What Are The Chances Of Uterine Fibroid To Be Malignant?

I woke up one morning with severe abdominal pain, was taken to emergency where I had an MRI and it was found i had a large fibroid growing on the outside of my uterus at the top. It was fairly large. Six weeks later i went back for another MRI to discover that the fibroid had grown significantly. I am a 56 yr old and stopped menstruating in October of 2011. My gynecologist is going to do a hysterectomy. So worried about cancer. What are the chances