear Trouble 72, There is not an absolute way of differentiating a cyst from cancer on a mammogram, however, there are certain findings that may be more suspicious one way or another. For example, a well defined area may be more likely a cyst. They may decide to do an ultrasound which is used to help differentiate between a fluid-filled structure (cyst) and solid area.
a cyst is a fluid filled area, and they are typically very round with smooth borders. Cancer has irregular borders which are fuzzy, sort of star-shaped, because it tends to grow outward in all directions. So a very round shadow nearly always is non-cancerous; you can't tell if it's a cyst for sure on a mammogram, because you can't tell if it's liquid. The other common very round shadow in a breast is a harmless tumor called a fibroadenoma. The way to tell for sure, if it can't be felt to poke a needle in it to look for fluid, is with an ultrasound.