I'm glad you are going to spay her! That's a wise and responsible decision. Cancer or pyometra (infection of the uterus) is open to all breeds.

Shepherds ARE, however, a very unhealthy breed in general -- subject to hip and elbow dysplasia, skin disorders, cancer, bloat --- pretty much everything. You said before your dog is a GSD mix though, so there's a great chance she will be fine. Shepherds were pretty much ruined by bad breeding after the war, when everyone wanted one. They bred them very carelessly and that's why they often have health and temperment problems today. It's difficult to get a healthy specimen of a purebred GSD unless you go to a good, reputable breeder.

Female dogs do have a higher chance of cancer than males from not being altered. Males can get testicular or prostate cancer, but it's not a huge chance whereas the chance with females is alot greater.