Usually once a cyst is identified it is merely followed to ensure that it does not continue to enlarge. When she went in for her normal ultrasound they discovered that she has several benign cysts within her left ovary. Cysts form in the ovary every month after puberty. In pre-puberty females cysts can also form. They are rarely studied since young girls rarely need ultra-sounds.
Benign cysts have a different appearance on ultra-sound than cancerous growths.
However, the only absolute way is to insert a needle into the cyst. I wouldn't he too concerned. A follow up ultra-sound will demonstrate either no change or regression of the cysts. Ovarian cancer is almost unheard of before 18. I've had a cyst in my lower left arm for most of my life.